Sunday, June 27, 2010

nevrvous system & musculr

The Muscular System
Chapter Outline
Muscle Structure
Muscle Arrangements
Antagonistic muscles
Synergistic muscles
The Role of the Brain
Muscle Tone
Muscle Sense
Energy Sources for Muscle Contraction
Muscle Fiber—Microscopic Structure
Contraction—The Sliding Filament Mechanism
Responses to Exercise—Maintaining
Aging and the Muscular System
Major Muscles of the Body
Muscles of the Head and Neck
Muscles of the Trunk
Muscles of the Shoulder and Arm
Muscles of the Hip and Leg
Student Objectives
• Name the organ systems directly involved in
movement, and state how they are involved.
• Describe muscle structure in terms of muscle cells,
tendons, and bones.
• Describe the difference between antagonistic and
synergistic muscles, and explain why such arrangements
are necessary.
• Explain the role of the brain with respect to skeletal
• Define muscle tone and explain its importance.
• Explain the difference between isotonic and isometric
• Define muscle sense and explain its importance.
• Name the energy sources for muscle contraction,
and state the simple equation for cell respiration.
• Explain the importance of hemoglobin and myoglobin,
oxygen debt, and lactic acid.
• Describe the neuromuscular junction and state the
function of each part.
• Describe the structure of a sarcomere.
• Explain the following in terms of ions and charges:
polarization, depolarization, and repolarization.
• Describe the sliding filament mechanism of muscle
• Describe some of the body’s responses to exercise
and explain how each maintains homeostasis.
• Learn the major muscles of the body and their
The Muscular System
New Terminology
Actin (AK-tin)
Antagonistic muscles (an-TAG-on-ISS-tik MUSSuhls)
Creatine phosphate (KREE-ah-tin FOSS-fate)
Depolarization (DE-poh-lahr-i-ZAY-shun)
Fascia (FASH-ee-ah)
Insertion (in-SIR-shun)
Isometric (EYE-so-MEH-trik)
Isotonic (EYE-so-TAHN-ik)
Lactic acid (LAK-tik ASS-id)
Muscle fatigue (MUSS-uhl fah-TEEG)
Muscle sense (MUSS-uhl SENSE)
Muscle tone (MUSS-uhl TONE)
Myoglobin (MYE-oh-GLOW-bin)
Myosin (MYE-oh-sin)
Neuromuscular junction (NYOOR-oh-MUSS-kyooler
Origin (AHR-i-jin)
Oxygen debt (AHKS-ah-jen DET)
Polarization (POH-lahr-i-ZAY-shun)
Prime mover (PRIME MOO-ver)
Sarcolemma (SAR-koh-LEM-ah)
Sarcomeres (SAR-koh-meers)
Synergistic muscles (SIN-er-JIS-tik MUSS-uhls)
Tendon (TEN-dun)
Related Clinical Terminology
Anabolic steroids (an-a-BOLL-ik STEER-oyds)
Atrophy (AT-ruh-fee)
Botulism (BOTT-yoo-lizm)
Hypertrophy (high-PER-truh-fee)
Intramuscular injection (IN-trah-MUSS-kyoo-ler in-
Muscular dystrophy (MUSS-kyoo-ler DIS-truh-fee)
Myalgia (my-AL-jee-ah)
Myasthenia gravis (MY-ass-THEE-nee-yuh GRAHviss)
Myopathy (my-AH-puh-thee)
Paralysis (pah-RAL-i-sis)
Range-of-motion exercises (RANJE-of-MOH-shun
Sex-linked trait (SEX LINKED TRAYT)
Tetanus (TET-uh-nus)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
Do you like to dance? Most of us do, or we may
simply enjoy watching good dancers. The grace and
coordination involved in dancing result from the
interaction of many of the organ systems, but the one
you think of first is probably the muscular system.
There are more than 600 muscles in the human
body. Most of these muscles are attached to the bones
of the skeleton by tendons, although a few muscles
are attached to the undersurface of the skin. The primary
function of the muscular system is to move
the skeleton. The muscle contractions required for
movement also produce heat, which contributes to the
maintenance of a constant body temperature. The
other body systems directly involved in movement
are the nervous, respiratory, and circulatory systems.
The nervous system transmits the electrochemical
impulses that cause muscle cells to contract. The respiratory
system exchanges oxygen and carbon dioxide
between the air and blood. The circulatory system
brings oxygen to the muscles and takes carbon
dioxide away.
These interactions of body systems are covered in
this chapter, which focuses on the skeletal muscles.
You may recall from Chapter 4 that there are two
other types of muscle tissue: smooth muscle and cardiac
muscle. These types of muscle tissue will be discussed
in other chapters in relation to the organs of
which they are part. Before you continue, you may
find it helpful to go back to Chapter 4 and review the
structure and characteristics of skeletal muscle tissue.
In this chapter we will begin with the gross (large)
anatomy and physiology of muscles, then discuss the
microscopic structure of muscle cells and the biochemistry
of muscle contraction.
All muscle cells are specialized for contraction. When
these cells contract, they shorten and pull a bone to
produce movement. Each skeletal muscle is made of
thousands of individual muscle cells, which also may
be called muscle fibers (see Fig. 7–3 later in this
chapter). Depending on the work a muscle is required
to do, variable numbers of muscle fibers contract.
When picking up a pencil, for example, only a small
portion of the muscle fibers in each finger muscle will
contract. If the muscle has more work to do, such as
picking up a book, more muscle fibers will contract to
accomplish the task.
Muscles are anchored firmly to bones by tendons.
Most tendons are rope-like, but some are flat; a flat
tendon is called an aponeurosis. (See Fig. 7–9 later in
this chapter for the epicranial aponeurosis, but before
you look, decide what epicranial means.) Tendons are
made of fibrous connective tissue, which, you may
remember, is very strong and merges with the fascia
that covers the muscle and with the periosteum, the
fibrous connective tissue membrane that covers bones.
A muscle usually has at least two tendons, each
attached to a different bone. The more immobile or
stationary attachment of the muscle is its origin; the
more movable attachment is called the insertion. The
muscle itself crosses the joint of the two bones to
which it is attached, and when the muscle contracts it
pulls on its insertion and moves the bone in a specific
Muscles are arranged around the skeleton so as to
bring about a variety of movements. The two general
types of arrangements are the opposing antagonists
and the cooperative synergists.
Antagonistic Muscles
Antagonists are opponents, so we use the term antagonistic
muscles for muscles that have opposing or
opposite functions. An example will be helpful here—
refer to Fig. 7–1 as you read the following. The biceps
brachii is the muscle on the front of the upper arm.
The origin of the biceps is on the scapula (there are
actually two tendons, hence the name biceps), and the
insertion is on the radius. When the biceps contracts,
it flexes the forearm, that is, bends the elbow (see
Table 7–2 later in this chapter). Recall that when a
muscle contracts, it gets shorter and pulls. Muscles
cannot push, for when they relax they exert no force.
Therefore, the biceps can bend the elbow but cannot
straighten it; another muscle is needed. The triceps
brachii is located on the back of the upper arm. Its origins
(the prefix tri tells you that there are three of
them) are on the scapula and humerus, and its insertion
is on the ulna. When the triceps contracts and
pulls, it extends the forearm, that is, straightens the
138 The Muscular System
Joints that are capable of a variety of movements
have several sets of antagonists. Notice how many
ways you can move your upper arm at the shoulder, for
instance. Abducting (laterally raising) the arm is the
function of the deltoid. Adducting the arm is brought
about by the pectoralis major and latissimus dorsi.
Flexion of the arm (across the chest) is also a function
of the pectoralis major, and extension of the arm
(behind the back) is also a function of the latissimus
dorsi. All of these muscles are described and depicted
in the tables and figures later in the chapter. Without
antagonistic muscles, this variety of movements would
not be possible.
You may be familiar with range-of-motion (or
ROM) exercises that are often recommended for
patients confined to bed. Such exercises are designed
to stretch and contract the antagonistic muscles of a
joint to preserve as much muscle function and joint
mobility as possible.
Synergistic Muscles
Synergistic muscles are those with the same function,
or those that work together to perform a particular
function. Recall that the biceps brachii flexes the
forearm. The brachioradialis, with its origin on the
humerus and insertion on the radius, also flexes
the forearm. There is even a third flexor of the forearm,
the brachialis. You may wonder why we need
three muscles to perform the same function, and the
explanation lies in the great mobility of the hand. If
the hand is palm up, the biceps does most of the work
of flexing and may be called the prime mover. When
the hand is thumb up, the brachioradialis is in position
to be the prime mover, and when the hand is palm
down, the brachialis becomes the prime mover. If you
have ever tried to do chin-ups, you know that it is
much easier with your palms toward you than with
palms away from you. This is because the biceps is a
larger, and usually much stronger, muscle than is the
Muscles may also be called synergists if they help to
stabilize or steady a joint to make a more precise
movement possible. If you drink a glass of water, the
biceps brachii may be the prime mover to flex the
forearm. At the same time, the muscles of the shoulder
keep that joint stable, so that the water gets to
your mouth, not over your shoulder or down your
chin. The shoulder muscles are considered synergists
for this movement because their contribution makes
the movement effective.
Even our simplest movements require the interaction
of many muscles, and the contraction of skeletal muscles
depends on the brain. The nerve impulses for
movement come from the frontal lobes of the cerebrum.
The cerebrum is the largest part of the brain;
the frontal lobes are beneath the frontal bone. The
The Muscular System 139
Ulna Humerus
Triceps (contracted)
Biceps (relaxed)
Triceps (relaxed)
Biceps (contracted)
A Extension B Flexion
Figure 7–1. Antagonistic muscles. (A) Extension of the forearm. (B) Flexion of the
QUESTION: When the biceps contracts, what happens to its length, and what kind of force
does it exert?
motor areas of the frontal lobes generate electrochemical
impulses that travel along motor nerves to
muscle fibers, causing the muscle fibers to contract.
For a movement to be effective, some muscles must
contract while others relax. When walking, for example,
antagonistic muscles on the front and back of the
thigh or the lower leg will alternate their contractions
and relaxations, and our steps will be smooth and efficient.
This is what we call coordination, and we do not
have to think about making it happen. Coordination
takes place below the level of conscious thought and is
regulated by the cerebellum, which is located below
the occipital lobes of the cerebrum.
Except during certain stages of sleep, most of our
muscles are in a state of slight contraction; this is what
is known as muscle tone. When sitting upright, for
example, the tone of your neck muscles keeps your
head up, and the tone of your back muscles keeps your
back straight. This is an important function of muscle
tone for human beings, because it helps us to maintain
an upright posture. For a muscle to remain slightly
contracted, only a few of the muscle fibers in that
muscle must contract. Alternate fibers contract so that
the muscle as a whole does not become fatigued. This
is similar to a pianist continuously rippling her fingers
over the keys of the piano—some notes are always
sounding at any given moment, but the notes that are
sounding are always changing. This contraction of
alternate fibers, muscle tone, is also regulated by the
cerebellum of the brain.
Muscle fibers need the energy of ATP (adenosine
triphosphate) in order to contract. When they produce
ATP in the process of cell respiration, muscle
fibers also produce heat. The heat generated by normal
muscle tone is approximately 25% of the total
body heat at rest. During exercise, of course, heat production
increases significantly.
Good muscle tone improves coordination. When
muscles are slightly contracted, they can react more
rapidly if and when greater exertion is necessary.
Muscles with poor tone are usually soft and flabby, but
exercise will improve muscle tone.
There are two general types of exercise: isotonic
and isometric. In isotonic exercise, muscles contract
and bring about movement. Jogging, swimming, and
weight lifting are examples. Isotonic exercise improves
muscle tone, muscle strength, and, if done repetitively
against great resistance (as in weight lifting), muscle
size. This type of exercise also improves cardiovascular
and respiratory efficiency, because movement
exerts demands on the heart and respiratory muscles.
If done for 30 minutes or longer, such exercise may be
called aerobic, because it strengthens the heart and respiratory
muscles as well as the muscles attached to the
Isotonic contractions are of two kinds, concentric
or eccentric. A concentric contraction is the shortening
of a muscle as it exerts force. An eccentric
contraction is the lengthening of a muscle as it still
exerts force. Imagine lifting a book straight up (or try
it); the triceps brachii contracts and shortens to
straighten the elbow and raise the book, a concentric
contraction. Now imagine slowly lowering the book.
The triceps brachii is still contracting even as it is
lengthening, exerting force to oppose gravity (which
would make the book drop quickly). This is an eccentric
Isometric exercise involves contraction without
movement. If you put your palms together and push
one hand against the other, you can feel your arm
muscles contracting. If both hands push equally, there
will be no movement; this is isometric contraction.
Such exercises will increase muscle tone and muscle
strength but are not considered aerobic. When the
body is moving, the brain receives sensory information
about this movement from the joints involved,
and responds with reflexes that increase heart rate and
respiration. Without movement, the brain does not
get this sensory information, and heart rate and
breathing do not increase nearly as much as they
would during an equally strenuous isotonic exercise.
Many of our actions involve both isotonic and isometric
contractions. Pulling open a door requires isotonic
contractions of arm muscles, but if the door is
then held open for someone else, those contractions
become isometric. Picking up a pencil is isotonic;
holding it in your hand is isometric. Walking uphill
involves concentric isotonic contractions, and may be
quite strenuous. Walking downhill seems easier, but is
no less complex. The eccentric isotonic contractions
involved make each step a precisely aimed and con-
140 The Muscular System
trolled fall against gravity. Without such control
(which we do not have to think about) a downhill walk
would quickly become a roll. These various kinds of
contractions are needed for even the simplest activities.
(With respect to increasing muscle strength, see
Box 7–1: Anabolic Steroids.)
When you walk up a flight of stairs, do you have to
look at your feet to be sure each will get to the next
step? Most of us don’t (an occasional stumble doesn’t
count), and for this freedom we can thank our
muscle sense. Muscle sense (proprioception) is the
brain’s ability to know where our muscles are and what
they are doing, without our having to consciously
look at them.
Within muscles are receptors called stretch receptors
(proprioceptors or muscle spindles). The general
function of all sensory receptors is to detect changes.
The function of stretch receptors is to detect changes
in the length of a muscle as it is stretched. The sensory
impulses generated by these receptors are interpreted
by the brain as a mental “picture” of where the
muscle is.
We can be aware of muscle sense if we choose to be,
but usually we can safely take it for granted. In fact,
that is what we are meant to do. Imagine what life
would be like if we had to watch every move to be sure
that a hand or foot performed its intended action.
Even simple activities such as walking or eating would
require our constant attention.
At times, we may become aware of our muscle
sense. Learning a skill such as typing or playing the
guitar involves very precise movements of the fingers,
and beginners will often watch their fingers to be sure
they are moving properly. With practice, however,
the movements simply “feel” right, which means that
the brain has formed a very good mental picture of the
task. Muscle sense again becomes unconscious, and
the experienced typist or guitarist need not watch
every movement.
All sensation is a function of brain activity, and
muscle sense is no exception. The impulses for muscle
sense are integrated in the parietal lobes of the cerebrum
(conscious muscle sense) and in the cerebellum
(unconscious muscle sense) to be used to promote
Before discussing the contraction process itself, let us
look first at how muscle fibers obtain the energy they
need to contract. The direct source of energy for muscle
contraction is ATP. ATP, however, is not stored in
large amounts in muscle fibers and is depleted in a few
The secondary energy sources are creatine phosphate
and glycogen. Creatine phosphate is, like
ATP, an energy-transferring molecule. When it is broken
down (by an enzyme) to creatine, phosphate, and
energy, the energy is used to synthesize more ATP.
Most of the creatine formed is used to resynthesize
creatine phosphate, but some is converted to creatinine,
a nitrogenous waste product that is excreted by
the kidneys.
The most abundant energy source in muscle fibers
The Muscular System 141
will increase muscle size, but there are hazards,
some of them very serious. Side effects of such
self-medication include liver damage, kidney damage,
disruption of reproductive cycles, and
mental changes such as irritability and aggressiveness.
Female athletes may develop increased growth
of facial and body hair and may become sterile as a
result of the effects of a male hormone on their own
hormonal cycles.
Anabolic steroids are synthetic drugs very similar
in structure and action to the male hormone testosterone.
Normal secretion of testosterone, beginning
in males at puberty, increases muscle size and
is the reason men usually have larger muscles than
do women.
Some athletes, both male and female, both
amateur and professional, take anabolic steroids to
build muscle mass and to increase muscle strength.
There is no doubt that the use of anabolic steroids
is glycogen. When glycogen is needed to provide
energy for sustained contractions (more than a few
seconds), it is first broken down into the glucose molecules
of which it is made. Glucose is then further broken
down in the process of cell respiration to produce
ATP, and muscle fibers may continue to contract.
Recall from Chapter 2 our simple reaction for cell
Glucose O2 → CO2 H2O ATP heat
Look first at the products of this reaction. ATP will
be used by the muscle fibers for contraction. The heat
produced will contribute to body temperature, and if
exercise is strenuous, will increase body temperature.
The water becomes part of intracellular water, and the
carbon dioxide is a waste product that will be exhaled.
Now look at what is needed to release energy from
glucose: oxygen. Muscles have two sources of oxygen.
The blood delivers a continuous supply of oxygen
from the lungs, which is carried by the hemoglobin in
red blood cells. Within muscle fibers themselves there
is another protein called myoglobin, which stores
some oxygen within the muscle cells. Both hemoglobin
and myoglobin contain the mineral iron, which
enables them to bond to oxygen. (Iron also makes
both molecules red, and it is myoglobin that gives
muscle tissue a red or dark color.)
During strenuous exercise, the oxygen stored in
myoglobin is quickly used up, and normal circulation
may not deliver oxygen fast enough to permit the
completion of cell respiration. Even though the respiratory
rate increases, the muscle fibers may literally
run out of oxygen. This state is called oxygen debt,
and in this case, glucose cannot be completely broken
down into carbon dioxide and water. If oxygen is not
present (or not present in sufficient amounts), glucose
is converted to an intermediate molecule called lactic
acid, which causes muscle fatigue.
In a state of fatigue, muscle fibers cannot contract
efficiently, and contraction may become painful. To be
in oxygen debt means that we owe the body some oxygen.
Lactic acid from muscles enters the blood and
circulates to the liver, where it is converted to pyruvic
acid, a simple carbohydrate (three carbons, about
half a glucose molecule). This conversion requires
ATP, and oxygen is needed to produce the necessary
ATP in the liver. This is why, after strenuous exercise,
the respiratory rate and heart rate remain high for a
time and only gradually return to normal. Another
name proposed for this state is recovery oxygen
uptake, which is a little longer but also makes sense.
Oxygen uptake means a faster and deeper respiratory
rate. What is this uptake for? For recovery from strenuous
We will now look more closely at a muscle fiber, keeping
in mind that there are thousands of these cylindrical
cells in one muscle. Each muscle fiber has its own
motor nerve ending; the neuromuscular junction is
where the motor neuron terminates on the muscle
fiber (Fig. 7–2). The axon terminal is the enlarged tip
of the motor neuron; it contains sacs of the neurotransmitter
acetylcholine (ACh). The membrane of
the muscle fiber is the sarcolemma, which contains
receptor sites for acetylcholine, and an inactivator
called cholinesterase. The synapse (or synaptic cleft)
is the small space between the axon terminal and the
Within the muscle fiber are thousands of individual
contracting units called sarcomeres, which are
arranged end to end in cylinders called myofibrils.
The structure of a sarcomere is shown in Fig. 7–3:
The Z lines are the end boundaries of a sarcomere.
Filaments of the protein myosin are in the center of
the sarcomere, and filaments of the protein actin are
at the ends, attached to the Z lines. Myosin filaments
are anchored to the Z lines by the protein titin.
Myosin and actin are the contractile proteins of a
muscle fiber. Their interactions produce muscle contraction.
Also present are two inhibitory proteins, troponin
and tropomyosin, which are part of the actin
filaments and prevent the sliding of actin and myosin
when the muscle fiber is relaxed.
Surrounding the sarcomeres is the sarcoplasmic
reticulum, the endoplasmic reticulum of muscle cells.
The sarcoplasmic reticulum is a reservoir for calcium
ions (Ca 2), which are essential for the contraction
All of these parts of a muscle fiber are involved in
the contraction process. Contraction begins when a
nerve impulse arrives at the axon terminal and stimulates
the release of acetylcholine. Acetylcholine generates
electrical changes (the movement of ions) at the
sarcolemma of the muscle fiber. These electrical
changes initiate a sequence of events within the muscle
fiber that is called the sliding filament mechanism
of muscle contraction. We will begin our
discussion with the sarcolemma.
142 The Muscular System
When a muscle fiber is relaxed, the sarcolemma is
polarized (has a resting potential), which refers to a
difference in electrical charges between the outside
and the inside. During polarization, the outside of
the sarcolemma has a positive charge relative to the
inside, which is said to have a negative charge. Sodium
ions (Na ) are more abundant outside the cell, and
potassium ions (K ) and negative ions are more abundant
inside (Fig. 7–4).
The Na ions outside tend to diffuse into the cell,
and the sodium pump transfers them back out. The
K ions inside tend to diffuse outside, and the potas-
The Muscular System 143
Vesicles of
Axon terminal
Motor neuron
ACh receptor
T tubule
Figure 7–2. Structure of the neuromuscular junction, showing an axon terminal adjacent
to the sarcolemma of a muscle fiber. Schematic of events: (1) Acetylcholine is about
to bond to the ACh receptor in the sarcolemma. (2) Channel opens to allow Na ions into
the muscle cell. (3) Cholinesterase inactivates acetylcholine.
QUESTION: What event opens a sodium channel in the sarcolemma?
B Bundles of
muscle cells
Muscle cells (fibers)
Fascia and
connective tissue
C Muscle fiber
Z line
Myosin filaments
Myosin cross bridges
E Muscle filaments
D Sarcomere
Z line
Myosin filament
A Entire muscle
Figure 7–3. Structure of skeletal muscle. (A) Entire muscle. (B) Bundles of muscle cells
within a muscle. (C) Single muscle fiber, microscopic structure. (D) A sarcomere.
(E) Structure of muscle filaments.
QUESTION: What is the unit of contraction of a muscle fiber?
sium pump returns them inside. Both of these pumps
are active transport mechanisms, which, you may
recall, require ATP. Muscle fibers use ATP to maintain
a high concentration of Na ions outside the cell and
a high concentration of K inside. The pumps, therefore,
maintain polarization and relaxation until a nerve
impulse stimulates a change.
When a nerve impulse arrives at the axon terminal, it
causes the release of acetylcholine, which diffuses
across the synapse and bonds to ACh receptors on
the sarcolemma. By doing so, acetylcholine makes the
sarcolemma very permeable to Na ions, which rush
into the cell. This makes the inside of the sarcolemma
positive relative to the outside, which is now considered
negative. This reversal of charges is called depolarization.
The electrical impulse thus generated
(called an action potential) then spreads along the
entire sarcolemma of a muscle fiber. The sarcolemma
has inward folds called T tubules (transverse tubules,
shown in Fig. 7–2), which carry the action potential to
the interior of the muscle cell. Depolarization initiates
changes within the cell that bring about contraction.
The electrical changes that take place at the sarcolemma
are summarized in Table 7–1 and shown in
Fig. 7–4.
The Muscular System 145
Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+
Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+
K+ K+
K+ K+ K+ K+
K+ K+ K+ K+
K+ K+ K+ K+
Figure 7–4. Electrical charges and ion concentrations
at the sarcolemma. (A) Polarization, when the muscle
fiber is relaxed. (B) Wave of depolarization in response to
stimulus of acetylcholine. (C) Wave of repolarization.
QUESTION: Which ion enters the cell during depolarization?
Which ion leaves during repolarization?
State or Event Description
Resting Potential
Action Potential
• Sarcolemma has a ( ) charge
outside and a ( ) charge inside.
• Na ions are more abundant
outside the cell; as they diffuse
inward, the sodium pump
returns them outside.
• K ions are more abundant
Inside the cell; as they diffuse
out, the potassium pump
returns them inside.
• ACh makes the sarcolemma very
permeable to Na ions, which
rush into the cell.
• Reversal of charges on the sarcolemma:
now ( ) outside and
( ) inside.
• The reversal of charges spreads
along the entire sarcolemma
• Cholinesterase at the sarcolemma
inactivates ACh.
• Sarcolemma becomes very permeable
to K ions, which rush
out of the cell.
• Restoration of charges on the
sarcolemma: ( ) outside and
( ) inside.
• The sodium and potassium
pumps return Na ions outside
and K ions inside.
• The muscle fiber is now able to
respond to ACh released by
another nerve impulse arriving
at the axon terminal.
All of the parts of a muscle fiber and the electrical
changes described earlier are involved in the contraction
process, which is a precise sequence of events
called the sliding filament mechanism.
In summary, a nerve impulse causes depolarization
of a muscle fiber, and this electrical change enables the
myosin filaments to pull the actin filaments toward the
center of the sarcomere, making the sarcomere
shorter. All of the sarcomeres shorten and the muscle
fiber contracts. A more detailed description of this
process is the following:
1. A nerve impulse arrives at the axon terminal;
acetylcholine is released and diffuses across the
2. Acetylcholine makes the sarcolemma more permeable
to Na ions, which rush into the cell.
3. The sarcolemma depolarizes, becoming negative
outside and positive inside. The T tubules bring
the reversal of charges to the interior of the muscle
4. Depolarization stimulates the release of Ca 2 ions
from the sarcoplasmic reticulum. Ca 2 ions bond
to the troponin–tropomyosin complex, which
shifts it away from the actin filaments.
5. Myosin splits ATP to release its energy; bridges
on the myosin attach to the actin filaments and
pull them toward the center of the sarcomere,
thus making the sarcomere shorter (Fig. 7–5).
6. All of the sarcomeres in a muscle fiber shorten—
the entire muscle fiber contracts.
7. The sarcolemma repolarizes: K ions leave the
cell, restoring a positive charge outside and a negative
charge inside. The pumps then return Na
ions outside and K ions inside.
8. Cholinesterase in the sarcolemma inactivates
9. Subsequent nerve impulses will prolong contraction
(more acetylcholine is released).
10. When there are no further impulses, the muscle
fiber will relax and return to its original length.
Steps 1 through 8 of this sequence describe a single
muscle fiber contraction (called a twitch) in response to
a single nerve impulse. Because all of this takes place
in less than a second, useful movements would not be
possible if muscle fibers relaxed immediately after
contracting. Normally, however, nerve impulses arrive
in a continuous stream and produce a sustained contraction
called tetanus, which is a normal state not to
be confused with the disease tetanus (see Box 7–2:
Tetanus and Botulism). When in tetanus, muscle
fibers remain contracted and are capable of effective
movements. In a muscle such as the biceps brachii that
flexes the forearm, an effective movement means that
many of its thousands of muscle fibers are in tetanus,
a sustained contraction.
As you might expect with such a complex process,
muscle contraction may be impaired in many different
ways. Perhaps the most obvious is the loss of nerve
impulses to muscle fibers, which can occur when
nerves or the spinal cord are severed, or when a stroke
146 The Muscular System
tetanus the cause of death is spasm of the respiratory
Botulism is usually a type of food poisoning, but
it is not characterized by typical food poisoning
symptoms such as diarrhea or vomiting. The neurotoxin
produced by the botulism bacteria (Clostridium
botulinum) prevents the release of acetylcholine at
neuromuscular junctions. Without acetylcholine,
muscle fibers cannot contract, and muscles become
paralyzed. Early symptoms of botulism include
blurred or double vision and difficulty speaking or
swallowing. Weakness and paralysis spread to other
muscle groups, eventually affecting all voluntary
muscles. Without rapid treatment with the antitoxin
(the specific antibody to this toxin), botulism is fatal
because of paralysis of the respiratory muscles.
Some bacteria cause disease by producing toxins. A
neurotoxin is a chemical that in some way disrupts
the normal functioning of the nervous system.
Because skeletal muscle contraction depends on
nerve impulses, the serious consequences for the
individual may be seen in the muscular system.
Tetanus is characterized by the inability of muscles
to relax. The toxin produced by the tetanus
bacteria (Clostridium tetani) affects the nervous system
in such a way that muscle fibers receive too
many impulses, and muscles go into spasms. Lockjaw,
the common name for tetanus, indicates one
of the first symptoms, which is difficulty opening
the mouth because of spasms of the masseter muscles.
Treatment requires the antitoxin (an antibody
to the toxin) to neutralize the toxin. In untreated
(cerebrovascular accident) occurs in the frontal
lobes of the cerebrum. Without nerve impulses, skeletal
muscles become paralyzed, unable to contract.
Paralyzed muscles eventually atrophy, that is, become
smaller from lack of use. Other disorders that affect
muscle functioning are discussed in Box 7–3: Muscular
Dystrophy and Box 7–4: Myasthenia Gravis.
Although entire textbooks are devoted to exercise
physiology, we will discuss it only briefly here as an
example of the body’s ability to maintain homeostasis.
Engaging in moderate or strenuous exercise is a physiological
stress situation, a change that the body must
cope with and still maintain a normal internal environment,
that is, homeostasis.
Some of the body’s responses to exercise are diagrammed
in Fig. 7–6; notice how they are related to
cell respiration. As you can see, the respiratory and
cardiovascular systems make essential contributions to
exercise. The integumentary system also has a role,
since it eliminates excess body heat. Although not
shown, the nervous system is also directly involved, as
we have seen. The brain generates the impulses for
muscle contraction, coordinates those contractions,
and regulates heart rate, breathing rate, and the diameter
of blood vessels. The next time you run up a flight
The Muscular System 147
A Relaxed muscle
B Contracted muscle
T tubule
Calcium ions released from
sarcoplasmic reticulum
Calcium ions
bonded to
Myosin cross bridges
attach to actin
Myosin-binding site
Figure 7–5. Sliding filament mechanism. (A) Sarcomere in relaxed muscle fiber.
(B) Sarcomere in contracted muscle fiber. See text for description.
QUESTION: During contraction, which filaments do the pulling?
Muscular dystrophy is really a group of genetic
diseases in which muscle tissue is replaced by
fibrous connective tissue or by fat. Neither of these
tissues is capable of contraction, and the result is
progressive loss of muscle function. The most common
form is Duchenne’s muscular dystrophy, in
which the loss of muscle function affects not only
skeletal muscle but also cardiac muscle. Death usually
occurs before the age of 20 due to heart failure,
and at present there is no cure.
Duchenne’s muscular dystrophy is a sexlinked
(or X-linked) trait, which means that the
gene for it is on the X chromosome and is recessive.
The female sex chromosomes are XX. If one X chromosome
has a gene for muscular dystrophy, and
the other X chromosome has a dominant gene for
normal muscle function, the woman will not have
muscular dystrophy but will be a carrier who may
pass the muscular dystrophy gene to her children.
The male sex chromosomes are XY, and the Y has
no gene at all for muscle function, that is, no gene
to prevent the expression of the gene on the X
chromosome. If the X chromosome has a gene for
muscular dystrophy, the male will have the disease.
This is why Duchenne’s muscular dystrophy is more
common in males; the presence of only one gene
means the disease will be present.
The muscular dystrophy gene on the X chromosome
has been located, and the protein the gene
codes for has been named dystrophin. Dystrophin is
necessary for the stability of the sarcolemma and
the proper movement of ions. Treatments for muscular
dystrophy that are being investigated include
the injection of normal muscle cells or stem cells
into affected muscles, and the insertion (using
viruses) of normal genes for dystrophin into
affected muscle cells.
Increased muscle
Increased cell
Increased ATP
Increased need
for O2
Increased CO2
Increased heat
Increased respiration
Increased heart rate
Vasodilation in muscles
Figure 7–6. Responses of the
body during exercise.
QUESTION: Name all the organ
systems depicted here.
which acetylcholine bonds and stimulates the entry
of Na ions. Without these receptors, the acetylcholine
released by the axon terminal cannot cause
depolarization of a muscle fiber.
Treatment of myasthenia gravis may involve
anticholinesterase medications. Recall that cholinesterase
is present in the sarcolemma to inactivate
acetylcholine and prevent continuous, unwanted
impulses. If this action of cholinesterase is inhibited,
acetylcholine remains on the sarcolemma for a
longer time and may bond to any remaining receptors
to stimulate depolarization and contraction.
Myasthenia gravis is an autoimmune disorder
characterized by extreme muscle fatigue even after
minimal exertion. Women are affected more often
than are men, and symptoms usually begin in middle
age. Weakness may first be noticed in the facial or
swallowing muscles and may progress to other muscles.
Without treatment, the respiratory muscles will
eventually be affected, and respiratory failure is the
cause of death.
In myasthenia gravis, the autoantibodies (selfantibodies)
destroy the acetylcholine receptors
on the sarcolemma. These receptors are the sites to
of stairs, hurry to catch a bus, or just go dancing, you
might reflect a moment on all of the things that are
actually happening to your body . . . after you catch
your breath.
With age, muscle cells die and are replaced by fibrous
connective tissue or by fat. Regular exercise, however,
delays atrophy of muscles. Although muscles become
slower to contract and their maximal strength decreases,
exercise can maintain muscle functioning at a
level that meets whatever a person needs for daily
activities. The lifting of small weights is recommended
as exercise for elderly people, women as well
as men. Such exercise also benefits the cardiovascular,
respiratory, and skeletal systems.
The loss of muscle fibers also contributes to a loss
of proprioception, because the brain is getting less
information about where and how the body is positioned.
The loss of muscle sense contributes to
unsteadiness in elderly people and to an impaired
sense of balance, which in turn may lead to a fall.
Simple awareness of this may help an elderly person
prevent such accidents.
The actions that muscles perform are listed in Table
7–2 and some are shown in Fig. 7–7. Most are in pairs
as antagonistic functions.
After the brief summaries of the muscles of each
body area that follow, the major muscles are shown in
Fig. 7–8. They are listed, according to body area, in
Tables 7–3 through 7–7, with associated Figs. 7–9
through 7–13, respectively. When you study the diagrams
of these muscles, and the tables that accompany
them, keep in mind the types of joints formed by the
bones of their origins and insertions. Muscles pull
bones to produce movement, and if you can remember
the joints involved, you can easily learn the locations
and actions of the muscles.
The name of the muscle may also be helpful, and
again, many of the terms are ones you have already
learned. Some examples: “abdominis” refers to an
abdominal muscle, “femoris” to a thigh muscle,
“brachii” to a muscle of the upper arm, “oculi” to an
The Muscular System 149
eye muscle, and so on. Other parts of muscle names
may be words such as “longus” or “maximus” that
tell you about size, or “flexor” that tells you about
Muscles that are sites for intramuscular injections
are shown in Box 7–5.
Intramuscular injections are used when rapid
absorption is needed, because muscle has a
good blood supply. Common sites are the buttock
(gluteus medius), the lateral thigh (vastus lateralis),
and the shoulder (deltoid). These sites are
shown; also shown are the large nerves to be
avoided when giving such injections.
Box Figure 7–A Sites for intramuscular injections.
Posterior view of right side of body.
150 The Muscular System
Three general groups of muscles are found in the head
and neck: those that move the head or neck, the muscles
of facial expression, and the muscles for chewing.
The muscles that turn or bend the head, such as the
sternocleidomastoids (flexion) and the pair of splenius
capitis muscles (extension), are anchored to the skull
and to the clavicle and sternum anteriorly or the vertebrae
posteriorly. The muscles for smiling or frowning
or raising our eyebrows in disbelief are anchored
to the bones of the head or to the undersurface of the
skin of the face. The masseter is an important chewing
muscle in that it raises the mandible (closes the jaw).
Figure 7–7. Actions of muscles.
QUESTION: Crossing the arm in front of the chest would
be which of these actions?
Action Definition
Plantar flexion
Most are grouped in pairs of antagonistic functions.
• To decrease the angle of a joint
• To increase the angle of a joint
• To move closer to the midline
• To move away from the midline
• To turn the palm down
• To turn the palm up
• To elevate the foot
• To lower the foot (point the toes)
• To move a bone around its
longitudinal axis
The Muscular System 151
The muscles of the trunk cannot be described with
one or two general functions. Some form the wall of
the trunk and bend the trunk, such as the rectus abdominis
(f lexion) and the sacrospinalis group (extension).
The trapezius (both together form the shape of
a trapezoid) is a large muscle that can raise (shrug) the
shoulder or pull it back, and can help extend the head.
Other muscles found on the trunk help move the arm
at the shoulder. The pectoralis major is a large muscle
of the chest that pulls the arm across the chest (flexion
and adduction). On the posterior side of the trunk, the
latissimus dorsi pulls the arm downward and behind
the back (extension and adduction). These muscles
have their origins on the bones of the trunk, the sternum,
the or vertebrae, which are strong, stable
anchors. Another set of muscles forms the pelvic floor,
where the muscles support the pelvic organs and assist
with urination and defecation. Yet another category is
the muscles that are concerned with breathing. These
are the intercostal muscles between the ribs and the
diaphragm that separates the thoracic and abdominal
cavities (see Fig. 15–6).
The triangular deltoid muscle covers the point of the
shoulder like a cap, and can pull the humerus to the
side (abduction), forward (flexion), or backward
(extension). You already know the functions of the
biceps brachii and triceps brachii, the muscles that
form the bulk of the upper arm. Other muscles partially
in the upper arm help bend the elbow (flexion).
The muscles that form the bulk of the forearm are the
flexors and extensors of the hand and fingers. You can
demonstrate this yourself by clasping the middle of
your right forearm with your left hand, then moving
your right hand at the wrist and closing and opening a
fist; you can both feel and see the hand and finger
muscles at work.
The hip muscles that move the thigh are anchored to
the pelvic bone and cross the hip joint to the femur.
Among these are the gluteus maximus (extension),
gluteus medius (abduction), and iliopsoas (flexion).
The muscles that form the thigh include the quadriceps
group anteriorly and the hamstring group
posteriorly. For most people, the quadriceps is
stronger than the hamstrings, which is why athletes
more often have a “pulled hamstring” rather than a
“pulled quadriceps.” Movement of the knee joint
depends on thigh muscles and lower leg muscles.
Movement of the foot depends on lower leg muscles
such as the gastrocnemius (dorsiflexion or flexion) and
the tibialis anterior (plantar flexion or extension).
brachii Brachialis
Latissimus dorsi
External oblique
Gluteus medius
Gluteus maximus
Vastus lateralis
Biceps femoris
Achilles tendon
Teres major
Triceps brachii
Adductor magnus
Figure 7–8. Major muscles of the body. (A) Posterior view.
Pectoralis major
Tibialis anterior
Vastus medialis
Vastus lateralis
Rectus femoris
Adductor longus
Rectus abdominis
External oblique
Figure 7–8. Major muscles of the body. (B) Anterior view.
QUESTION: Find a muscle named for: shape, size, location, a bone it is near, and function.
Levator labii
Orbicularis oris
Anterior—left lateral view
Orbicularis oculi
Figure 7–9. Muscles of the
head and neck in anterior, left-lateral
QUESTION: In what way are both
orbicularis muscles similar?
Muscle Function Origin Insertion
Orbicularis oculi
Orbicularis oris
Semispinalis capitis
(a deep muscle)
Splenius capitis
Raises eyebrows, wrinkles skin
of forehead
Closes eye
Puckers lips
Closes jaw
Pulls corners of mouth laterally
Turns head to opposite side
(both—flex head and neck)
Turns head to same side (both—
extend head and neck)
Turns head to same side (both—
extend head)
• epicranial aponeurosis
• medial side of orbit
• encircles mouth
• maxilla and zygomatic
• maxilla and mandible
• sternum and clavicle
• 7th cervical and first 6
thoracic vertebrae
• 7th cervical and first 4
thoracic vertebrae
• skin above supraorbital
• encircles eye
• skin at corners of mouth
• mandible
• orbicularis oris
• temporal bone (mastoid
• occipital bone
• occipital bone
Muscle Function Origin Insertion
External intercostals
Internal intercostals
Rectus abdominis
External oblique
Sacrospinalis group
(deep muscles)
Raises, lowers, and adducts
Pull ribs up and out (inhalation)
Pull ribs down and in (forced
Flattens (down) to enlarge
chest cavity for inhalation
Flexes vertebral column, compresses
Rotates and flexes vertebral column,
compresses abdomen
Extends vertebral column
• occipital bone and all
thoracic vertebrae
• superior rib
• inferior rib
• last 6 costal cartilages
and lumbar vertebrae
• pubic bones
• lower 8 ribs
• ilium, lumbar, and
some thoracic vertebrae
• spine of scapula and
• inferior rib
• superior rib
• central tendon
• 5th–7th costal cartilages
and xiphoid process
• iliac crest and linea alba
• ribs, cervical, and
thoracic vertebrae
External oblique
Internal oblique
Rectus abdominis
Pectoralis major
Serratus anterior
Splenius capitis
Teres major
Gluteus maximus
Figure 7–10. Muscles of the trunk. (A) Anterior view. (B) Posterior view.
QUESTION: Which muscles of the trunk move the arm? Why are they on the trunk?
156 The Muscular System
Extensor carpi
radialis longus
Extensor carpi
radialis longus
carpi radialis
Extensor carpi
radialis brevis
Palmaris longus
Flexor pollicis longus
Extensor carpi
radialis brevis
Flexor digitorum
pollicis longus
pollicis brevis
Flexor carpi ulnaris
Extensor carpi ulnaris
Extensor digiti minimi
Figure 7–11. Muscles of the arm. (A) Anterior view. (B) Posterior view.
QUESTION: Where are the muscles that flex the fingers located? How did you know?
The Muscular System 157
Muscle Function Origin Insertion
Pectoralis major
Latissimus dorsi
Teres major
Triceps brachii
Biceps brachii
Abducts the humerus
Flexes and adducts the
Extends and adducts
the humerus
Extends and adducts
the humerus
Extends the forearm
Flexes the forearm
Flexes the forearm
• scapula and clavicle
• clavicle, sternum, 2nd–6th costal
• last 6 thoracic vertebrae, all lumbar
vertebrae, sacrum, iliac crest
• scapula
• humerus and scapula
• scapula
• humerus
• humerus
• humerus
• humerus
• humerus
• ulna
• radius
• radius
Muscle Function Origin Insertion
Gluteus maximus
Gluteus medius
Quadriceps femoris group:
Rectus femoris
Vastus lateralis
Vastus medialis
Vastus intermedius
Hamstring group
Biceps femoris
Adductor group
Tibialis anterior
Flexes femur
Extends femur
Abducts femur
Flexes femur and extends
lower leg
Extends femur and flexes
lower leg
Adducts femur
Flexes femur and lower leg
Plantar flexes foot
Plantar flexes foot
Dorsiflexes foot
• ilium, lumbar vertebrae
• iliac crest, sacrum, coccyx
• ilium
• ilium and femur
• ischium
• ischium and pubis
• ilium
• femur
• tibia and fibula
• tibia
• femur
• femur
• femur
• tibia
• tibia and fibula
• femur
• tibia
• calcaneus (Achilles tendon)
• calcaneus (Achilles tendon)
• metatarsals
Rectus femoris
Vastus lateralis
Peroneus longus
Tibialis anterior
Extensor digitorum brevis
Extensor digitorum longus
hallucis brevis
Vastus medialis
digitorum longus
Peroneus brevis
Extensor hallucis longus
Gluteus maximus
Vastus lateralis
Peroneus longus
Biceps femoris
Figure 7–12. Muscles of the leg. (A) Anterior view. (B) Posterior view.
QUESTION: How does the gastrocnemius compare in size to the tibialis anterior? What is
the reason for this difference?
The Muscular System 159
Transverse perineus
Levator ani
Figure 7–13. Muscles of the female pelvic floor.
QUESTION: In women, what organs are directly supported by this “floor” of muscles?
Muscle Function Origin Insertion
Levator ani
Transverse perineus
(superficial and deep)
External anal sphincter
Supports pelvic organs, especially during
defecation, urination, coughing,
and forced exhalation; constricts
anus, urethra, and vagina
Supports pelvic organs, especially during
defecation, urination, coughing,
and forced exhalation
Erection of clitoris in female, penis in
Assists urination; erection in female;
erection and ejaculation in male
Assists urination in female; urination and
ejaculation in male
Closes anus
• pubis and ischium
• ischium
• ischium and pubis
• central tendon of
• ischium
• anococcygeal
• coccyx, anal canal,
• coccyx and sacrum
• clitoris or penis
• fasciae, pubic arch,
clitoris, or penis
• central tendon of
• central tendon of
Organ Systems Involved in Movement
1. Muscular—moves the bones.
2. Skeletal—bones are moved, at their joints, by muscles.
3. Nervous—transmits impulses to muscles to cause
4. Respiratory—exchanges O2 and CO2 between the
air and blood.
5. Circulatory—transports O2 to muscles and removes
Muscle Structure
1. Muscle fibers (cells) are specialized to contract,
shorten, and produce movement.
2. A skeletal muscle is made of thousands of
muscle fibers. Varying movements require contrac-
tion of variable numbers of muscle fibers in a
3. Tendons attach muscles to bone; the origin is the
more stationary bone, the insertion is the more
movable bone. A tendon merges with the fascia of
a muscle and the periosteum of a bone; all are made
of fibrous connective tissue.
Muscle Arrangements
1. Antagonistic muscles have opposite functions. A
muscle pulls when it contracts, but exerts no force
when it relaxes and it cannot push. When one muscle
pulls a bone in one direction, another muscle is
needed to pull the bone in the other direction (see
also Table 7–2 and Fig. 7–1).
2. Synergistic muscles have the same function and
alternate as the prime mover depending on the
position of the bone to be moved. Synergists also
stabilize a joint to make a more precise movement
3. The frontal lobes of the cerebrum generate the
impulses necessary for contraction of skeletal muscles.
The cerebellum regulates coordination.
Muscle Tone—the state of slight contraction
present in muscles
1. Alternate fibers contract to prevent muscle fatigue;
regulated by the cerebellum.
2. Good tone helps maintain posture, produces 25%
of body heat (at rest), and improves coordination.
3. Isotonic exercise involves contraction with movement;
improves tone and strength and improves
cardiovascular and respiratory efficiency (aerobic
• Concentric contraction—muscle exerts force
while shortening.
• Eccentric contraction—muscle exerts force
while lengthening.
4. Isometric exercise involves contraction without
movement; improves tone and strength but is not
Muscle Sense—proprioception: knowing
where our muscles are without looking
at them
1. Permits us to perform everyday activities without
having to concentrate on muscle position.
2. Stretch receptors (proprioceptors) in muscles
respond to stretching and generate impulses that
the brain interprets as a mental “picture” of where
the muscles are. Parietal lobes: conscious muscle
sense; cerebellum: unconscious muscle sense used
to promote coordination.
Energy Sources for Muscle Contraction
1. ATP is the direct source; the ATP stored in muscles
lasts only a few seconds.
2. Creatine phosphate is a secondary energy source; is
broken down to creatine phosphate energy.
The energy is used to synthesize more ATP. Some
creatine is converted to creatinine, which must be
excreted by the kidneys. Most creatine is used for
the resynthesis of creatine phosphate.
3. Glycogen is the most abundant energy source and
is first broken down to glucose. Glucose is broken
down in cell respiration:
Glucose O2 → CO2 H2O ATP heat
ATP is used for contraction; heat contributes to
body temperature; H2O becomes part of intracellular
fluid; CO2 is eventually exhaled.
4. Oxygen is essential for the completion of cell respiration.
Hemoglobin in red blood cells carries
oxygen to muscles; myoglobin stores oxygen in
muscles; both of these proteins contain iron, which
enables them to bond to oxygen.
5. Oxygen debt (recovery oxygen uptake): Muscle
fibers run out of oxygen during strenuous exercise,
and glucose is converted to lactic acid, which causes
fatigue. Breathing rate remains high after exercise
to deliver more oxygen to the liver, which converts
lactic acid to pyruvic acid, a simple carbohydrate
(ATP required).
Muscle Fiber—microscopic structure
1. Neuromuscular junction: axon terminal and sarcolemma;
the synapse is the space between. The
axon terminal contains acetylcholine (a neurotransmitter),
and the sarcolemma contains cholinesterase
(an inactivator) (see Fig. 7–2).
2. Sarcomeres are the contracting units of a muscle
fiber. Myosin and actin filaments are the contracting
proteins of sarcomeres. Troponin and tropomyosin
are proteins that inhibit the sliding of
myosin and actin when the muscle fiber is relaxed
(see Figs. 7–3 and 7–5).
3. The sarcoplasmic reticulum surrounds the sarcomeres
and is a reservoir for calcium ions.
4. Polarization (resting potential): When the muscle
fiber is relaxed, the sarcolemma has a ( ) charge
160 The Muscular System
1. Name the organ systems directly involved in
movement, and for each state how they are
involved. (p. 138)
2. State the function of tendons. Name the part of a
muscle and a bone to which a tendon is attached.
(p. 138)
3. State the term for: (pp. 138–139)
a. Muscles with the same function
b. Muscles with opposite functions
c. The muscle that does most of the work in a
4. Explain why antagonistic muscle arrangements
are necessary. Give two examples. (p. 138)
5. State three reasons why good muscle tone is
important. (p. 140)
6. Explain why muscle sense is important. Name the
receptors involved and state what they detect.
(p. 141)
7. With respect to muscle contraction, state the
functions of the cerebellum and the frontal lobes
of the cerebrum. (p. 140)
8. Name the direct energy source for muscle contraction.
Name the two secondary energy sources.
Which of these is more abundant? (p. 141)
9. State the simple equation of cell respiration and
what happens to each of the products of this reaction.
(p. 142)
10. Name the two sources of oxygen for muscle
fibers. State what the two proteins have in common.
(p. 142)
11. Explain what is meant by oxygen debt. What is
needed to correct oxygen debt, and where does it
come from? (p. 142)
12. Name these parts of the neuromuscular junction:
(p. 142)
a. The membrane of the muscle fiber
b. The end of the motor neuron
c. The space between neuron and muscle cell
State the locations of acetylcholine and cholinesterase.
13. Name the contracting proteins of sarcomeres, and
describe their locations in a sarcomere. Where is
the sarcoplasmic reticulum and what does it contain?
(p. 142)
The Muscular System 161
outside and a ( ) charge inside. Na ions are more
abundant outside the cell and K ions are more
abundant inside the cell. The Na and K pumps
maintain these relative concentrations on either
side of the sarcolemma (see Table 7–1 and Fig.
5. Depolarization: This process is started by a nerve
impulse. Acetylcholine released by the axon terminal
makes the sarcolemma very permeable to Na
ions, which enter the cell and cause a reversal of
charges to ( ) outside and ( ) inside. The depolarization
spreads along the entire sarcolemma and
initiates the contraction process. Folds of the sarcolemma
called T tubules carry the depolarization
into the interior of the muscle cell.
Contraction—the sliding filament mechanism
(see Fig. 7–5)
1. Depolarization stimulates a sequence of events
that enables myosin filaments to pull the actin filaments
to the center of the sarcomere, which shortens.
2. All of the sarcomeres in a muscle fiber contract in
response to a nerve impulse; the entire cell contracts.
3. Tetanus is a sustained contraction brought about by
continuous nerve impulses; all our movements
involve tetanus.
4. Paralysis: Muscles that do not receive nerve
impulses are unable to contract and will atrophy.
Paralysis may be the result of nerve damage, spinal
cord damage, or brain damage.
Responses to Exercise—maintaining homeostasis
See section in chapter and Fig. 7–6.
Major Muscles
See Tables 7–2 through 7–7 and Figs. 7–7 through
1. In an accident with farm machinery, Mr. R. had his
left arm severed just below the elbow. Mrs. R.
stopped the bleeding, called for an ambulance,
and packed the severed arm in ice for the EMTs
to take to the hospital. Will Mr. R. ever be able
to move the fingers of his left hand again? What
structures must be reattached, and what has to
2. Name all of the muscles you can think of that move
the thigh at the hip. Group them as synergists, if
possible. Then pair those groups or individual
muscles as antagonists.
3. Muscle contraction is important for posture.
Muscles oppose each other, contracting equally to
keep us upright. Picture the body in anatomic position,
and describe what would happen if each of
these muscles relaxed completely:
Semispinalis capitis
Rectus abdominis
Quadriceps femoris
Gluteus maximus
4. An exercise for skiers involves sitting against a wall
as if you were sitting in a chair, but without a chair.
Thighs should be parallel to the floor and the knees
should make a 90o angle. Try it. What kind of
exercise is this? Which muscles are doing most of
the work (which ones begin to hurt)? Which do
you think would be easier: 3 minutes of this exercise
or 3 minutes of jogging? Can you think of an
5. Can you juggle? Don’t just say “no”—have you
ever tried? Find some old tennis balls and try juggling
two balls with one hand, or three balls with
two hands. Explain how muscle sense is involved in
Now try to imagine what it would be like to be
without muscle sense. Some people do not have
muscle sense in certain parts of their bodies. Who
are these people, and what has happened that cost
them their muscle sense (and muscle contraction)?
162 The Muscular System
14. In terms of ions and charges, describe: (p. 145)
a. Polarization
b. Depolarization
c. Repolarization
15. With respect to the sliding filament mechanism,
explain the function of: (p. 146)
a. Acetylcholine
b. Calcium ions
c. Myosin and actin
d. Troponin and tropomyosin
e. Cholinesterase
16. State three of the body’s physiological responses
to exercise, and explain how each helps maintain
homeostasis. (pp. 147–148)
17. Find the major muscles on yourself, and state a
function of each muscle
The Nervous System
Chapter Outline
Nervous System Divisions
Nerve Tissue
Types of Neurons
Nerves and Nerve Tracts
The Nerve Impulse
The Spinal Cord
Spinal Nerves
Spinal Cord Reflexes
Reflex arc
The Brain
Frontal lobes
Parietal lobes
Temporal lobes
Occipital lobes
Association areas
Basal ganglia
Corpus callosum
Meninges and Cerebrospinal Fluid
Cranial Nerves
The Autonomic Nervous System
Autonomic Pathways
Sympathetic Division
Parasympathetic Division
Aging and the Nervous System
Student Objectives
• Name the divisions of the nervous system and the
parts of each, and state the general functions of
the nervous system.
• Name the parts of a neuron and state the function
of each.
• Explain the importance of Schwann cells in the
peripheral nervous system and neuroglia in the
central nervous system.
• Describe the electrical nerve impulse, and describe
impulse transmission at synapses.
• Describe the types of neurons, nerves, and nerve
• State the names and numbers of the spinal nerves,
and their destinations.
• Explain the importance of stretch reflexes and
flexor reflexes.
• State the functions of the parts of the brain; be
able to locate each part on a diagram.
• Name the meninges and describe their locations.
The Nervous System
Student Objectives (Continued)
• State the locations and functions of cerebrospinal
• Name the cranial nerves, and state their functions.
• Explain how the sympathetic division of the autonomic
nervous system enables the body to adapt
to a stress situation.
• Explain how the parasympathetic division of the
autonomic nervous system promotes normal body
functioning in relaxed situations.
New Terminology
Afferent (AFF-uh-rent)
Autonomic nervous system (AW-toh-NOM-ik)
Cauda equina (KAW-dah ee-KWHY-nah)
Cerebral cortex (se-REE-bruhl KOR-teks)
Cerebrospinal fluid (se-REE-broh-SPY-nuhl)
Choroid plexus (KOR-oid PLEK-sus)
Corpus callosum (KOR-pus kuh-LOH-sum)
Cranial nerves (KRAY-nee-uhl NERVS)
Efferent (EFF-uh-rent)
Gray matter (GRAY MA-TUR)
Neuroglia (new-ROG-lee-ah)
Neurolemma (NYOO-ro-LEM-ah)
Parasympathetic (PAR-uh-SIM-puh-THET-ik)
Reflex (REE-fleks)
Somatic (soh-MA-tik)
Spinal nerves (SPY-nuhl NERVS)
Sympathetic (SIM-puh-THET-ik)
Ventricles of brain (VEN-trick’ls)
Visceral (VISS-er-uhl)
White matter (WIGHT MA-TUR)
Related Clinical Terminology
Alzheimer’s disease (ALZ-high-mer’s)
Aphasia (ah-FAY-zee-ah)
Blood–brain barrier (BLUHD BRAYNE)
Cerebrovascular accident (CVA) (se-REE-broh-
Lumbar puncture (LUM-bar PUNK-chur)
Meningitis (MEN-in-JIGH-tis)
Multiple sclerosis (MS) (MULL-ti-puhl skle-
Neuralgia (new-RAL-jee-ah)
Neuritis (new-RYE-tis)
Neuropathy (new-RAH-puh-thee)
Parkinson’s disease (PAR-kin-son’s)
Remission (ree-MISH-uhn)
Spinal shock (SPY-nuhl SHAHK)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
Most of us can probably remember being told,
when we were children, not to touch the stove or some
other source of potential harm. Because children are
curious, such warnings often go unheeded. The
result? Touching a hot stove brings about an immediate
response of pulling away and a vivid memory of
painful fingers. This simple and familiar experience
illustrates the functions of the nervous system:
1. To detect changes and feel sensations
2. To initiate appropriate responses to changes
3. To organize information for immediate use and
store it for future use
The nervous system is one of the regulating systems
(the endocrine system is the other and is discussed
in Chapter 10). Electrochemical impulses of
the nervous system make it possible to obtain information
about the external or internal environment
and do whatever is necessary to maintain homeostasis.
Some of this activity is conscious, but much of it happens
without our awareness.
The nervous system has two divisions. The central
nervous system (CNS) consists of the brain and
spinal cord. The peripheral nervous system (PNS)
consists of cranial nerves and spinal nerves. The PNS
includes the autonomic nervous system (ANS).
The peripheral nervous system relays information
to and from the central nervous system, and the brain
is the center of activity that integrates this information,
initiates responses, and makes us the individuals
we are.
Nerve tissue was briefly described in Chapter 4, so we
will begin by reviewing what you already know and
then add to it.
Nerve cells are called neurons, or nerve fibers.
Whatever their specific functions, all neurons have the
same physical parts. The cell body contains the
nucleus (Fig. 8–1) and is essential for the continued
life of the neuron. As you will see, neuron cell bodies
are found in the central nervous system or close to it
in the trunk of the body. In these locations, cell bodies
are protected by bone. There are no cell bodies in the
arms and legs, which are much more subject to injury.
Dendrites are processes (extensions) that transmit
impulses toward the cell body. The one axon of a neuron
transmits impulses away from the cell body. It is
the cell membrane of the dendrites, cell body, and
axon that carries the electrical nerve impulse.
In the peripheral nervous system, axons and dendrites
are “wrapped” in specialized cells called
Schwann cells (see Fig. 8–1). During embryonic
development, Schwann cells grow to surround the
neuron processes, enclosing them in several layers of
Schwann cell membrane. These layers are the myelin
sheath; myelin is a phospholipid that electrically insulates
neurons from one another. Without the myelin
sheath, neurons would short-circuit, just as electrical
wires would if they were not insulated (see Box 8–1:
Multiple Sclerosis).
The spaces between adjacent Schwann cells, or segments
of the myelin sheath, are called nodes of
Ranvier (neurofibril nodes). These nodes are the parts
of the neuron cell membrane that depolarize when an
electrical impulse is transmitted (see “The Nerve
Impulse” section, on pages 171–172).
The nuclei and cytoplasm of the Schwann cells are
wrapped around the outside of the myelin sheath and
are called the neurolemma, which becomes very
important if nerves are damaged. If a peripheral nerve
is severed and reattached precisely by microsurgery,
the axons and dendrites may regenerate through the
tunnels formed by the neurolemmas. The Schwann
cells are also believed to produce a chemical growth
factor that stimulates regeneration. Although this regeneration
may take months, the nerves may eventually
reestablish their proper connections, and the
person may regain some sensation and movement in
the once-severed limb.
In the central nervous system, the myelin sheaths
are formed by oligodendrocytes, one of the neuroglia
(glial cells), the specialized cells found only in
the brain and spinal cord. Because no Schwann cells
are present, however, there is no neurolemma, and
regeneration of neurons does not occur. This is why
severing of the spinal cord, for example, results in permanent
loss of function. Another kind of neuroglia are
microglia,which are constantly moving, phagocytizing
cellular debris, damaged cells, and pathogens.
166 The Nervous System
Yet another type of glial cell is the astrocyte (literally,
“star cell”). In the embryo, these cells provide a
framework for the migrating neurons that will form
the brain. Thereafter, the extensions of astrocytes are
wrapped around brain capillaries and contribute to the
blood–brain barrier, which prevents potentially
harmful waste products in the blood from diffusing
out into brain tissue. These waste products are normal
in the blood and tissue fluid, but brain tissue is much
more sensitive to even low levels of them than are
other tissues such as muscle tissue or connective tissue.
The capillaries of the brain also contribute to this
barrier, because they are less permeable than are other
capillaries. A disadvantage of the blood–brain barrier
is that some useful medications cannot cross it, and
the antibodies produced by lymphocytes cross only
with difficulty. This becomes an important consideration
when treating brain infections or other diseases
The Nervous System 167
Afferent (sensory) neuron
Axon terminal
Cell body
Functional dendrite
Myelin sheath
Axon terminal
Efferent (motor) neuron
Cell body
Schwann cell nucleus
Myelin sheath
Node of
Schwann cell
Layers of myelin sheath
Figure 8–1. Neuron structure.
(A) A typical sensory neuron.
(B) A typical motor neuron.
The arrows indicate the direction
of impulse transmission.
(C) Details of the myelin sheath
and neurolemma formed by
Schwann cells.
QUESTION: The axon terminal
of the motor neuron would be
found at what kinds of effectors?
or disorders (Table 8–1 summarizes the functions of
the neuroglia).
Neurons that transmit impulses to other neurons do
not actually touch one another. The small gap or space
between the axon of one neuron and the dendrites or
cell body of the next neuron is called the synapse.
Within the synaptic knob (terminal end) of the presynaptic
axon is a chemical neurotransmitter that is
released into the synapse by the arrival of an electrical
nerve impulse (Fig. 8–2). The neurotransmitter diffuses
across the synapse, combines with specific receptor
sites on the cell membrane of the postsynaptic
neuron, and there generates an electrical impulse that
is, in turn, carried by this neuron’s axon to the next
synapse, and so forth. A chemical inactivator at the
cell body or dendrite of the postsynaptic neuron
quickly inactivates the neurotransmitter. This prevents
unwanted, continuous impulses, unless a new
impulse from the first neuron releases more neurotransmitter.
Many synapses are termed excitatory, because the
neurotransmitter causes the postsynaptic neuron to
depolarize (become more negative outside as Na ions
enter the cell) and transmit an electrical impulse to
another neuron, muscle cell, or gland. Some synapses,
however, are inhibitory, meaning that the neurotransmitter
causes the postsynaptic neuron to hyperpolarize
(become even more positive outside as K ions
leave the cell or Cl ions enter the cell) and therefore
not transmit an electrical impulse. Such inhibitory
synapses are important, for example, for slowing the
heart rate, and for balancing the excitatory impulses
transmitted to skeletal muscles. With respect to the
skeletal muscles, this inhibition prevents excessive
contraction and is important for coordination.
168 The Nervous System
protect the axon. Because loss of myelin may
occur in many parts of the central nervous system,
the symptoms vary, but they usually include muscle
weakness or paralysis, numbness or partial loss
of sensation, double vision, and loss of spinal
cord reflexes, including those for urination and
The first symptoms usually appear between the
ages of 20 and 40 years, and the disease may
progress either slowly or rapidly. Some MS patients
have remissions, periods of time when their symptoms
diminish, but remissions and progression of
the disease are not predictable. There is still no cure
for MS, but therapies include suppression of the
immune response, and interferon, which seems to
prolong remissions in some patients. The possibility
of stimulating remyelination of neurons is also
being investigated.
Multiple sclerosis (MS) is a demyelinating disease;
that is, it involves deterioration of the myelin
sheath of neurons in the central nervous system.
Without the myelin sheath, the impulses of these
neurons are short-circuited and do not reach their
proper destinations, and the neuron axons are
damaged and gradually die.
Multiple sclerosis is an autoimmune disorder
that may be triggered by a virus or bacterial infection.
Research has also uncovered a genetic component
to some clusters of MS cases in families.
Exactly how such genes would increase a person’s
susceptibility to an autoimmune disease is not
yet known. In MS, the autoantibodies destroy
the oligodendrocytes, the myelin-producing neuroglia
of the central nervous system, which results
in the formation of scleroses, or plaques of scar
tissue, that do not provide electrical insulation or
Name Function
• Produce the myelin sheath to
electrically insulate neurons of
the CNS.
• Capable of movement and
phagocytosis of pathogens
and damaged tissue.
• Support neurons, help maintain
K level, contribute to the
blood–brain barrier.
• Line the ventricles of the
brain; many of the cells have
cilia; involved in circulation of
cerebrospinal fluid.
One important consequence of the presence of
synapses is that they ensure one-way transmission of
impulses in a living person. A nerve impulse cannot go
backward across a synapse because there is no neurotransmitter
released by the dendrites or cell body.
Neurotransmitters can be released only by a neuron’s
axon, which does not have receptor sites for it, as does
the postsynaptic membrane. Keep this in mind when
we discuss the types of neurons later in the chapter.
An example of a neurotransmitter is acetylcholine,
which is found at neuromuscular junctions, in the
CNS, and in much of the peripheral nervous system.
Acetylcholine usually makes a postsynaptic membrane
more permeable to Na ions, which brings about
depolarization of the postsynaptic neuron. Cholinesterase
is the inactivator of acetylcholine. There are
many other neurotransmitters, especially in the central
nervous system. These include dopamine, GABA,
norepinephrine, glutamate, and serotonin. Each of
these neurotransmitters has its own chemical inactivator.
Some neurotransmitters are reabsorbed into the
neurons that secreted them; this process is called
reuptake and also terminates the effect of the transmitter.
The complexity and variety of synapses make them
frequent targets of medications. For example, drugs
that alter mood or behavior often act on specific neurotransmitters
in the brain, and antihypertensive
drugs affect synapse transmission at the smooth muscle
of blood vessels.
The Nervous System 169
Axon of presynaptic
Vesicles of neurotransmitter Receptor site
Dendrite of
Figure 8–2. Impulse transmission at a synapse. The arrow indicates the direction of the
electrical impulse.
QUESTION: Is this an excitatory synapse or an inhibitory synapse? Explain your answer.
Neurons may be classified into three groups: sensory
neurons, motor neurons, and interneurons (Fig. 8–3).
Sensory neurons (or afferent neurons) carry impulses
from receptors to the central nervous system.
Receptors detect external or internal changes and
send the information to the CNS in the form of
impulses by way of the afferent neurons. The central
nervous system interprets these impulses as a sensation.
Sensory neurons from receptors in skin, skeletal
muscles, and joints are called somatic; those from
receptors in internal organs are called visceral sensory
Motor neurons (or efferent neurons) carry
impulses from the central nervous system to effectors.
The two types of effectors are muscles and glands. In
response to impulses, muscles contract or relax and
glands secrete. Motor neurons linked to skeletal muscle
are called somatic; those to smooth muscle, cardiac
muscle, and glands are called visceral.
170 The Nervous System
Dorsal root
Dorsal root ganglion
Cell body of
sensory neuron
Dendrite of
sensory neuron
Receptor Ventral root
Axon of motor neuron
Synaptic knobs
Effector muscle
Cell body of motor neuron
Gray matter
White matter
Spinothalamic tract
Rubrospinal tract
Corticospinal tract
Dorsal column
Central canal
Figure 8–3. Cross-section of the spinal cord and the three types of neurons. Spinal nerve
roots and their neurons are shown on the left side. Spinal nerve tracts are shown in the
white matter on the right side. All tracts and nerves are bilateral (both sides).
QUESTION: The dorsal column is an ascending tract, and the corticospinal tract is
descending. Explain what this means.
Sensory and motor neurons make up the peripheral
nervous system. Visceral motor neurons form the
autonomic nervous system, a specialized subdivision
of the PNS that will be discussed later in this chapter.
Interneurons are found entirely within the central
nervous system. They are arranged so as to carry only
sensory or motor impulses, or to integrate these functions.
Some interneurons in the brain are concerned
with thinking, learning, and memory.
A neuron carries impulses in only one direction.
This is the result of the neuron’s structure and location,
as well as its physical arrangement with other
neurons and the resulting pattern of synapses. The
functioning nervous system, therefore, is an enormous
network of “one-way streets,” and there is no danger
of impulses running into and canceling one another
A nerve is a group of axons and/or dendrites of many
neurons, with blood vessels and connective tissue.
Sensory nerves are made only of sensory neurons.
The optic nerves for vision and olfactory nerves for
smell are examples of nerves with a purely sensory
function. Motor nerves are made only of motor neurons;
autonomic nerves are motor nerves. A mixed
nerve contains both sensory and motor neurons. Most
of our peripheral nerves, such as the sciatic nerves in
the legs, are mixed nerves.
The term nerve tract refers to groups of neurons
within the central nervous system. All the neurons in
a nerve tract are concerned with either sensory or
motor activity. These tracts are often referred to as
white matter; the myelin sheaths of the neurons give
them a white color.
The events of an electrical nerve impulse are the same
as those of the electrical impulse generated in muscle
fibers, which is discussed in Chapter 7. Stated simply,
a neuron not carrying an impulse is in a state of polarization,
with Na ions more abundant outside the
cell, and K ions and negative ions more abundant
inside the cell. The neuron has a positive charge on
the outside of the cell membrane and a relative negative
charge inside. A stimulus (such as a neurotransmitter)
makes the membrane very permeable to Na
ions, which rush into the cell. This brings about
depolarization, a reversal of charges on the membrane.
The outside now has a negative charge, and the
inside has a positive charge.
As soon as depolarization takes place, the neuron
membrane becomes very permeable to K ions, which
rush out of the cell. This restores the positive charge
outside and the negative charge inside, and is called
repolarization. (The term action potential refers to
depolarization followed by repolarization.) Then the
sodium and potassium pumps return Na ions outside
and K ions inside, and the neuron is ready to respond
to another stimulus and transmit another impulse. An
action potential in response to a stimulus takes place
very rapidly and is measured in milliseconds. An individual
neuron is capable of transmitting hundreds of
action potentials (impulses) each second. A summary
of the events of nerve impulse transmission is given in
Table 8–2.
Transmission of electrical impulses is very rapid.
The presence of an insulating myelin sheath increases
the velocity of impulses, since only the nodes of
Ranvier depolarize. This is called saltatory conduction.
Many of our neurons are capable of transmitting
impulses at a speed of many meters per second.
Imagine a person 6 feet (about 2 meters) tall who stubs
his toe; sensory impulses travel from the toe to the
brain in less than a second (crossing a few synapses
along the way). You can see how the nervous system
can communicate so rapidly with all parts of the body,
and why it is such an important regulatory system.
At synapses, nerve impulse transmission changes
from electrical to chemical and depends on the release
of neurotransmitters. Although diffusion across
synapses is slow, the synapses are so small that this
does not significantly affect the velocity of impulses in
a living person.
The spinal cord transmits impulses to and from the
brain and is the integrating center for the spinal cord
reflexes. Although this statement of functions is very
brief and sounds very simple, the spinal cord is of
great importance to the nervous system and to the
body as a whole.
Enclosed within the vertebral canal and the meninges,
the spinal cord is well protected from mechanical
The Nervous System 171
injury. In length, the spinal cord extends from the
foramen magnum of the occipital bone to the disc
between the first and second lumbar vertebrae.
A cross-section of the spinal cord is shown in Fig.
8–3; refer to it as you read the following. The internal
gray matter is shaped like the letter H; gray matter
consists of the cell bodies of motor neurons and
interneurons. The external white matter is made of
myelinated axons and dendrites of interneurons.
These nerve fibers are grouped into nerve tracts based
on their functions. Ascending tracts (such as the dorsal
columns and spinothalamic tracts) carry sensory
impulses to the brain. Descending tracts (such as the
corticospinal and rubrospinal tracts) carry motor
impulses away from the brain. Lastly, find the central
canal; this contains cerebrospinal fluid and is continuous
with cavities in the brain called ventricles.
There are 31 pairs of spinal nerves, those that emerge
from the spinal cord. The nerves are named according
to their respective vertebrae: 8 cervical pairs, 12 thoracic
pairs, 5 lumbar pairs, 5 sacral pairs, and 1 very
small coccygeal pair. These are shown in Fig. 8–4;
notice that each nerve is designated by a letter and a
number. The 8th cervical nerve is C8, the 1st thoracic
nerve is T1, and so on.
In general, the cervical nerves supply the back of
the head, neck, shoulders, arms, and diaphragm (the
phrenic nerves). The first thoracic nerve also contributes
to nerves in the arms. The remaining thoracic
nerves supply the trunk of the body. The lumbar and
sacral nerves supply the hips, pelvic cavity, and legs.
Notice that the lumbar and sacral nerves hang below
the end of the spinal cord (in order to reach their
proper openings to exit from the vertebral canal); this
is called the cauda equina, literally, the “horse’s tail.”
Some of the important peripheral nerves and their
destinations are listed in Table 8–3.
Each spinal nerve has two roots, which are neurons
entering or leaving the spinal cord (see Fig. 8–3). The
dorsal root is made of sensory neurons that carry
impulses into the spinal cord. The dorsal root ganglion
is an enlarged part of the dorsal root that contains
the cell bodies of the sensory neurons. The term
ganglion means a group of cell bodies outside the
CNS. These cell bodies are within the vertebral canal
and are thereby protected from injury (see Box 8–2:
The ventral root is the motor root; it is made of
the axons of motor neurons carrying impulses from
the spinal cord to muscles or glands. The cell bodies
of these motor neurons, as mentioned previously, are
in the gray matter of the spinal cord. When the two
nerve roots merge, the spinal nerve thus formed is a
mixed nerve.
When you hear the term reflex, you may think of an
action that “just happens,” and in part this is so. A
reflex is an involuntary response to a stimulus, that is,
172 The Nervous System
State or Event Description
(the neuron
is not carrying
an electrical
by a stimulus)
Propagation of
the impulse
from point
of stimulus
• Neuron membrane has a ( )
charge outside and a ( ) charge
• Na ions are more abundant outside
the cell.
• K ions and negative ions are
more abundant inside the cell.
Sodium and potassium pumps
maintain these ion concentrations.
• Neuron membrane becomes very
permeable to Na ions, which
rush into the cell.
• The neuron membrane then has a
( ) charge outside and a ( )
charge inside.
• Depolarization of part of the
membrane makes adjacent membrane
very permeable to Na ions,
and subsequent depolarization,
which similarly affects the next
part of the membrane, and so on.
• The depolarization continues
along the membrane of the neuron
to the end of the axon.
• Neuron membrane becomes very
permeable to K ions, which rush
out of the cell. This restores the
( ) charge outside and ( )
charge inside the membrane.
• The Na ions are returned outside
and the K ions are returned
inside by the sodium and potassium
• The neuron is now able to
respond to another stimulus and
generate another impulse.
Spinal cord
Phrenic nerve
Intercostal nerves
Radial nerve
Median nerve
Ulnar nerve
Cauda equina
Femoral nerve
Sciatic nerve
Cervical plexus
Brachial plexus
Lumbar plexus
Sacral plexus
Figure 8–4. The spinal cord
and spinal nerves. The distribution
of spinal nerves is shown
only on the left side. The nerve
plexuses are labeled on the
right side. A nerve plexus is a
network of neurons from several
segments of the spinal cord
that combine to form nerves to
specific parts of the body. For
example, the radial and ulnar
nerves to the arm emerge from
the brachial plexus (see also
Table 8–3).
QUESTION: Where does the
spinal cord end? Why is this
important clinically?
an automatic action stimulated by a specific change of
some kind. Spinal cord reflexes are those that do not
depend directly on the brain, although the brain may
inhibit or enhance them. We do not have to think
about these reflexes, which is very important, as you
will see.
Reflex Arc
A reflex arc is the pathway that nerve impulses travel
when a reflex is elicited, and there are five essential
1. Receptors—detect a change (the stimulus) and
generate impulses.
2. Sensory neurons—transmit impulses from receptors
to the CNS.
3. Central nervous system—contains one or more
synapses (interneurons may be part of the pathway).
4. Motor neurons—transmit impulses from the
CNS to the effector.
5. Effector—performs its characteristic action.
Let us now look at the reflex arc of a specific reflex,
the patellar (or knee-jerk) reflex, with which you are
174 The Nervous System
Spinal Nerves
Nerve That Contribute Distribution
C5–C8, T1
C5–C8, T1
C8, T1
• Diaphragm
• Skin and muscles of posterior arm, forearm, and hand; thumb and first 2 fingers
• Skin and muscles of anterior arm, forearm, and hand
• Skin and muscles of medial arm, forearm, and hand; little finger and ring finger
• Intercostal muscles, abdominal muscles; skin of trunk
• Skin and muscles of anterior thigh, medial leg, and foot
• Skin and muscles of posterior thigh, leg and foot
Shingles is caused by the same virus that causes
chickenpox: the herpes varicella-zoster virus.
Varicella is chickenpox, which many of us probably
had as children (there is now a vaccine). When a
person recovers from chickenpox, the virus may
survive in a dormant (inactive) state in the dorsal
root ganglia of some spinal nerves. For most people,
the immune system is able to prevent reactivation
of the virus. With increasing age, however, the
immune system is not as effective, and the virus
may become active and cause zoster, or shingles.
The virus is present in sensory neurons, often
those of the trunk, but the damage caused by the
virus is seen in the skin over the affected nerve. The
raised, red lesions of shingles are often very painful
and follow the course of the nerve on the skin external
to it. Pain may continue even after the rash
heals; this is postherpetic neuralgia. Occasionally
the virus may affect a cranial nerve and cause facial
paralysis called Bell’s palsy (7th cranial) or extensive
facial lesions, or, rarely, blindness. Although not a
cure, some antiviral medications lessen the duration
of the illness. A vaccine is being developed for
adults. Though it may not completely prevent shingles,
it is expected to lessen the chance of postherpetic
Box Figure 8–A Lesions of shingles on skin of trunk. (From
Goldsmith, LA, Lazarus, GS, and Tharp, MD: Adult and Pediatric
Dermatology: A Color Guide to Diagnosis and Treatment. FA
Davis, Philadelphia, 1997, p 307, with permission.)
probably familiar. In this reflex, a tap on the patellar
tendon just below the kneecap causes extension of the
lower leg. This is a stretch reflex, which means that a
muscle that is stretched will automatically contract.
Refer now to Fig. 8–5 as you read the following:
In the quadriceps femoris muscle are (1) stretch
receptors that detect the stretching produced by striking
the patellar tendon. These receptors generate
impulses that are carried along (2) sensory neurons in
the femoral nerve to (3) the spinal cord. In the spinal
cord, the sensory neurons synapse with (4) motor neurons
(this is a two-neuron reflex). The motor neurons
in the femoral nerve carry impulses back to (5) the
quadriceps femoris, the effector, which contracts and
extends the lower leg.
The patellar reflex is one of many used clinically to
determine whether the nervous system is functioning
properly. If the patellar reflex were absent in a patient,
the problem could be in the thigh muscle, the femoral
nerve, or the spinal cord. Further testing would be
needed to determine the precise break in the reflex
arc. If the reflex is normal, however, that means that
all parts of the reflex arc are intact. So the testing of
reflexes may be a first step in the clinical assessment of
neurologic damage.
You may be wondering why we have such reflexes,
these stretch reflexes. What is their importance in our
everyday lives? Imagine a person standing upright—is
the body perfectly still? No, it isn’t, because gravity
exerts a downward pull. However, if the body tilts to
the left, the right sides of the leg and trunk are
stretched, and these stretched muscles automatically
contract and pull the body upright again. This is the
purpose of stretch reflexes; they help keep us upright
without our having to think about doing so. If the
brain had to make a decision every time we swayed a
bit, all our concentration would be needed just to
remain standing. Since these are spinal cord reflexes,
the brain is not directly involved. The brain may
become aware that a reflex has taken place, but that
involves another set of neurons carrying impulses to
the brain.
Flexor reflexes (or withdrawal reflexes) are
another type of spinal cord reflex. The stimulus is
something painful and potentially harmful, and the
response is to pull away from it. If you inadvertently
touch a hot stove, you automatically pull your hand
away. Flexor reflexes are three-neuron reflexes,
because sensory neurons synapse with interneurons in
the spinal cord, which in turn synapse with motor
neurons. Again, however, the brain does not have to
make a decision to protect the body; the flexor reflex
does that automatically (see Box 8–3: Spinal Cord
Injuries). The brain may know that the reflex has
taken place, and may even learn from the experience,
but that requires different neurons, not the reflex arc.
The Nervous System 175
Gray matter
(4) Motor neuron
(3) Synapse in
spinal cord
Ventral root
(1) Stretch receptor
(5) Quadriceps femoris muscle
Femoral nerve Dorsal root
(2) Sensory neuron Dorsal root ganglion
Figure 8–5. Patellar reflex. The
reflex arc is shown. See text for
QUESTION: Why is this reflex
called a stretch reflex?
The brain consists of many parts that function as an
integrated whole. The major parts are the medulla,
pons, and midbrain (collectively called the brain
stem), the cerebellum, the hypothalamus, the thalamus,
and the cerebrum. These parts are shown in Fig.
8–6. We will discuss each part separately, but keep in
mind that they are all interconnected and work
The ventricles are four cavities within the brain: two
lateral ventricles, the third ventricle, and the fourth
ventricle (Fig. 8–7). Each ventricle contains a capillary
network called a choroid plexus, which forms cerebrospinal
fluid (CSF) from blood plasma. Cerebrospinal
fluid is the tissue fluid of the central nervous
system; its circulation and functions will be discussed
in the section on meninges.
The medulla extends from the spinal cord to the pons
and is anterior to the cerebellum. Its functions are
those we think of as vital (as in “vital signs”). The
medulla contains cardiac centers that regulate heart
rate, vasomotor centers that regulate the diameter of
blood vessels and, thereby, blood pressure, and respiratory
centers that regulate breathing. You can see
why a crushing injury to the occipital bone may be
rapidly fatal—we cannot survive without the medulla.
Also in the medulla are reflex centers for coughing,
sneezing, swallowing, and vomiting.
The pons bulges anteriorly from the upper part of the
medulla. Within the pons are two respiratory centers
that work with those in the medulla to produce a normal
breathing rhythm. (The function of all the respiratory
centers is discussed in Chapter 15.) The many
other neurons in the pons ( pons is from the Latin for
“bridge”) connect the medulla with other parts of the
The midbrain extends from the pons to the hypothalamus
and encloses the cerebral aqueduct, a tunnel
that connects the third and fourth ventricles. Several
176 The Nervous System
need to urinate or defecate. Nor will voluntary control
of these reflexes be possible, because inhibiting
impulses from the brain can no longer reach the
lower segments of the spinal cord.
Potentially less serious injuries are those in which
the spinal cord is crushed rather than severed, and
treatment is aimed at preserving whatever function
remains. Minimizing inflammation and stimulating
the production of nerve growth factors are aspects
of such treatment.
Perhaps the most challenging research is the
attempt to stimulate severed spinal cords to regenerate.
Partial success has been achieved in rats
and mice, with Schwann cells transplanted from
their peripheral nerves and nerve growth factors
produced by genetically engineered cells. The use
of stem cells has also been successful in rats. The
researchers caution, however, that it will take some
time before their procedures will be tested on
Injuries to the spinal cord are most often caused by
auto accidents, falls, and gunshot wounds. The
most serious injury is transection, or severing, of the
spinal cord. If, for example, the spinal cord is severed
at the level of the 8th thoracic segment, there
will be paralysis and loss of sensation below that
level. Another consequence is spinal shock, the atleast-
temporary loss of spinal cord reflexes. In this
example, the spinal cord reflexes of the lower trunk
and legs will not occur. The stretch reflexes and
flexor reflexes of the legs will be at least temporarily
abolished, as will the urination and defecation
reflexes. Although these reflexes do not depend
directly on the brain, spinal cord neurons depend
on impulses from the brain to enhance their own
ability to generate impulses.
As spinal cord neurons below the injury recover
their ability to generate impulses, these reflexes,
such as the patellar reflex, often return. Urination
and defecation reflexes may also be reestablished,
but the person will not have an awareness of the
Frontal lobe
Corpus callosum
Parietal lobe
Occipital lobe
Choroid plexus in
fourth ventricle
Spinal cord
Temporal lobe
Pituitary gland
Optic nerve
Choroid plexus in
third ventricle
Longitudinal fissure
Cerebral cortex
Corpus callosum
Lateral ventricle
Third ventricle
Optic tracts
Figure 8–6. (A) Midsagittal section of the brain as seen from the left side. This medial
plane shows internal anatomy as well as the lobes of the cerebrum. (B) Frontal section of
the brain in anterior view.
QUESTION: Find the corpus callosum in parts A and B, and describe its shape. What is its
different kinds of reflexes are integrated in the midbrain,
including visual and auditory reflexes. If you see
a wasp flying toward you, you automatically duck or
twist away; this is a visual reflex, as is the coordinated
movement of the eyeballs. Turning your head (ear) to
a sound is an example of an auditory reflex. The midbrain
is also concerned with what are called righting
reflexes, those that keep the head upright and maintain
balance or equilibrium.
The cerebellum is separated from the medulla and
pons by the fourth ventricle and is inferior to the
occipital lobes of the cerebrum. As you already know,
many of the functions of the cerebellum are concerned
with movement. These include coordination, regulation
of muscle tone, the appropriate trajectory and
endpoint of movements, and the maintenance of posture
and equilibrium. Notice that these are all involuntary;
that is, the cerebellum functions below the
level of conscious thought. This is important to permit
the conscious brain to work without being overburdened.
If you decide to pick up a pencil, for example,
the impulses for arm movement come from the cerebrum.
The cerebellum then modifies these impulses so
that your arm and finger movements are coordinated,
and you don’t reach past the pencil.
The cerebellum seems also to be involved in certain
sensory functions. For example, if you close your eyes
and someone places a tennis ball in one hand and a
baseball in the other, could you tell which was which?
Certainly you could, by the “feel” of each: the texture
and the weight or heft. If you pick up a plastic container
of coffee (with a lid on it) could you tell if the
cup is full, half-full, or empty? Again, you certainly
could. Do you have to think about it? No. The cerebellum
is, in part, responsible for this ability.
To regulate equilibrium, the cerebellum (and midbrain)
uses information about gravity and movement
provided by receptors in the inner ears. These receptors
are discussed further in Chapter 9.
Located superior to the pituitary gland and inferior to
the thalamus, the hypothalamus is a small area of the
brain with many diverse functions:
178 The Nervous System
Lateral ventricles
Parietal lobe
Occipital lobe
Cerebral aqueduct
Fourth ventricle
Central canal of spinal cord
Temporal lobe
Third ventricle
Figure 8–7. Ventricles of the brain as projected into the interior of the brain, which is
seen from the left side.
QUESTION: Describe the extent of each lateral ventricle.
1. Production of antidiuretic hormone (ADH) and
oxytocin; these hormones are then stored in the
posterior pituitary gland. ADH enables the kidneys
to reabsorb water back into the blood and thus
helps maintain blood volume. Oxytocin causes contractions
of the uterus to bring about labor and
2. Production of releasing hormones (also called
releasing factors) that stimulate the secretion of
hormones by the anterior pituitary gland. Because
these hormones are covered in Chapter 10, a single
example will be given here: The hypothalamus produces
growth hormone releasing hormone
(GHRH), which stimulates the anterior pituitary
gland to secrete growth hormone (GH).
3. Regulation of body temperature by promoting
responses such as sweating in a warm environment
or shivering in a cold environment (see Chapter
4. Regulation of food intake; the hypothalamus is
believed to respond to changes in blood nutrient
levels, to chemicals secreted by fat cells, and to hormones
secreted by the gastrointestinal tract. For
example, during a meal, after a certain duration of
digestion, the small intestine produces a hormone
that circulates to the hypothalamus and brings
about a sensation of satiety, or fullness, and we tend
to stop eating.
5. Integration of the functioning of the autonomic
nervous system, which in turn regulates the activity
of organs such as the heart, blood vessels, and
intestines. This will be discussed in more detail
later in this chapter.
6. Stimulation of visceral responses during emotional
situations. When we are angry, heart rate usually
increases. Most of us, when embarrassed, will blush,
which is vasodilation in the skin of the face. These
responses are brought about by the autonomic
nervous system when the hypothalamus perceives a
change in emotional state. The neurologic basis of
our emotions is not well understood, and the visceral
responses to emotions are not something most
of us can control.
7. Regulation of body rhythms such as secretion of
hormones, sleep cycles, changes in mood, or mental
alertness. This is often referred to as our biological
clock, the rhythms as circadian rhythms,
meaning “about a day.” If you have ever had to stay
awake for 24 hours, you know how disorienting it
can be, until the hypothalamic biological clock has
been reset.
The thalamus is superior to the hypothalamus and
inferior to the cerebrum. The third ventricle is a narrow
cavity that passes through both the thalamus and
hypothalamus. Many of the functions of the thalamus
are concerned with sensation. Sensory impulses to the
brain (except those for the sense of smell) follow neuron
pathways that first enter the thalamus, which
groups the impulses before relaying them to the cerebrum,
where sensations are felt. For example, holding
a cup of hot coffee generates impulses for heat, touch
and texture, and the shape of the cup (muscle sense),
but we do not experience these as separate sensations.
The thalamus integrates the impulses from the cutaneous
receptors and from the cerebellum, that is, puts
them together in a sort of electrochemical package, so
that the cerebrum feels the whole and is able to interpret
the sensation quickly.
Some sensations, especially unpleasant ones such as
pain, are believed to be felt by the thalamus. However,
the thalamus cannot localize the sensation; that is, it
does not know where the painful sensation is. The
sensory areas of the cerebrum are required for localization
and precise awareness.
The thalamus may also suppress unimportant
sensations. If you are reading an enjoyable book, you
may not notice someone coming into the room. By
temporarily blocking minor sensations, the thalamus
permits the cerebrum to concentrate on important
Parts of the thalamus are also involved in alertness
and awareness (being awake and knowing we are), and
others contribute to memory. For these functions, as
for others, the thalamus works very closely with the
The largest part of the human brain is the cerebrum,
which consists of two hemispheres separated by the
longitudinal fissure. At the base of this deep groove is
the corpus callosum, a band of 200 million neurons
that connects the right and left hemispheres. Within
each hemisphere is a lateral ventricle.
The surface of the cerebrum is gray matter called
the cerebral cortex. Gray matter consists of cell bodies
of neurons, which carry out the many functions of
the cerebrum. Internal to the gray matter is white
matter, made of myelinated axons and dendrites that
connect the lobes of the cerebrum to one another and
to all other parts of the brain.
The Nervous System 179
In the human brain the cerebral cortex is folded
extensively. The folds are called convolutions or
gyri and the grooves between them are fissures or
sulci (you can see the folding of the cortex in the
frontal section of the brain in Fig. 8–6). This folding
permits the presence of millions more neurons in
the cerebral cortex. The cerebral cortex of an animal
such as a dog or cat does not have this extensive
folding. This difference enables us to read, speak,
do long division, write poetry and songs, and do
so many other “human” things that dogs and cats cannot
The cerebral cortex is divided into lobes that have
the same names as the cranial bones external to them.
Therefore, each hemisphere has a frontal lobe, parietal
lobe, temporal lobe, and occipital lobe (Fig. 8–8).
These lobes have been mapped; that is, certain areas
are known to be associated with specific functions. We
will discuss the functions of the cerebrum according to
these mapped areas.
Frontal Lobes
Within the frontal lobes are the motor areas that
generate the impulses for voluntary movement. The
largest portions are for movement of the hands and
face, those areas with many muscles capable of very
fine or precise movements. It is the large size of the
motor area devoted to them that gives these muscles
their precision. The left motor area controls movement
on the right side of the body, and the right
motor area controls the left side of the body. This is
why a patient who has had a cerebrovascular accident,
or stroke, in the right frontal lobe will have paralysis
of muscles on the left side (see Box 8–4: Cerebrovascular
Anterior to the motor areas are the premotor
areas, which are concerned with learned motor skills
that require a sequence of movements. Tying shoelaces,
for example, seems almost automatic to us; we
forget having learned it. It is not a reflex, however;
180 The Nervous System
Frontal lobe
Premotor area
Motor area
General sensory area
Sensory association
Parietal lobe
Occipital lobe
Visual area
Auditory area
Temporal lobe
Visual association
Motor speech area
Figure 8–8. Left cerebral hemisphere showing some of the functional areas that have
been mapped.
QUESTION: What sensations are felt in the general sensory area?
rather the premotor cortex has learned the sequence
so well that we are able to repeat it without consciously
thinking about it.
The parts of the frontal lobes just behind the eyes
are the prefrontal or orbitofrontal cortex. This area
is concerned with things such as keeping emotional
responses appropriate to the situation, realizing that
there are standards of behavior (laws or rules of a
game or simple courtesy) and following them, and
anticipating and planning for the future. An example
may be helpful to put all this together: Someone with
damage to the prefrontal area might become enraged
if his pen ran out of ink during class, might throw the
pen at someone, and might not think that a pen will be
needed tomorrow and that it is time to go buy one. As
you can see, the prefrontal cortex is very important for
social behavior, and greatly contributes to what makes
us human.
Also in the frontal lobe, usually only the left lobe
for most right-handed people, is Broca’s motor
speech area, which controls the movements of the
mouth involved in speaking.
Parietal Lobes
The general sensory areas in the parietal lobes
receive impulses from receptors in the skin and feel
and interpret the cutaneous sensations. The left area is
for the right side of the body and vice versa. These
areas also receive impulses from stretch receptors in
muscles for conscious muscle sense. The largest portions
of these areas are for sensation in the hands and
face, those parts of the body with the most cutaneous
receptors and the most muscle receptors. The taste
areas, which overlap the parietal and temporal lobes,
receive impulses from taste buds on the tongue and
elsewhere in the oral cavity.
Temporal Lobes
The olfactory areas in the temporal lobes receive
impulses from receptors in the nasal cavities for the
sense of smell. The olfactory association area learns
the meaning of odors such as the smell of sour milk, or
fire, or brownies baking in the oven, and enables the
thinking cerebrum to use that information effectively.
The Nervous System 181
they cause is very widespread or affects vital centers
in the medulla or pons.
For CVAs of the thrombus type, a clot-dissolving
drug may help reestablish blood flow. To be effective,
however, the drug must be administered
within 3 hours of symptom onset (see also Box
Recovery from a CVA depends on its location
and the extent of damage, as well as other factors.
One of these is the redundancy of the brain.
Redundancy means repetition or exceeding what
is necessary; the cerebral cortex has many more
neurons than we actually use in daily activities.
The characteristic of plasticity means that these
neurons are available for use, especially in younger
people (less than 50 years of age). When a patient
recovers from a disabling stroke, what has often
happened is that the brain has established new
pathways, with previously little-used neurons now
carrying impulses “full time.” Such recovery is
highly individual and may take months. Yet another
important factor is that CVA patients be started on
rehabilitation therapy as soon as their condition
A cerebrovascular accident (CVA), or stroke, is
damage to a blood vessel in the brain, resulting in
lack of oxygen to that part of the brain. Possible
types of vessel damage are thrombosis or hemorrhage.
A thrombus is a blood clot, which most often is
a consequence of atherosclerosis, abnormal lipid
deposits in cerebral arteries. The rough surface
stimulates clot formation, which obstructs the
blood flow to the part of the brain supplied by
the artery. The symptoms depend on the part of the
brain affected and may be gradual in onset if clot
formation is slow. Approximately 80% of CVAs are
of this type.
A hemorrhage, the result of arteriosclerosis or
aneurysm of a cerebral artery, allows blood out
into brain tissue, which destroys brain neurons by
putting excessive pressure on them as well as
depriving them of oxygen. Onset of symptoms in
this type of CVA is usually rapid.
If, for example, the CVA is in the left frontal lobe,
paralysis of the right side of the body will occur.
Speech may also be affected if the speech areas are
involved. Some CVAs are fatal because the damage
The auditory areas, as their name suggests, receive
impulses from receptors in the inner ear for hearing.
The auditory association area is quite large. Part of it
is concerned with the meanings of words we hear, that
is, with speech. Other parts are for the interpretation
of sounds such as thunder during a storm, an ambulance
siren, or a baby crying. Without proper interpretation,
we would hear the sound but would not know
what it meant, and could not respond appropriately.
Also in the temporal and parietal lobes in the left
hemisphere (for most of us) are other speech areas
concerned with the thought that precedes speech.
Each of us can probably recall (and regret) times when
we have “spoken without thinking,” but in actuality
that is not possible. The thinking takes place very rapidly
and is essential in order to be able to speak (see
Box 8–5: Aphasia).
Occipital Lobes
Impulses from the retinas of the eyes travel along the
optic nerves to the visual areas in the occipital lobes.
These areas “see.” The visual association areas interpret
what is seen, and enable the thinking cerebrum to
use the information. Imagine looking at a clock.
Seeing the clock is far different from being able to
interpret it. At one time we learned to interpret the
clock face and hands, and now we do not have to consciously
decide what time the clock is reading. We can
simply use that information, such as hurrying a bit so
as not to be late to class. Other parts of the occipital
lobes are concerned with spatial relationships; things
such as judging distance and seeing in three dimensions,
or the ability to read a map and relate it to the
physical world.
The cerebral cortex has the characteristic of neural
plasticity, the ability to adapt to changing needs, to
recruit different neurons for certain functions, as may
occur during childhood or recovery from a stroke.
Another example is the visual cortex of a person who
is born blind. The neurons in the occipital lobes that
would have been used for vision will often be used for
another function; some may become part of an auditory
area that is used to localize sounds and estimate
their distance. Those of us who can see may not rely
on hearing for localization; we simply look at where
we think the sound came from. A blind person cannot
do this, and may have an extensive mental catalogue of
sounds, meanings of sounds, distances of sounds, and
so on, some of these in the part of the cortex that normally
is for vision.
The younger the person, the more plastic the brain.
The brains of children are extraordinarily adaptable.
As we get older, this ability diminishes, but is still
Association Areas
As you can see in Fig. 8–8, many parts of the cerebral
cortex are not concerned with movement or a particu-
182 The Nervous System
Auditory aphasia is “word deafness,” caused
by damage to an interpretation area. The person
can still hear but cannot comprehend what the
words mean. Visual aphasia is “word blindness”;
the person can still see perfectly well, but cannot
make sense of written words (the person retains
the ability to understand spoken words). Imagine
how you would feel if wms qsbbcljw jmqr rfc
yzgjgrw rm pcyb. Frustrating isn’t it? You know
that those symbols are letters, but you cannot
“decode” them right away. Those “words”
were formed by shifting the alphabet two letters
(A C, B D, C E, etc.), and would normally
be read as: “you suddenly lost the ability to read.”
That may give you a small idea of what word blindness
is like.
Our use of language sets us apart from other
animals and involves speech, reading, and writing.
Language is the use of symbols (words) to designate
objects and to express ideas. Damage to
the speech areas or interpretation areas of the
cerebrum may impair one or more aspects of a person’s
ability to use language; this is called aphasia.
Aphasia may be a consequence of a cerebrovascular
accident, or of physical trauma to the skull
and brain such as a head injury sustained in an
automobile accident. If the motor speech (Broca’s)
area is damaged, the person is still able to understand
written and spoken words and knows what
he wants to say, but he cannot say it. Without coordination
and impulses from the motor speech area,
the muscles used for speech cannot contract to
form words properly.
lar sensation. These may be called association areas
and perhaps are what truly make us individuals. It is
probably these areas that give each of us a personality,
a sense of humor, and the ability to reason and use
logic. Learning and memory are also functions of
these areas.
Although much has been learned about the formation
of memories, the processes are still incompletely
understood and mostly beyond the scope of this book.
Briefly, however, we can say that memories of things
such as people or books or what you did last summer
involve the hippocampus (from the Greek for
“seahorse,” because of its shape), part of the temporal
lobe on the floor of the lateral ventricle. The two hippocampi
seem to collect information from many areas
of the cerebral cortex. When you meet a friend, for
example, the memory emerges as a whole: “Here’s
Fred,” not in pieces. People whose hippocampi are
damaged cannot form new memories that last more
than a few seconds.
The right hippocampus is also believed to be
involved in spatial cognition (literally: “space thinking”).
For example, if you are in school and a friend
asks you the shortest way to your home, you will probably
quickly form a mental map. You can see how
much memory that involves (streets, landmarks, and
so on), but the hippocampus can take it a step further
and make your memories three-dimensional and mentally
visible. You can see your way home. That is spatial
It is believed that most, if not all, of what we have
experienced or learned is stored somewhere in the
brain. Sometimes a trigger may bring back memories;
a certain scent or a song could act as possible triggers.
Then we find ourselves recalling something from the
past and wondering where it came from.
The loss of personality due to destruction of
brain neurons is perhaps most dramatically seen in
Alzheimer’s disease (see Box 8–6: Alzheimer’s Disease).
Basal Ganglia
The basal ganglia are paired masses of gray matter
within the white matter of the cerebral hemispheres
(see Fig. 8–6). Their functions are certain subconscious
aspects of voluntary movement, and they work
with the cerebellum. The basal ganglia help regulate
muscle tone, and they coordinate accessory movements
such as swinging the arms when walking or gesturing
while speaking. The most common disorder of
the basal ganglia is Parkinson’s disease (see Box 8–7:
Parkinson’s Disease).
Corpus Callosum
As mentioned previously, the corpus callosum is a
band of nerve fibers that connects the left and right
cerebral hemispheres. This enables each hemisphere
to know of the activity of the other. This is especially
important for people because for most of us, the left
hemisphere contains speech areas and the right hemisphere
does not. The corpus callosum, therefore, lets
The Nervous System 183
of another protein called beta-amyloid that are
damaging to neurons.
A defective gene has been found in some
patients who have late-onset Alzheimer’s disease,
the most common type. Yet another gene seems to
trigger increased synthesis of beta-amyloid. Some
research is focused on the interaction of these
genes and on inflammation as a contributing factor
to this type of brain damage.
It is likely that the treatment of Alzheimer’s disease
will one day mean delaying its onset with a
variety of medications, each targeted at a different
aspect of this complex disease. Early diagnosis will
be very important, and this is yet another avenue of
In the United States, Alzheimer’s disease, a progressive,
incurable form of mental deterioration,
affects approximately 5 million people and is the
cause of 100,000 deaths each year. The first symptoms,
which usually begin after age 65, are memory
lapses and slight personality changes. As the
disease progresses, there is total loss of memory,
reasoning ability, and personality, and those with
advanced disease are unable to perform even the
simplest tasks or self-care.
Structural changes in the brains of Alzheimer’s
patients may be seen at autopsy. Neurofibrillary
tangles are abnormal fibrous proteins found in cells
of the cerebral cortex in areas important for memory
and reasoning. Also present are plaques made
the left hemisphere know what the right hemisphere is
thinking about, and the right hemisphere know what
the left hemisphere is thinking and talking about. A
brief example may be helpful. If you put your left hand
behind your back and someone places a pencil in your
hand (you are not looking at it) and asks you what it is,
would you be able to say? Yes, you would. You would
feel the shape and weight of the pencil, find the point
and the eraser. The sensory impulses from your left
hand are interpreted as “pencil” by the general sensory
area in your right parietal lobe. Your right hemisphere
probably cannot speak, but its thoughts can be conveyed
by way of the corpus callosum to the left hemisphere,
which does have speech areas. Your left
hemisphere can say that you are holding a pencil.
Other aspects of the “division of labor” of our cerebral
hemispheres are beyond the scope of this book, but it
is a fascinating subject that you may wish to explore
The connective tissue membranes that cover the brain
and spinal cord are called meninges; the three layers
are illustrated in Fig. 8–9. The thick outermost layer,
made of fibrous connective tissue, is the dura mater
(Latin for “tough mother”), which lines the skull and
vertebral canal. The middle arachnoid membrane
(arachnids are spiders) is made of web-like strands of
connective tissue. The innermost pia mater (Latin for
“gentle mother”) is a very thin membrane on the surface
of the spinal cord and brain. Between the arachnoid
and the pia mater is the subarachnoid space,
which contains cerebrospinal fluid (CSF), the tissue
fluid of the central nervous system.
Recall the ventricles (cavities) of the brain: two lateral
ventricles, the third ventricle, and the fourth ventricle.
Each contains a choroid plexus, a capillary
network that forms cerebrospinal fluid from blood
plasma. This is a continuous process, and the cerebrospinal
fluid then circulates in and around the central
nervous system (Fig. 8–10).
From the lateral and third ventricles, cerebrospinal
fluid flows through the fourth ventricle, then to the
central canal of the spinal cord, and to the cranial and
spinal subarachnoid spaces. As more cerebrospinal
fluid is formed, you might expect that some must be
reabsorbed, and that is just what happens. From the
cranial subarachnoid space, cerebrospinal fluid is reabsorbed
through arachnoid villi into the blood in
cranial venous sinuses (large veins within the
double-layered cranial dura mater). The cerebrospinal
fluid becomes blood plasma again, and the rate of
reabsorption normally equals the rate of production.
Since cerebrospinal fluid is tissue fluid, one of its
functions is to bring nutrients to CNS neurons and to
remove waste products to the blood as the fluid is
reabsorbed. The other function of cerebrospinal fluid
is to act as a cushion for the central nervous system.
The brain and spinal cord are enclosed in fluid-filled
membranes that absorb shock. You can, for example,
shake your head vigorously without harming your
brain. Naturally, this protection has limits; very sharp
or heavy blows to the skull will indeed cause damage
to the brain.
Examination of cerebrospinal fluid may be used in
the diagnosis of certain diseases (see Box 8–8: Lumbar
184 The Nervous System
Parkinson’s disease is a disorder of the basal
ganglia whose cause is unknown, and though
there is a genetic component in some families, it
is probably not the only factor. The disease usually
begins after the age of 60. Neurons in the
basal ganglia that produce the neurotransmitter
dopamine begin to degenerate and die, and the
deficiency of dopamine causes specific kinds
of muscular symptoms. Tremor, or involuntary
shaking, of the hands is probably the most common
symptom. The accessory movements regulated
by the basal ganglia gradually diminish,
and the affected person walks slowly without
swinging the arms. A mask-like face is characteristic
of this disease, as the facial muscles become
rigid. Eventually all voluntary movements become
slower and much more difficult, and balance
is seriously impaired.
Dopamine itself cannot be used to treat
Parkinson’s disease because it does not cross the
blood–brain barrier. A substance called L-dopa
does cross and can be converted to dopamine by
brain neurons. Unfortunately, L-dopa begins to
lose its therapeutic effectiveness within a few
Other medications in use do not provide a
cure. Some researchers suggest that implants of
stem cells may offer the best hope of meaningful
Central canal
Pia mater
Gray matter
White matter
Spinal nerve roots
Dorsal root ganglion
Spinal nerve
Dura mater
Superior sagittal sinus
Cerebral cortex
(white matter)
Arachnoid membrane
Subarachnoid space
Pia mater
Arachnoid villi
Figure 8–9. Structure of the meninges. (A) Meninges of the spinal cord. (B) Frontal section
through the top of the skull showing the double-layered cranial dura mater and one
of the cranial venous sinuses.
QUESTION: Describe the structural difference between the spinal dura mater and the cranial
dura mater.
The 12 pairs of cranial nerves emerge from the brain
stem or other parts of the brain—they are shown in
Fig. 8–11. The name cranial indicates their origin, and
many of them do carry impulses for functions involving
the head. Some, however, have more far-reaching
The impulses for the senses of smell, taste, sight,
hearing, and equilibrium are all carried by cranial
nerves to their respective sensory areas in the brain.
Some cranial nerves carry motor impulses to muscles
of the face and eyes or to the salivary glands. The
186 The Nervous System
Subarachnoid space
Dura mater
Fourth ventricle
Arachnoid villus
Choroid plexus of
fourth ventricle
Cerebral aqueduct
Pia mater
Cranial venous sinus
Subarachnoid space
Central canal
Spinal cord
Spinal meninges
Pia mater
Dura mater
Subarachnoid space
Third ventricle
Choroid plexus of
third ventricle
Choroid plexus of
lateral ventricle
Figure 8–10. Formation, circulation, and reabsorption of cerebrospinal fluid. See text for
QUESTION: In this pathway, where is the CSF reabsorbed, and into what?
vagus nerves (vagus means “wanderer”) branch extensively
to the larynx, heart, stomach and intestines, and
bronchial tubes.
The functions of the cranial nerves are summarized
in Table 8–4.
The autonomic nervous system (ANS) is actually
part of the peripheral nervous system in that it consists
of motor portions of some cranial and spinal nerves.
Because its functioning is so specialized, however, the
autonomic nervous system is usually discussed as a
separate entity, as we will do here.
Making up the autonomic nervous system are visceral
motor neurons to smooth muscle, cardiac muscle,
and glands. These are the visceral effectors;
muscle will either contract or relax, and glands will
either increase or decrease their secretions.
The ANS has two divisions: sympathetic and
parasympathetic. Often, they function in opposition
to each other, as you will see. The activity of both divisions
is integrated by the hypothalamus, which
ensures that the visceral effectors will respond appropriately
to the situation.
An autonomic nerve pathway from the central nervous
system to a visceral effector consists of two motor
neurons that synapse in a ganglion outside the CNS
(Fig. 8–12). The first neuron is called the preganglionic
neuron, from the CNS to the ganglion. The
second neuron is called the postganglionic neuron,
from the ganglion to the visceral effector. The ganglia
are actually the cell bodies of the postganglionic
Another name for the sympathetic division is thoracolumbar
division, which tells us where the sympathetic
preganglionic neurons originate. Their cell
The Nervous System 187
Box Figure 8–B Cerebrospinal fluid from a patient with
meningitis. The bacteria are streptococci, found in pairs. The
large cells are WBCs. ( 500) (From Sacher, RA, and
McPherson, RA: Widmann’s Clinical Interpretation of
Laboratory Tests, ed. 11. FA Davis, Philadelphia, 2000, Plate
52, with permission.)
A lumbar puncture (spinal tap) is a diagnostic
procedure that involves the removal of cerebrospinal
fluid to determine its pressure and constituents.
As the name tells us, the removal, using a
syringe, is made in the lumbar area. Because the
spinal cord ends between the 1st and 2nd lumbar
vertebrae, the needle is usually inserted between
the 4th and 5th lumbar vertebrae. The meningeal
sac containing cerebrospinal fluid extends to the
end of the lumbar vertebrae, permitting access to
the cerebrospinal fluid with little chance of damaging
the spinal cord.
Cerebrospinal fluid is a circulating fluid and has
a normal pressure of 70 to 200 mmH2O. An abnormal
pressure usually indicates an obstruction in circulation,
which may be caused by infection, a
tumor, or mechanical injury. Other diagnostic
tests would be needed to determine the precise
Perhaps the most common reason for a lumbar
puncture is suspected meningitis, which may be
caused by several kinds of bacteria. If the patient
does have meningitis, the cerebrospinal fluid will be
cloudy rather than clear and will be examined for
the presence of bacteria and many white blood
cells. A few WBCs in CSF is normal, because WBCs
are found in all tissue fluid.
Another abnormal constituent of cerebrospinal
fluid is red blood cells. Their presence indicates
bleeding somewhere in the central nervous system.
There may be many causes, and again, further testing
would be necessary.
188 The Nervous System
Figure 8–11. Cranial
nerves and their distributions.
The brain is shown in an inferior
view. See Table 8–4 for
QUESTION: Which cranial
nerves bring about secretion
of saliva? Which nerve brings
about gastric and intestinal
bodies are in the thoracic segments and some of the
lumbar segments of the spinal cord. Their axons
extend to the sympathetic ganglia, most of which are
located in two chains just outside the spinal column
(see Fig. 8–12). Within the ganglia are the synapses
between preganglionic and postganglionic neurons;
the postganglionic axons then go to the visceral effectors.
One preganglionic neuron often synapses with
many postganglionic neurons to many effectors. This
anatomic arrangement has physiological importance:
The sympathetic division brings about widespread
responses in many organs.
The sympathetic division is dominant in stressful
situations, which include anger, fear, or anxiety, as
well as exercise. For our prehistoric ancestors, stressful
situations often involved the need for intense physical
activity—the “fight or flight response.” Our
nervous systems haven’t changed very much in 50,000
years, and if you look at Table 8–5, you will see the
kinds of responses the sympathetic division stimulates.
The heart rate increases, vasodilation in skeletal muscles
supplies them with more oxygen, the bronchioles
dilate to take in more air, and the liver changes glycogen
to glucose to supply energy. At the same time,
digestive secretions decrease and peristalsis slows;
these are not important in a stress situation.
Vasoconstriction in the skin and viscera shunts blood
to more vital organs such as the heart, muscles, and
brain. All of these responses enabled our ancestors to
stay and fight or to get away from potential danger.
Even though we may not always be in life-threatening
situations during stress (such as figuring out our
income taxes), our bodies are prepared for just that.
The other name for the parasympathetic division is
the craniosacral division. The cell bodies of parasympathetic
preganglionic neurons are in the brain stem
and the sacral segments of the spinal cord. Their axons
are in cranial nerve pairs 3, 7, 9, and 10 and in some
sacral nerves and extend to the parasympathetic ganglia.
These ganglia are very close to or actually in the
visceral effector (see Fig. 8–12), and contain the postganglionic
cell bodies, with very short axons to the
cells of the effector.
In the parasympathetic division, one preganglionic
neuron synapses with just a few postganglionic neurons
to only one effector. With this anatomic arrangement,
very localized (one organ) responses are possible.
The parasympathetic division dominates in relaxed
(non-stress) situations to promote normal functioning
of several organ systems. Digestion will be efficient,
with increased secretions and peristalsis; defecation
and urination may occur; and the heart will beat at a
normal resting rate. Other functions of this division
are listed in Table 8–5.
Notice that when an organ receives both sympathetic
and parasympathetic impulses, the responses are
opposites. Such an arrangement makes maintaining an
appropriate level of activity quite simple, as in changing
the heart rate to meet the needs of a situation.
Notice also that some visceral effectors receive only
sympathetic impulses. In such cases, the opposite
response is brought about by a decrease in sympathetic
impulses. Secretion by the sweat glands is an example.
Recall that neurotransmitters enable nerve impulses to
cross synapses. In autonomic pathways there are two
synapses: one between preganglionic and postganglionic
neurons, and the second between postganglionic
neurons and visceral effectors.
Acetylcholine is the transmitter released by all
preganglionic neurons, both sympathetic and para-
The Nervous System 189
Number and Name Function(s)
I Olfactory
II Optic
III Oculomotor
IV Trochlear
V Trigeminal
VI Abducens
VII Facial
VIII Acoustic (vestibulocochlear)
IX Glossopharyngeal
X Vagus
XI Accessory
XII Hypoglossal
• Sense of smell
• Sense of sight
• Movement of the eyeball; constriction of pupil in bright light or for near vision
• Movement of eyeball
• Sensation in face, scalp, and teeth; contraction of chewing muscles
• Movement of the eyeball
• Sense of taste; contraction of facial muscles; secretion of saliva
• Sense of hearing; sense of equilibrium
• Sense of taste; sensory for cardiac, respiratory, and blood pressure reflexes;
contraction of pharynx; secretion of saliva
• Sensory in cardiac, respiratory, and blood pressure reflexes; sensory and motor
to larynx (speaking); decreases heart rate; contraction of alimentary tube
(peristalsis); increases digestive secretions
• Contraction of neck and shoulder muscles; motor to larynx (speaking)
• Movement of the tongue
Eye Ciliary ganglion
Vagus nerve
Preganglionic neurons
Celiac ganglion
Adrenal gland
Chain of
Figure 8–12. The autonomic nervous system. The sympathetic division is shown on the
left, and the parasympathetic division is shown on the right (both divisions are bilateral).
QUESTION: Do both or just one division of the ANS supply the heart? What is the purpose
of this arrangement?
sympathetic; it is inactivated by cholinesterase in
postganglionic neurons. Parasympathetic postganglionic
neurons all release acetylcholine at the
synapses with their visceral effectors. Most sympathetic
postganglionic neurons release the transmitter
norepinephrine at the synapses with the effector cells.
Norepinephrine is inactivated by either catechol-Omethyl
transferase (COMT) or monoamine oxidase
(MAO), or it may be removed from the synapse by
The aging brain does lose neurons, but this is only a
small percentage of the total and not the usual cause of
mental impairment in elderly people. (Far more common
causes are depression, malnutrition, hypotension,
and the side effects of medications.) Some forgetfulness
is to be expected, however, as is a decreased ability
for rapid problem solving, but most memory
should remain intact. Voluntary movements become
slower, as do reflexes and reaction time. Think of driving
a car, an ability most of us take for granted. For
elderly people, with their slower perceptions and reaction
times, greater consciousness of driving is necessary.
As the autonomic nervous system ages, dry eyes and
constipation may become problems. Transient hypotension
may be the result of decreased sympathetic
stimulation of vasoconstriction. In most cases, however,
elderly people who are aware of these aspects of
aging will be able to work with their physicians or
nurses to adapt to them.
The nervous system regulates many of our simplest
and our most complex activities. The impulses generated
and carried by the nervous system are an example
of the chemical level of organization of the body.
These nerve impulses then regulate the functioning of
tissues, organs, and organ systems, which permits us to
perceive and respond to the world around us and the
changes within us. The detection of such changes is
the function of the sense organs, and they are the subject
of our next chapter.
The Nervous System 191
Organ Sympathetic Response Parasympathetic Response
Heart (cardiac muscle)
Bronchioles (smooth muscle)
Iris (smooth muscle)
Salivary glands
Stomach and intestines (smooth muscle)
Stomach and intestines (glands)
Internal anal sphincter
Urinary bladder (smooth muscle)
Internal urethral sphincter
Sweat glands
Blood vessels in skin and viscera
(smooth muscle)
Blood vessels in skeletal muscle
(smooth muscle)
Adrenal glands
• Increase rate
• Dilate
• Pupil dilates
• Decrease secretion
• Decrease peristalsis
• Decrease secretion
• Contracts to prevent defecation
• Relaxes to prevent urination
• Contracts to prevent urination
• Changes glycogen to glucose
• Secretes glucagon
• Increase secretion
• Constrict
• Dilate
• Increase secretion of epinephrine
and norepinephrine
• Decrease rate (to normal)
• Constrict (to normal)
• Pupil constricts (to normal)
• Increase secretion (to normal)
• Increase peristalsis for normal digestion
• Increase secretion for normal digestion
• Relaxes to permit defecation
• Contracts for normal urination
• Relaxes to permit urination
• None
• Secretes insulin and digestive enzymes
• None
• None
• None
• None
192 The Nervous System
Functions of the Nervous System
1. Detect changes and feel sensations.
2. Initiate responses to changes.
3. Organize and store information.
Nervous System Divisions
1. Central nervous system (CNS)—brain and spinal
2. Peripheral nervous system (PNS)—cranial nerves
and spinal nerves.
Nerve Tissue—neurons (nerve fibers) and
specialized cells (Schwann, neuroglia)
1. Neuron cell body contains the nucleus; cell bodies
are in the CNS or in the trunk and are protected by
2. Axon carries impulses away from the cell body;
dendrites carry impulses toward the cell body.
3. Schwann cells in PNS: Layers of cell membrane
form the myelin sheath to electrically insulate neurons;
nodes of Ranvier are spaces between adjacent
Schwann cells. Nuclei and cytoplasm of Schwann
cells form the neurolemma, which is essential for
regeneration of damaged axons or dendrites.
4. Oligodendrocytes in CNS form the myelin
sheaths; microglia phagocytize pathogens and
damaged cells; astrocytes contribute to the
blood–brain barrier (see Table 8–1).
5. Synapse—the space between the axon of one neuron
and the dendrites or cell body of the next neuron.
A neurotransmitter carries the impulse across
a synapse and is then destroyed by a chemical inactivator.
Synapses make impulse transmission one
way in the living person.
Types of Neurons—nerve fibers
1. Sensory—carry impulses from receptors to the
CNS; may be somatic (from skin, skeletal muscles,
joints) or visceral (from internal organs).
2. Motor—carry impulses from the CNS to effectors;
may be somatic (to skeletal muscle) or visceral (to
smooth muscle, cardiac muscle, or glands). Visceral
motor neurons make up the autonomic nervous
3. Interneurons—entirely within the CNS.
Nerves and Nerve Tracts
1. Sensory nerve—made only of sensory neurons.
2. Motor nerve—made only of motor neurons.
3. Mixed nerve—made of both sensory and motor
4. Nerve tract—a nerve within the CNS; also called
white matter.
The Nerve Impulse—see Table 8–2
1. Polarization—neuron membrane has a ( ) charge
outside and a ( ) charge inside.
2. Depolarization—entry of Na ions and reversal of
charges on either side of the membrane.
3. Impulse transmission is rapid, often several meters
per second.
• Saltatory conduction—in a myelinated neuron
only the nodes of Ranvier depolarize; increases
speed of impulses.
The Spinal Cord
1. Functions: transmits impulses to and from the
brain, and integrates the spinal cord reflexes.
2. Location: within the vertebral canal; extends from
the foramen magnum to the disc between the 1st
and 2nd lumbar vertebrae.
3. Cross-section: internal H-shaped gray matter contains
cell bodies of motor neurons and interneurons;
external white matter is the myelinated axons
and dendrites of interneurons.
4. Ascending tracts carry sensory impulses to the
brain; descending tracts carry motor impulses away
from the brain.
5. Central canal contains cerebrospinal fluid and is
continuous with the ventricles of the brain.
Spinal Nerves—see Table 8–3 for major
peripheral nerves
1. Eight cervical pairs to head, neck, shoulder, arm,
and diaphragm; 12 thoracic pairs to trunk; 5 lumbar
pairs and 5 sacral pairs to hip, pelvic cavity, and
leg; 1 very small coccygeal pair.
2. Cauda equina—the lumbar and sacral nerves that
extend below the end of the spinal cord.
3. Each spinal nerve has two roots: dorsal or sensory
root; dorsal root ganglion contains cell bodies of
sensory neurons; ventral or motor root; the two
roots unite to form a mixed spinal nerve.
Spinal Cord Reflexes—do not depend directly
on the brain
1. A reflex is an involuntary response to a stimulus.
2. Reflex arc—the pathway of nerve impulses during a
reflex: (1) receptors, (2) sensory neurons, (3) CNS
with one or more synapses, (4) motor neurons,
(5) effector that responds.
3. Stretch reflex—a muscle that is stretched will contract;
these reflexes help keep us upright against
gravity. The patellar reflex is also used clinically to
assess neurologic functioning, as are many other
reflexes (Fig. 8–5).
4. Flexor reflex—a painful stimulus will cause withdrawal
of the body part; these reflexes are protective.
The Brain—many parts that function as an
integrated whole; see Figs. 8–6 and 8–8 for
1. Ventricles—four cavities: two lateral, 3rd, 4th; each
contains a choroid plexus that forms cerebrospinal
fluid (Figs. 8–6 and 8–7).
2. Medulla—regulates the vital functions of heart
rate, breathing, and blood pressure; regulates
reflexes of coughing, sneezing, swallowing, and
3. Pons—contains respiratory centers that work with
those in the medulla.
4. Midbrain—contains centers for visual reflexes,
auditory reflexes, and righting (equilibrium)
5. Cerebellum—regulates coordination of voluntary
movement, muscle tone, stopping movements, and
equilibrium; contributes to sensations involving
texture and weight.
6. Hypothalamus—produces antidiuretic hormone
(ADH), which increases water reabsorption by the
kidneys; produces oxytocin, which promotes uterine
contractions for labor and delivery; produces
releasing hormones that regulate the secretions of
the anterior pituitary gland; regulates body temperature;
regulates food intake; integrates the functioning
of the autonomic nervous system (ANS);
promotes visceral responses to emotional situations;
acts as a biological clock that regulates body
7. Thalamus—groups sensory impulses as to body
part before relaying them to the cerebrum; awareness
of pain but inability to localize; suppresses
unimportant sensations to permit concentration;
contributes to alertness and awareness, and to
8. Cerebrum—two hemispheres connected by the
corpus callosum, which permits communication
between the hemispheres. The cerebral cortex is
the surface gray matter, which consists of cell bodies
of neurons and is folded extensively into convolutions.
The internal white matter consists of nerve
tracts that connect the lobes of the cerebrum to one
another and to other parts of the brain.
• Frontal lobes—motor areas initiate voluntary
movement; premotor area regulates sequences of
movements for learned skills; prefrontal area for
aspects of social behavior; Broca’s motor speech
area (left hemisphere) regulates the movements
involved in speech.
• Parietal lobes—general sensory area feels and
interprets the cutaneous senses and conscious
muscle sense; taste area extends into temporal
lobe, for sense of taste; speech areas (left hemisphere)
for thought before speech.
• Temporal lobes—auditory areas for hearing and
interpretation; olfactory areas for sense of smell
and interpretation; speech areas for thought
before speech.
• Occipital lobes—visual areas for vision; interpretation
areas for spatial relationships.
• Association areas—in all lobes, for abstract
thinking, reasoning, learning, memory, and
personality. The hippocampi are essential for
the formation of memories. Neural plasticity is
the ability of the brain to adapt to changing
• Basal ganglia—gray matter within the cerebral
hemispheres; regulate accessory movements and
muscle tone.
Meninges and Cerebrospinal Fluid (CSF) (see
Figs. 8–9 and 8–10)
1. Three meningeal layers made of connective tissue:
outer—dura mater; middle—arachnoid membrane;
inner—pia mater; all three enclose the brain and
spinal cord.
2. Subarachnoid space contains CSF, the tissue fluid
of the CNS.
3. CSF is formed continuously in the ventricles
of the brain by choroid plexuses, from blood
4. CSF circulates from the ventricles to the central
canal of the spinal cord and to the cranial and
spinal subarachnoid spaces.
5. CSF is reabsorbed from the cranial subarachnoid
space through arachnoid villi into the blood in the
cranial venous sinuses. The rate of reabsorption
equals the rate of production.
The Nervous System 193
6. As tissue fluid, CSF brings nutrients to CNS neurons
and removes waste products. CSF also acts as
a shock absorber to cushion the CNS.
Cranial Nerves—12 pairs of nerves that
emerge from the brain (see Fig. 8–11)
1. Concerned with vision, hearing and equilibrium,
taste and smell, and many other functions.
2. See Table 8–4 for the functions of each pair.
The Autonomic Nervous System (ANS) (see
Fig. 8–12 and Table 8–5)
1. Has two divisions: sympathetic and parasympathetic;
their functioning is integrated by the hypothalamus.
2. Consists of motor neurons to visceral effectors:
smooth muscle, cardiac muscle, and glands.
3. An ANS pathway consists of two neurons that
synapse in a ganglion:
• Preganglionic neurons—from the CNS to the
• Postganglionic neurons—from the ganglia to the
• Most sympathetic ganglia are in two chains
just outside the vertebral column; parasympathetic
ganglia are very near or in the visceral
4. Neurotransmitters: acetylcholine is released by
all preganglionic neurons and by parasympathetic
postganglionic neurons; the inactivator is
cholinesterase. Norepinephrine is released by most
sympathetic postganglionic neurons; the inactivator
is COMT or MAO.
5. Sympathetic division—dominates during stress situations;
responses prepare the body to meet physical
6. Parasympathetic division—dominates in relaxed
situations to permit normal functioning.
194 The Nervous System
1. Name the divisions of the nervous system and state
the parts of each. (p. 166)
2. State the function of the following parts of nerve
tissue: (pp. 166–167)
a. Axon
b. Dendrites
c. Myelin sheath
d. Neurolemma
e. Microglia
f. Astrocytes
3. Explain the difference between: (pp. 170–171)
a. Sensory neurons and motor neurons
b. Interneurons and nerve tracts
4. Describe an electrical nerve impulse in terms of
charges on either side of the neuron membrane.
Describe how a nerve impulse crosses a synapse.
(pp. 168–169, 171)
5. With respect to the spinal cord: (p. 172)
a. Describe its location
b. State what gray matter and white matter are
made of
c. State the function of the dorsal root, ventral
root, and dorsal root ganglion
6. State the names and number of pairs of spinal
nerves. State the part of the body supplied by
the phrenic nerves, radial nerves, and sciatic nerves.
(pp. 172, 174)
7. Define reflex, and name the five parts of a reflex
arc. (pp. 172, 174)
8. Define stretch reflexes, and explain their practical
importance. Define flexor reflexes, and explain
their practical importance. (p. 175)
9. Name the part of the brain concerned with each of
the following: (pp. 176–179)
a. Regulates body temperature
b. Regulates heart rate
c. Suppresses unimportant sensations
d. Regulates respiration (two parts)
e. Regulates food intake
f. Regulates coordination of voluntary movement
g. Regulates secretions of the anterior pituitary
h. Regulates coughing and sneezing
i. Regulates muscle tone
j. Regulates visual and auditory reflexes
k. Regulates blood pressure
10. Name the part of the cerebrum concerned with
each of the following: (pp. 179–183)
a. Feels the cutaneous sensations
b. Contains the auditory areas
c. Contains the visual areas
d. Connects the cerebral hemispheres
e. Regulates accessory movements
f. Contains the olfactory areas
g. Initiates voluntary movement
h. Contains the speech areas (for most people)
11. Name the three layers of the meninges, beginning
with the outermost. (p. 184)
12. State all the locations of cerebrospinal fluid. What
is CSF made from? Into what is CSF reabsorbed?
State the functions of CSF. (p. 184)
13. State a function of each of the following cranial
nerves: (p. 189)
a. Glossopharyngeal
b. Olfactory
c. Trigeminal
d. Facial
e. Vagus (three functions)
14. Explain how the sympathetic division of the ANS
helps the body adapt to a stress situation; give
three specific examples. (pp. 188–189)
15. Explain how the parasympathetic division of the
ANS promotes normal body functioning; give
three specific examples. (pp. 189, 191)
The Nervous System 195
1. Your friend Fred was telling a story, with eloquent
gestures, while making a salad. He missed the
tomato with the knife, cut his hand badly, and
needed quite a few stitches. A local anesthetic was
used. How might a local anesthetic stop nerve
impulses? (Remember that a nerve impulse is very
simple.) What part of Fred’s brain got him into
2. Some pesticides kill insects by interfering with
cholinesterase. We have cholinesterase too, and
may be adversely affected. What would be the
symptoms of such pesticide poisoning?
3. We cannot live without a central nervous system.
Describe all the ways in which the central nervous
system is protected.
4. Older drivers are sometimes said to have “lost their
reflexes.” Is this really true? Explain.
5. Look at Question Figure 8–A. Starting at the top
of column A, read the words down as fast as you
can. For column B, start at the top and name the
colors as fast as you can. Did you have any trouble?
Now column C: Start at the top and name the colors—
do not read the words—as fast as you can.
Was there any difference? Explain why.
Question Figure 8–A
Chapter Outline
Sensory Pathway
Characteristics of Sensations
Cutaneous Senses
Referred Pain
Muscle Sense
Sense of Taste
Sense of Smell
Hunger and Thirst
The Eye
Eyelids and the Lacrimal Apparatus
Layers of the eyeball
Cavities of the eyeball
Physiology of Vision
The Ear
Outer Ear
Middle Ear
Inner Ear
Utricle and saccule
Semicircular canals
Arterial Receptors
Aging and the Senses
Student Objectives
• Explain the general purpose of sensations.
• Name the parts of a sensory pathway, and state the
function of each.
• Describe the characteristics of sensations.
• Name the cutaneous senses, and explain their
• Explain referred pain and its importance.
• Explain the importance of muscle sense.
• Describe the pathways for the senses of taste and
smell, and explain how these senses are interrelated.
• Name the parts of the eye and their functions.
• Describe the physiology of vision.
• Name the parts of the ear and their functions.
• Describe the physiology of hearing.
• Describe the physiology of equilibrium.
• Explain the importance of the arterial pressoreceptors
and chemoreceptors.
The Senses
New Terminology
Adaptation (A-dap-TAY-shun)
After-image (AFF-ter-im-ije)
Aqueous humor (AY-kwee-us HYOO-mer)
Cochlea (KOK-lee-ah)
Cones (KOHNES)
Conjunctiva (KON-junk-TIGH-vah)
Contrast (KON-trast)
Cornea (KOR-nee-ah)
Eustachian tube (yoo-STAY-shee-un TOOB)
Iris (EYE-ris)
Lacrimal glands (LAK-ri-muhl)
Olfactory receptors (ohl-FAK-toh-ree)
Organ of Corti (KOR-tee)
Projection (proh-JEK-shun)
Referred pain (ree-FURRD PAYNE)
Retina (RET-i-nah)
Rhodopsin (roh-DOP-sin)
Rods (RAHDS)
Sclera (SKLER-ah)
Semicircular canals (SEM-ee-SIR-kyoo-lur)
Tympanic membrane (tim-PAN-ik)
Vitreous humor (VIT-ree-us HYOO-mer)
Related Clinical Terminology
Age-related macular degeneration (MAK-yoo-lar
Amblyopia (am-blee-OH-pee-uh)
Astigmatism (uh-STIG-mah-TIZM)
Cataract (KAT-uh-rakt)
Color blindness (KUHL-or BLIND-ness)
Conjunctivitis (kon-JUNK-ti-VIGH-tis)
Deafness (DEFF-ness)
Detached retina (dee-TACHD)
Glaucoma (glaw-KOH-mah)
Hyperopia (HIGH-per-OH-pee-ah)
Motion sickness (MOH-shun)
Myopia (my-OH-pee-ah)
Night blindness (NITE BLIND-ness)
Otitis media (oh-TIGH-tis MEE-dee-ah)
Phantom pain (FAN-tum)
Presbyopia (PREZ-bee-OH-pee-ah)
Strabismus (strah-BIZ-miss)
Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.
Our senses constantly provide us with information
about our surroundings: We see, hear, and touch. The
senses of taste and smell enable us to enjoy the flavor
of our food or warn us that food has spoiled and may
be dangerous to eat. Our sense of equilibrium keeps us
upright. We also get information from our senses
about what is happening inside the body. The pain of
a headache, for example, prompts us to do something
about it, such as take aspirin. In general, this is the
purpose of sensations: to enable the body to respond
appropriately to ever-changing situations and maintain
The impulses involved in sensations follow very precise
pathways, which all have the following parts:
1. Receptors—detect changes (stimuli) and generate
impulses. Receptors are usually very specific with
respect to the kinds of changes they respond to.
Those in the retina detect light rays, those in the
nasal cavities detect vapors, and so on. Once a specific
stimulus has affected receptors, however, they
all respond in the same way by generating electrical
nerve impulses.
2. Sensory neurons—transmit impulses from receptors
to the central nervous system. These sensory
neurons are found in both spinal nerves and cranial
nerves, but each carries impulses from only one
type of receptor.
3. Sensory tracts—white matter in the spinal cord or
brain that transmits the impulses to a specific part
of the brain.
4. Sensory areas—most are in the cerebral cortex.
These areas feel and interpret the sensations.
Learning to interpret sensations begins in infancy,
without our awareness of it, and continues throughout
Certain characteristics of sensations will help you
understand how the sensory areas work with information
from the receptors.
1. Projection—the sensation seems to come from the
area where the receptors were stimulated. If you
touch this book, the sensation of touch seems to be
in your hand but is actually being felt by your cerebral
cortex. That it is indeed the brain that feels
sensations is demonstrated by patients who feel
phantom pain after amputation of a limb. After
loss of a hand, for example, the person may still feel
that the hand is really there. Why does this happen?
The receptors in the hand are no longer present,
but the severed nerve endings continue to
generate impulses. These impulses arrive in the
parietal lobe area for the hand, and the brain does
what it has always done and creates the projection,
the feeling that the hand is still there. For most
amputees, phantom pain diminishes as the severed
nerves heal, but the person often experiences a
phantom “presence” of the missing part. This may
be helpful when learning to use an artificial limb.
2. Intensity—some sensations are felt more distinctly
and to a greater degree than are others. A weak
stimulus such as dim light will affect a small number
of receptors, but a stronger stimulus, such as
bright sunlight, will stimulate many more receptors.
When more receptors are stimulated, more
impulses will arrive in the sensory area of the brain.
The brain “counts” the impulses and projects a
more intense sensation.
3. Contrast—the effect of a previous or simultaneous
sensation on a current sensation, which may then
be exaggerated or diminished. Again, this is a function
of the brain, which constantly compares sensations.
If, on a very hot day, you jump into a
swimming pool, the water may feel quite cold at
first. The brain compares the new sensation to the
previous one, and since there is a significant difference
between the two, the water will seem colder
than it actually is.
4. Adaptation—becoming unaware of a continuing
stimulus. Receptors detect changes, but if the stimulus
continues it may not be much of a change, and
the receptors will generate fewer impulses. The
water in the swimming pool that seemed cold at
first seems to “warm up” after a few minutes. The
water has not changed temperature, and the receptors
for cold have no changes to detect, therefore
they generate fewer impulses. The sensation of
cold lessens, and we interpret or feel that as
increasing warmth. For another example, look at
your left wrist (or perhaps the right one). Many of
us wear a watch and are probably unaware of its
198 The Senses
presence on the arm most of the time. The cutaneous
receptors for touch or pressure adapt very
quickly to a continuing stimulus, and if there is no
change, there is nothing for the receptors to detect.
5. After-image—the sensation remains in the consciousness
even after the stimulus has stopped. A
familiar example is the bright after-image seen after
watching a flashbulb go off. The very bright light
strongly stimulates receptors in the retina, which
generate many impulses that are perceived as an
intense sensation that lasts longer than the actual
The dermis of the skin and the subcutaneous tissue
contain receptors for the sensations of touch, pressure,
heat, cold, and pain. The receptors for pain, heat, and
cold are free nerve endings, which also respond to
any intense stimulus. Intense pressure, for example,
may be felt as pain. The receptors for touch and pressure
are encapsulated nerve endings, meaning that
there is a cellular structure around the nerve ending
(Fig. 9–1).
The cutaneous senses provide us with information
about the external environment and also about
the skin itself. Much of the information about the
environment is not of great importance and is
processed at a subconscious level (suppressed by the
thalamus), though we can choose to be aware of it. For
example, could you distinguish a cotton T-shirt from
denim jeans by touch alone? Probably, but you might
not realize that you can do that until you try it by, say,
sorting laundry in the dark. If you were walking barefoot,
could you tell if you were walking on a carpet, a
The Senses 199
Free nerve
Merkel disc
(touch receptor)
Ruffini corpuscle
(pressure receptor) Meissner corpuscle
(touch receptor)
Pacinian corpuscle
(pressure receptor)
Free nerve endings
(pain receptor)
Figure 9–1. Cutaneous receptors in a section of the skin. Free nerve endings and encapsulated
nerve endings are shown.
QUESTION: In which layers are most of the cutaneous receptors located?
wood floor, concrete, or beach sand? Yes, you could.
But are we usually aware of the sensation from the
soles of our feet? If all is going well, probably not.
Some people with diabetes develop diabetic neuropathy,
damage to nerves that impairs sensation, and they
may say that a wood floor feels like walking on cotton
balls or that the buttons of a shirt feel too large or too
small. They are aware of such odd sensations simply
because the feelings are odd. For most of us, the touch
of the wood floor is not brought to awareness because
it is what the brain expects from past experience,
but if the floor has splinters or if the beach sand is hot,
we are certainly aware. This is information we can
bring to our conscious minds if necessary, but usually
do not.
As for the skin itself, if you have ever had poison ivy
or chickenpox, you may remember the itching sensation
of the rash. An itch is actually a mild pain sensation,
which may become real pain if not scratched.
Why does scratching help relieve some itches, besides
by removing an external irritant? One proposed
mechanism is that scratching is a bit more painful than
the itch, and the impulses it generates can distract the
brain from the impulses from the itch. Scratching will
not help relieve the itch of poison ivy, chickenpox, or
a mosquito bite, however, because the irritating chemicals
are in the skin, not on it. In such cases, scratching
may do more damage and worsen inflammation at
the site.
The sensory areas for the skin are in the parietal
lobes. You may recall from Chapter 5 that the sensitivity
of an area of skin is determined by the number
of receptors present. The number of receptors corresponds
to the size of the sensory area in the cerebral
cortex. The largest parts of this sensory cortex are for
the parts of the skin with the most receptors, that is,
the hands and face.
As mentioned previously, sensory areas are not
merely passive recipients of impulses. Consider the
sensation of wetness. It is a distinct sensation, but
there are no receptors for “wet” in the skin. Where
does the sensation come from? Where all sensation
comes from: the brain. The parietal lobes have learned
to associate the simultaneous reception of temperature
and pressure impulses with “wet.” You can demonstrate
this for yourself by putting on a plastic glove
and dunking your fingers in a cup of water. Your fingers
will feel wet, though they are perfectly dry inside
the glove. Wetness is a learned sensation, created by
the brain.
Free nerve endings are also found in internal organs.
The smooth muscle of the small intestine, for example,
has free nerve endings that are stimulated by
excessive stretching or contraction; the resulting pain
is called visceral pain. Sometimes pain that originates
in an internal organ may be felt in a cutaneous area;
this is called referred pain. The pain of a heart attack
(myocardial infarction) may be felt in the left arm and
shoulder, or the pain of gallstones may be felt in the
right shoulder.
This referred pain is actually a creation of the
brain. Within the spinal cord are sensory tracts
that are shared by cutaneous impulses and visceral
impulses. Cutaneous impulses are much more frequent,
and the brain correctly projects the sensation to
the skin. When the impulses come from an organ such
as the heart, however, the brain may still project the
sensation to the “usual” cutaneous area. The brain
projects sensation based on past experience, and cutaneous
pain is far more common than visceral pain.
Knowledge of referred pain, as in the examples mentioned
earlier, may often be helpful in diagnosis.
Muscle sense (also called proprioception or kinesthetic
sense) was discussed in Chapter 7 and will be
reviewed only briefly here. Stretch receptors (also
called proprioceptors or muscle spindles) detect
stretching of muscles and generate impulses, which
enable the brain to create a mental picture to know
where the muscles are and how they are positioned.
Conscious muscle sense is felt by the parietal lobes.
Unconscious muscle sense is used by the cerebellum
to coordinate voluntary movements. We do not have
to see our muscles to be sure that they are performing
their intended actions. Muscle sense also contributes
to our ability to distinguish the shape of objects.
The receptors for taste are found in taste buds, most
of which are in papillae on the tongue (Fig. 9–2).
These chemoreceptors detect chemicals in solution
in the mouth. The chemicals are foods and the solvent
is saliva (if the mouth is very dry, taste is very
200 The Senses
Figure 9–2. Structures concerned with the senses of smell and taste, shown in a midsagittal
section of the head.
QUESTION: If we sniff something pungent, why can we often taste it as well? (Follow the
inhaled air.)
indistinct). There are five (perhaps more) general
types of taste receptors: sweet, sour, salty, bitter, and
savory. Savory (also called umami or glutamate) is a
taste like grilled meat. We experience many more different
tastes, however, because foods are often complex
chemicals that stimulate different combinations
of receptors, and the sense of smell also contributes to
our perception of food.
Some taste preferences have been found to be
genetic. People with more than the average number of
taste buds find broccoli very bitter, whereas people
with fewer taste buds may like the taste.
The impulses from taste buds are transmitted by
the facial and glossopharyngeal (7th and 9th cranial)
nerves to the taste areas in the parietal-temporal cortex.
The sense of taste is important because it makes
eating enjoyable. Some medications may interfere
with the sense of taste, and this sense becomes less
acute as we get older. These may be contributing factors
to poor nutrition in certain patients and in the
The receptors for smell (olfaction) are chemoreceptors
that detect vaporized chemicals that have been
sniffed into the upper nasal cavities (see Fig. 9–2). Just
as there are specific taste receptors, there are also
specific scent receptors, and research indicates that
humans have several hundred different receptors.
When stimulated by vapor molecules, olfactory receptors
generate impulses carried by the olfactory
nerves (1st cranial) through the ethmoid bone to the
olfactory bulbs. The pathway for these impulses ends
in the olfactory areas of the temporal lobes. Vapors
may stimulate many combinations of receptors, and it
has been estimated that the human brain is capable of
distinguishing among 10,000 different scents.
That may seem impressive, but the human sense of
smell is very poorly developed compared to those
of other animals. Dogs, for example, have a sense of
smell about 2000 times more acute than that of people.
(It has been said that most people live in a world
of sights, whereas dogs live in a world of smells.) As
mentioned earlier, however, much of what we call taste
is actually the smell of food. If you have a cold and
your nasal cavities are stuffed up, food just doesn’t
taste as good as it usually does. Adaptation occurs relatively
quickly with odors. Pleasant scents may be
sharply distinct at first but rapidly seem to dissipate or
fade, and even unpleasant scents may fade with long
Hunger and thirst may be called visceral sensations,
in that they are triggered by internal changes. Hunger
is a sensation that seems to be far more complex than
was first thought, but thirst seems to be somewhat
simpler. The receptors for both senses are specialized
cells in the hypothalamus. Receptors for hunger are
believed to detect changes in blood nutrient levels, the
blood levels of hormones from the stomach and small
intestine, and a hormone released by adipose tissue; all
of these chemical signals are collected by the hypothalamus.
The receptors for thirst detect changes in
the body water content, which is actually the water-tosalt
Naturally we do not feel these sensations in the
hypothalamus: They are projected. Hunger is projected
to the stomach, which contracts. Thirst is projected
to the mouth and pharynx, and less saliva is
If not satisfied by eating, the sensation of hunger
gradually diminishes, that is, adaptation occurs. The
reason is that after blood nutrient levels decrease, they
become stable as fat in adipose tissue is used for
energy. With little or no digestive activity in the gastrointestinal
tract, secretion of hormones diminishes.
With no sharp fluctuations of the chemical signals, the
receptors in the hypothalamus have few changes to
detect, and hunger becomes much less intense.
In contrast, the sensation of thirst, if not satisfied by
drinking, continues to worsen. There is no adaptation.
As body water is lost, the amount keeps decreasing
and does not stabilize. Therefore, there are constant
changes for the receptors to detect, and prolonged
thirst may be painful.
The eye contains the receptors for vision and a
refracting system that focuses light rays on the receptors
in the retina. We will begin our discussion, however,
with the accessory structures of the eye, then
later return to the eye itself and the physiology of
202 The Senses
The eyelids contain skeletal muscle that enables the
eyelids to close and cover the front of the eyeball.
Eyelashes along the border of each eyelid help keep
dust out of the eyes. The eyelids are lined with a thin
membrane called the conjunctiva, which is also
folded over the white of the eye and merges with the
corneal epithelium. Inflammation of this membrane,
called conjunctivitis, may be caused by allergies or by
certain bacteria or viruses, and makes the eyes red,
itchy, and watery.
Tears are produced by the lacrimal glands,
located at the upper, outer corner of the eyeball,
within the orbit (Fig. 9–3). Secretion of tears occurs
constantly, but is increased by the presence of irritating
chemicals (onion vapors, for example) or dust,
and in certain emotional situations (sad or happy).
Small ducts take tears to the anterior of the eyeball,
and blinking spreads the tears and washes the surface
of the eye. Tears are mostly water, with about 1%
sodium chloride, similar to other body fluids. Tears
also contain lysozyme, an enzyme that inhibits the
growth of most bacteria on the wet, warm surface
of the eye. At the medial corner of the eyelids are
two small openings into the superior and inferior
lacrimal canals. These ducts take tears to the lacrimal
sac (in the lacrimal bone), which leads to the
nasolacrimal duct, which empties tears into the
nasal cavity. This is why crying often makes the nose
Most of the eyeball is within and protected by the
orbit, formed by the lacrimal, maxilla, zygomatic,
frontal, sphenoid, and ethmoid bones. The six extrinsic
muscles of the eye (Fig. 9–4) are attached to this
bony socket and to the surface of the eyeball. There
are four rectus (straight) muscles that move the eyeball
up and down or side to side; the name tells you which
direction. The medial rectus muscle, for example,
pulls the eyeball medially, as if to look at the nose. The
two oblique (slanted) muscles rotate the eye. The cranial
nerves that innervate these muscles are the oculomotor,
trochlear, and abducens (3rd, 4th, and 6th
cranial nerves, respectively). The very rapid and complex
coordination of these muscles in both eyes is, fortunately,
not something we have to think about. The
convergence of both eyes on an object is very important
to ensure a single image (that is, to prevent double
vision) and to give us depth perception and a
three-dimensional world.
The Senses 203
Lacrimal ducts
Lacrimal gland
Lacrimal canals
Lacrimal sac
Nasolacrimal duct
Nasal cavity
Figure 9–3. Lacrimal apparatus
shown in an anterior
view of the right eye.
QUESTION: Where do tears
usually end up?
Layers of the Eyeball
In its wall, the eyeball has three layers: the outer
sclera, middle choroid layer, and inner retina (Fig.
9–5). The sclera is the thickest layer and is made of
fibrous connective tissue that is visible as the white of
the eye. The most anterior portion is the cornea,
which differs from the rest of the sclera in that it is
transparent. The cornea has no capillaries, covers the
iris and pupil inside the eye, and is the first part of the
eye that refracts, or bends, light rays.
The choroid layer contains blood vessels and a
dark blue pigment (derived from melanin) that
absorbs light within the eyeball and thereby prevents
glare (just as does the black interior of a camera). The
anterior portion of the choroid is modified into more
specialized structures: the ciliary body and the iris.
The ciliary body (muscle) is a circular muscle that
surrounds the edge of the lens and is connected to the
lens by suspensory ligaments. The lens is made of a
transparent, elastic protein, and, like the cornea, has
no capillaries (see Box 9–1: Cataracts). The shape of
the lens is changed by the ciliary muscle, which
enables the eye to focus light from objects at varying
distances from the eye.
Just in front of the lens is the circular iris, the colored
part of the eye; its pigment is a form of melanin.
What we call “eye color” is the color of the iris and is
a genetic characteristic, just as skin color is. Two sets
of smooth muscle fibers in the iris change the diameter
of the pupil, the central opening. Contraction of
the radial fibers dilates the pupil; this is a sympathetic
response. Contraction of the circular fibers constricts
the pupil; this is a parasympathetic response (oculomotor
nerves). Pupillary constriction is a reflex that
protects the retina from intense light or that permits
more acute near vision, as when reading.
The retina lines the posterior two-thirds of the
eyeball and contains the visual receptors, the rods and
cones (Fig. 9–6). Rods detect only the presence of
light, whereas cones detect colors, which, as you may
know from physics, are the different wavelengths of
visible light. Rods are proportionally more abundant
toward the periphery, or edge, of the retina. Our best
vision in dim light or at night, for which we depend
on the rods, is at the sides of our visual fields. Cones
are most abundant in the center of the retina, especially
an area called the macula lutea directly behind
the center of the lens on what is called the visual axis.
The fovea, which contains only cones, is a small
depression in the macula and is the area for best color
An important cause of vision loss for people over
65 years of age is age-related macular degeneration
204 The Senses
Inferior rectus muscle
Superior rectus muscle
Optic nerve
Lateral rectus muscle
Inferior oblique muscle
Levator palpebrae
superioris muscle
Figure 9–4. Extrinsic muscles
of the eye. Lateral view of
left eye (the medial rectus
and superior oblique are not
QUESTION: Contraction of
the inferior rectus muscle will
have what effect on the eyeball?
(AMD), that is, loss of central vision, and some cases
seem to have a genetic component. In the dry form of
AMD, small fatty deposits impair circulation to the
macula, and cells die from lack of oxygen. In the wet
form of AMD, abnormal blood vessels begin leaking
into the retina, and cells in the macula die from the
damaging effects of blood outside its vessels. The
macula, the center of the visual field, is the part of
The Senses 205
Inferior rectus muscle
Canal of Schlemm
Suspensory ligament
Anterior cavity
Vitreous humor in
posterior cavity
Optic nerve
Optic disc
Retinal artery and vein
Fovea in macula lutea
Ciliary body (muscle)
Figure 9–5. Internal anatomy of the eyeball.
QUESTION: What is the function of the iris?
and blurry vision throughout the visual field is
the result. Small cataracts may be destroyed
by laser surgery. Artificial lenses are available, and
may be surgically implanted to replace an extensively
cloudy lens. The artificial lens is not
adjustable, however, and the person may require
glasses or contact lenses for vision at certain
The lens of the eye is normally transparent but may
become opaque; this cloudiness or opacity is called
a cataract. Cataract formation is most common
among elderly people. With age, the proteins of the
lens break down and lose their transparency. Longterm
exposure to ultraviolet light (sunlight) seems to
be a contributing factor, as is smoking.
The cloudy lens does not refract light properly,
the retina we use most: for reading, for driving, for
recognizing people, and for any kind of close work.
People of all ages should be aware of this condition
and that smoking and exposure to ultraviolet rays are
risk factors.
When light strikes the retina, the rods and cones
generate impulses. These impulses are carried by ganglion
neurons, which all converge at the optic disc
(see Figs. 9–5 and 9–6) and pass through the wall of
the eyeball as the optic nerve. There are no rods or
cones in the optic disc, so this part of the retina is
sometimes called the “blind spot.” We are not aware
of a blind spot in our field of vision, however, in part
because the eyes are constantly moving, and in part
because the brain “fills in” the blank spot to create a
“complete” picture.
206 The Senses
Pigment cells Choroid
Light waves
Optic nerve
Optic nerve
Figure 9–6. Microscopic structure of the retina in the area of the optic disc. See text for
QUESTION: Which type of neuron forms the optic nerve? Which cells are the photoreceptors?
Cavities of the Eyeball
There are two cavities within the eye: the posterior
cavity and the anterior cavity. The larger, posterior
cavity is found between the lens and retina and contains
vitreous humor (or vitreous body). This semisolid
substance keeps the retina in place. If the eyeball
is punctured and vitreous humor is lost, the retina may
fall away from the choroid; this is one possible cause
of a detached retina.
The anterior cavity is found between the back of
the cornea and the front of the lens, and contains
aqueous humor, the tissue fluid of the eyeball.
Aqueous humor is formed by capillaries in the ciliary
body, flows anteriorly through the pupil, and is reabsorbed
by the canal of Schlemm (small veins also
called the scleral venous sinus) at the junction of the
iris and cornea. Because aqueous humor is tissue fluid,
you would expect it to have a nourishing function, and
it does. Recall that the lens and cornea have no capillaries;
they are nourished by the continuous flow of
aqueous humor (see Box 9–2: Glaucoma).
For us to see, light rays must be focused on the retina,
and the resulting nerve impulses must be transmitted
to the visual areas of the cerebral cortex in the
Refraction of light rays is the deflection or bending
of a ray of light as it passes through one object and
into another object of greater or lesser density. The
refraction of light within the eye takes place in the following
pathway of structures: the cornea, aqueous
humor, lens, and vitreous humor. The lens is the only
adjustable part of the refraction system. When looking
at distant objects, the ciliary muscle is relaxed and the
lens is elongated and thin. When looking at near
objects, the ciliary muscle contracts to form a smaller
circle, the elastic lens recoils and bulges in the middle,
and has greater refractive power (see Box 9–3: Errors
of Refraction).
When light rays strike the retina, they stimulate
chemical reactions in the rods and cones. In rods, the
chemical rhodopsin breaks down to form scotopsin
and retinal (a derivative of vitamin A). This chemical
reaction generates an electrical impulse, and
rhodopsin is then resynthesized in a slower reaction.
Adaptation to darkness, such as going outside at night,
takes a little while because being in a well-lit area has
broken down most of the rhodopsin in the rods, and
resynthesis of rhodopsin is slow. The opposite situation,
perhaps being suddenly awakened by a bright
light, can seem almost painful. What happens is this:
In darkness the rods have resynthesized a full supply of
rhodopsin, and the sudden bright light breaks down
all the rhodopsin at the same time. The barrage of
The Senses 207
cannot easily integrate with the normal image of
the other eye. When both eyes are affected, the
person may not become aware of the gradual loss
of peripheral vision, because close work such as
reading does not require the edges of the visual
Glaucoma may often be controlled with medications
that constrict the pupil and flatten the iris,
thus opening up access to the canal of Schlemm. If
these or other medications are not effective, laser
surgery may be used to create a larger drainage
Anyone over the age of 40 should have a test for
glaucoma; anyone with a family history of glaucoma
should have this test annually, as should those
with diabetes or high blood pressure. If diagnosed
early, glaucoma is treatable, and blindness can usually
be prevented.
The presence of aqueous humor in the anterior cavity
of the eye creates a pressure called intraocular
pressure. An increase in this pressure is an important
risk factor for glaucoma, which is now defined
as a group of disorders that damage the optic nerve
and cause loss of vision. Other risk factors include
high blood pressure and diabetes. In the most
common form of glaucoma, aqueous humor is not
reabsorbed properly into the canal of Schlemm.
Increased pressure in the anterior cavity is transmitted
to the lens, the vitreous humor, and the retina
and optic nerve. As pressure on the retina increases,
halos may be seen around bright lights, and peripheral
vision is lost. Frequently, however, there are no
symptoms. A person with glaucoma may not notice
the shrinking visual field in one eye before vision
loss is far advanced. This happens because the brain
will suppress a faulty image from one eye that it
Normal visual acuity is referred to as 20/20; that is,
the eye should and does clearly see an object 20
feet away. Nearsightedness (myopia) means
that the eye sees near objects well but not distant
ones. If an eye has 20/80 vision, this means that
what the normal eye can see at 80 feet, the nearsighted
eye can see only if the object is brought to
20 feet away. The nearsighted eye focuses images
from distant objects in front of the retina, because
the eyeball is too long or the lens too thick. These
structural characteristics of the eye are hereditary.
Correction requires a concave lens to spread out
light rays before they strike the eye.
Farsightedness (hyperopia) means that the
eye sees distant objects well. Such an eye may have
an acuity of 20/10, that is, it sees at 20 feet what the
normal eye can see only at 10 feet. The farsighted
eye focuses light from near objects “behind” the
retina, because the eyeball is too short or the lens
too thin. Correction requires a convex lens to converge
light rays before they strike the eye.
As we get older, most of us will become more farsighted
(presbyopia). As the aging lens loses its
elasticity, it is not as able to recoil and thicken for near
vision, and glasses for reading are often necessary.
Astigmatism is another error of refraction,
caused by an irregular curvature of the cornea or
lens that scatters light rays and blurs the image on
the retina. Correction requires a lens ground specifically
for the curvature of the individual eye.
Normal eye
Box Figure 9–A Errors of refraction compared to normal eye. Corrective lenses are shown for nearsightedness
and farsightedness.
impulses generated is very intense, and the brain may
interpret any intense sensation as pain. A few minutes
later the bright light seems fine because the rods are
recycling their rhodopsin slowly, and it is not breaking
down all at once.
Chemical reactions in the cones, also involving
retinal, are brought about by different wavelengths of
light. It is believed that there are three types of cones:
red-absorbing, blue-absorbing, and green-absorbing
cones. Each type absorbs wavelengths over about a
third of the visible light spectrum, so red cones,
for example, absorb light of the red, orange, and
yellow wavelengths. The chemical reactions in cones
also generate electrical impulses (see Box 9–4: Night
Blindness and Color Blindness).
The impulses from the rods and cones are transmitted
to ganglion neurons (see Fig. 9–6); these converge
at the optic disc and become the optic nerve,
which passes posteriorly through the wall of the eyeball.
Ganglion neurons also seem to have a photoreceptor
chemical (called melanopsin) that may
contribute to the daily resetting of our biological
The optic nerves from both eyes come together at
the optic chiasma (or chiasm), just in front of the
pituitary gland (see Fig. 8–11). Here, the medial fibers
of each optic nerve cross to the other side. This crossing
permits each visual area to receive impulses from
both eyes, which is important for binocular vision.
The visual areas are in the occipital lobes of the
cerebral cortex. Although each eye transmits a slightly
different picture (look straight ahead and close one
eye at a time to see the difference between the two pictures),
the visual areas put them together, or integrate
them, to make a single image that has depth and three
dimensions. This is called binocular vision. The
visual areas also right the image, because the image on
the retina is upside down. The image on film in a camera
is also upside down, but we don’t even realize that
because we look at the pictures right side up. The
brain just as automatically ensures that we see our
world right side up.
Also for near vision, the pupils constrict to block
out peripheral light rays that would otherwise blur the
image, and the eyes converge even further to keep the
images on the corresponding parts of both retinas.
The Senses 209
color blindness on his X chromosome has no gene
at all for color vision on his Y chromosome and will
be color blind.
Night blindness, the inability to see well in dim
light or at night, is usually caused by a deficiency of
vitamin A, although some night blindness may
occur with aging. Vitamin A is necessary for the synthesis
of rhodopsin in the rods. Without sufficient
vitamin A, there is not enough rhodopsin present to
respond to low levels of light.
Color blindness is a genetic disorder in which
one of the three sets of cones is lacking or nonfunctional.
Total color blindness, the inability to see
any colors at all, is very rare. The most common
form is red-green color blindness, which is the
inability to distinguish between these colors. If
either the red cones or green cones are nonfunctional,
the person will still see most colors, but will
not have the contrast that the non-working set of
cones would provide. So red and green shades will
look somewhat similar, without the definite difference
most of us see. This is a sex-linked trait; the
recessive gene is on the X chromosome. A woman
with one gene for color blindness and a gene for
normal color vision on her other X chromosome will
not be color blind but may pass the gene for color
blindness to her children. A man with a gene for
Box Figure 9–B Example of color patterns used to
detect color blindness.
The importance of pupil constriction can be demonstrated
by looking at this page through a pinhole in a
piece of paper. You will be able to read with the page
much closer to your eye because the paper blocks out
light from the sides.
The importance of convergence can be demonstrated
by looking at your finger placed on the tip of
your nose. You can feel your eyes move medially
(“cross”) in maximum convergence. If the eyes don’t
converge, the result is double vision; the brain cannot
make the very different images into one, and settles
for two. This is temporary, however, because the brain
does not like seeing double and will eventually suppress
one image.
You have probably heard of the condition called
“lazy eye” (the formal name is strabismus), in which
a person’s eyes (the visual axis of each) cannot be
directed at precisely the same point. True convergence
is not possible and, if untreated, the brain simply will
not use the lazy eye image. That eye may stop focusing
and become functionally blind because the brain is
ignoring the nerve impulses from it. Such loss of
vision is called amblyopia. Correction of a lazy eye
may involve eye exercises (to make the lazy eye
straighten out), a patch over the good eye (to make the
lazy eye straighten out and take over), or surgery to
correct an imbalance of the extrinsic muscles. You can
show yourself the benefits of converging eyes the next
time you are a passenger in a car (not the driver). As
the car is moving, close one eye. Does the oncoming
landscape seem to flatten out, lose dimension? This is
loss of depth perception and some of the three dimensionality
that our brains provide.
The ear consists of three areas: the outer ear, the middle
ear, and the inner ear (Fig. 9–7). The ear contains
the receptors for two senses: hearing and equilibrium.
These receptors are all found in the inner ear.
The outer ear consists of the auricle and the ear
canal. The auricle, or pinna, is made of cartilage covered
with skin. For animals such as dogs, whose ears
are movable, the auricle may act as a funnel for sound
waves. For people, however, the flat and stationary
auricle is not important. Hearing would not be negatively
affected without it, although those of us who
wear glasses would have our vision impaired without
our auricles. The ear canal is lined with skin that contains
ceruminous glands. It may also be called the
external auditory meatus, and is a tunnel into the
temporal bone, curving slightly forward and down.
The middle ear is an air-filled cavity in the temporal
bone. The eardrum, or tympanic membrane, is
stretched across the end of the ear canal and vibrates
when sound waves strike it. These vibrations are
transmitted to the three auditory bones: the malleus,
incus, and stapes (see Fig. 9–7). The stapes then
transmits vibrations to the fluid-filled inner ear at the
oval window.
The eustachian tube (auditory tube) extends from
the middle ear to the nasopharynx and permits air to
enter or leave the middle ear cavity. The air pressure
in the middle ear must be the same as the external
atmospheric pressure in order for the eardrum to
vibrate properly. You may have noticed your ears
“popping” when in an airplane or when driving to a
higher or lower altitude. Swallowing or yawning creates
the “pop” by opening the eustachian tubes and
equalizing the air pressures.
The eustachian tubes of children are short and
nearly horizontal and may permit bacteria to spread
from the pharynx to the middle ear. This is why otitis
media may be a complication of a strep throat.
Within the temporal bone, the inner ear is a cavity
called the bony labyrinth (a labyrinth is a series of
interconnecting paths or tunnels, somewhat like a
maze but without dead ends; see Fig. 9–7), which is
lined with membrane called the membranous
labyrinth. Perilymph is the fluid found between bone
and membrane, and endolymph is the fluid within the
membranous structures of the inner ear. These structures
are the cochlea, concerned with hearing, and the
utricle, saccule, and semicircular canals, all concerned
with equilibrium (Fig. 9–8).
The cochlea is shaped like a snail shell with two-anda-
half structural turns. Internally, the cochlea is partitioned
into three fluid-filled canals. The medial canal
210 The Senses
Stapes Vestibular branch
Temporal bone Semicircular canals
External auditory meatus
Cochlear branch
Eustachian tube
Acoustic or
cranial nerve
Malleus Cochlea
Figure 9–7. (A) Outer, middle, and inner ear structures as shown in a frontal section
through the right temporal bone. (B) Section of temporal bone with the auditory bones.
(C) Section of temporal bone showing bony labyrinth of inner ear. The colors on the bone
are artificial. (Photographs by Dan Kaufman.)
QUESTION: What structure is first to vibrate when sound waves enter the ear canal? What
is second?
is the cochlear duct, the floor of which is the basilar
membrane that supports the receptors for hearing in
the organ of Corti (spiral organ). The receptors are
called hair cells (their projections are not “hair,” of
course, but rather are specialized microvilli called
stereocilia), which contain endings of the cochlear
branch of the 8th cranial nerve. Overhanging the hair
cells is the tectorial membrane (Fig. 9–9).
Very simply, the process of hearing involves the
transmission of vibrations and the generation of nerve
impulses. When sound waves enter the ear canal,
vibrations are transmitted by the following sequence
of structures: eardrum, malleus, incus, stapes, oval
window of the inner ear, and perilymph and
endolymph within the cochlea. Imagine the vibrations
in the fluids as ripples or waves. The basilar membrane
ripples and pushes the hair cells of the organ of
Corti against the tectorial membrane. When the hair
cells bend, they generate impulses that are carried by
the 8th cranial nerve to the brain. As you may recall,
the auditory areas are in the temporal lobes of the
cerebral cortex. It is here that sounds are heard and
interpreted (see Box 9–5: Deafness).
The auditory areas also enable us to determine
from which direction a sound is coming. Simply
stated, the auditory areas count and compare the number
of impulses coming from each inner ear. For
example, if more impulses arrive from the left cochlea
than from the right one, the sound will be projected to
the left. If the source of a sound is directly above your
head, the sound may seem to come from all directions,
because each auditory area is receiving approximately
the same number of impulses and cannot project the
sensation to one side or the other.
The final structure in the hearing pathway is the
round window (see Fig. 9–8). The membrane-covered
212 The Senses
Semicircular canals
Vestibular nerve
Cochlear nerve
Scala tympani
Cochlear duct
Scala vestibuli
Round window
Oval window
Figure 9–8. Inner ear structures. The arrows show the transmission of vibrations during
QUESTION: What is the function of the round window?
The Senses 213
Cochlear duct filled with endolymph
Vestibular membrane
Vestibular canal (from the oval window)
Supporting cells
Hair cells
Basilar membrane
Tympanic canal
(to the round window)
Semicircular canals
Oval window
Round window
Nerve fibers of
8th cranial nerve
Tectorial membrane
Figure 9–9. Organ of Corti. (A) Inner ear structures. (B) Magnification of organ of Corti
within the cochlea.
QUESTION: What do the canals and cochlear duct contain (air or fluid)? What are the
receptors for hearing?
round window, just below the oval window, is important
to relieve pressure. When the stapes pushes in the
fluid at the oval window, the round window bulges
out, which prevents damage to the hair cells.
Utricle and Saccule
The utricle and saccule are membranous sacs in an
area called the vestibule, between the cochlea and
semicircular canals. Within the utricle and saccule are
hair cells embedded in a gelatinous membrane with
tiny crystals of calcium carbonate called otoliths.
Gravity pulls on the otoliths and bends the hair cells
as the position of the head changes (Fig. 9–10). The
impulses generated by these hair cells are carried by
the vestibular portion of the 8th cranial nerve to the
cerebellum, the midbrain, and the temporal lobes of
the cerebrum.
The cerebellum and midbrain use this information
to maintain equilibrium at a subconscious level. We
can, of course, be aware of the position of the head,
and it is the cerebrum that provides awareness.
214 The Senses
measles). Deterioration of the hair cells in the
cochlea is a natural consequence of aging, and the
acuity of hearing diminishes as we get older. For
example, it may be more difficult for an elderly person
to distinguish conversation from background
noise. Chronic exposure to loud noise accelerates
degeneration of the hair cells and onset of this type
of deafness. Listening to music by way of earphones
is also believed to increase the risk of this type of
Central deafness—damage to the auditory areas
in the temporal lobes. This type of deafness is rare
but may be caused by a brain tumor, meningitis, or
a cerebrovascular accident in the temporal lobe.
Deafness is the inability to hear properly; the types
are classified according to the part of the hearing
process that is not functioning normally:
Conduction deafness—impairment of one of the
structures that transmits vibrations. Examples of this
type are a punctured eardrum, arthritis of the auditory
bones, or a middle ear infection in which fluid
fills the middle ear cavity.
Nerve deafness—impairment of the 8th cranial
nerve or the receptors for hearing in the cochlea.
The 8th cranial nerve may be damaged by some
antibiotics used to treat bacterial infections. Nerve
deafness is a rare complication of some viral infections
such as mumps or congenital rubella (German
Central deafness
Nerve deafness
Conduction deafness
Box Figure 9–C Types of deafness.
Semicircular Canals
The three semicircular canals are fluid-filled membranous
ovals oriented in three different planes. At the
base of each is an enlarged portion called the ampulla
(see Fig. 9–8), which contains hair cells (the crista) that
are affected by movement. As the body moves forward,
for example, the hair cells are bent backward
at first and then straighten (see Fig. 9–10). The bending
of the hair cells generates impulses carried by the
vestibular branch of the 8th cranial nerve to the
cerebellum, midbrain, and temporal lobes of the cerebrum.
These impulses are interpreted as starting or
stopping, and accelerating or decelerating, or changing
direction, and this information is used to maintain
equilibrium while we are moving (see Box 9–6: Motion
In summary then, the utricle and saccule provide
information about the position of the body at rest,
while the semicircular canals provide information
about the body in motion. Of course, there is some
overlap, and the brain puts all the information together
to create a single sense of body position.
The Senses 215
Hair cells
Head tilted
8th cranial
Head upright
8th cranial
At rest
At rest
Figure 9–10. Physiology of
equilibrium. (A) Utricle and
saccule. (B) Semicircular canals.
See text for description.
QUESTION: In part A, what
causes the hair cells to bend? In
part B, what causes the hair
cells to sway?
The aorta and carotid arteries contain receptors that
detect changes in the blood. The aortic arch, which
receives blood pumped by the left ventricle of the
heart, curves over the top of the heart. The left and
right carotid arteries are branches of the aortic arch
that take blood through the neck on the way to the
brain. In each of these vessels are pressoreceptors and
chemoreceptors (see Fig. 12–7).
Pressoreceptors in the carotid sinuses and aortic
sinus detect changes in blood pressure. Chemoreceptors
in the carotid bodies and the aortic body detect
changes in the oxygen and carbon dioxide content and
the pH of blood. The impulses generated by these
receptors do not give rise to sensations that we feel but
rather are information used to make any necessary
changes in respiration or circulation. We will return
to this in later chapters, so one example will suffice
for now.
If the blood level of oxygen decreases significantly,
this change (hypoxia) is detected by carotid and aortic
chemoreceptors. The sensory impulses are carried by
the glossopharyngeal (9th cranial) and vagus (10th
cranial) nerves to the medulla. Centers in the medulla
may then increase the respiratory rate and the heart
rate to obtain and circulate more oxygen. These are
the respiratory and cardiac reflexes that were mentioned
in Chapter 8 as functions of the glossopharyngeal
and vagus nerves. The importance of these
reflexes is readily apparent: to maintain normal blood
levels of oxygen and carbon dioxide and to maintain
normal blood pressure.
All of the senses may be diminished in old age. In the
eye, cataracts may make the lens opaque. The lens
also loses its elasticity and the eye becomes more
farsighted, a condition called presbyopia. The risk
of glaucoma increases, and elderly people should be
tested for it because treatment is available that can
prevent blindness. Macular degeneration, in which
central vision becomes impaired first, is a major cause
of vision loss for people over 65. Reading and close
work of any kind become difficult.
In the ear, cumulative damage to the hair cells in
the organ of Corti usually becomes apparent some
time after the age of 60. Hair cells that have been
damaged in a lifetime of noise cannot be replaced
(regrowth of cochlear hair cells has been stimulated in
guinea pigs, but not yet in people). The deafness of
old age ranges from slight to profound; very often
high-pitched sounds are lost first, while hearing
may still be adequate for low-pitched sounds. The
sense of equilibrium may be diminished; the body is
slower to react to tilting, and falls may become more
Both taste and smell become less acute with age,
which may contribute to poor nutrition in elderly
Changes take place all around us as well as within
us. If the body could not respond appropriately to
environmental and internal changes, homeostasis
would soon be disrupted, resulting in injury, illness, or
even death. To respond appropriately to changes, the
brain must know what they are. Conveying this information
to our brains is the function of our senses.
Although we may sometimes take our senses for
granted, we could not survive for very long without
You have just read about the great variety of internal
and external changes that are detected by the sense
organs. You are also familiar with the role of the nervous
system in the regulation of the body’s responses.
In the next chapter we will discuss the other regulatory
system, the endocrine system. The hormones
of the endocrine glands are produced in response to
changes, and their regulatory effects all contribute to
216 The Senses
Motion sickness is characterized by cold sweats,
hyperventilation, nausea, and vomiting when the
person is exposed to repetitive motion that is
unexpected or unfamiliar, or that cannot be controlled.
Seasickness is a type of motion sickness,
as is carsickness (why children are carsick more
often than adults is not known).
Some people are simply not affected by the
rolling of a ship or train; for others, the constant
stimulation of the receptors for position first
becomes uncomfortable, then nauseating. For
those who know they are susceptible to motion
sickness, medications are available for use before
traveling by plane, train, boat, or car.
Purpose of Sensations—to detect changes in
the external or internal environment to
enable the body to respond appropriately
to maintain homeostasis
Sensory Pathway—pathway of impulses for a
1. Receptors—detect a change (usually very specific)
and generate impulses.
2. Sensory neurons—transmit impulses from receptors
to the CNS.
3. Sensory tracts—white matter in the CNS.
4. Sensory area—most are in the cerebral cortex; feels
and interprets the sensation.
Characteristics of Sensations
1. Projection—the sensation seems to come from the
area where the receptors were stimulated, even
though it is the brain that truly feels the sensation.
2. Intensity—the degree to which a sensation is felt; a
strong stimulus affects more receptors, more
impulses are sent to the brain and are interpreted as
a more intense sensation.
3. Contrast—the effect of a previous or simultaneous
sensation on a current sensation as the brain compares
4. Adaptation—becoming unaware of a continuing
stimulus; if the stimulus remains constant, there is
no change for receptors to detect.
5. After-image—the sensation remains in the consciousness
after the stimulus has stopped.
Cutaneous Senses—provide information
about the external environment and the
skin itself
1. The dermis has free nerve endings that are receptors
for pain, heat, and cold, and encapsulated nerve
endings that are receptors for touch and pressure
(see Fig. 9–1).
2. Sensory areas are in parietal lobes.
3. Referred pain is visceral pain that is felt as cutaneous
pain. Common pathways in the CNS carry
both cutaneous and visceral impulses; the brain
usually projects sensation to the cutaneous area.
Muscle Sense—knowing where our muscles
are without looking at them
1. Stretch receptors in muscles detect stretching.
2. Sensory areas for conscious muscle sense are in
parietal lobes.
3. Cerebellum uses unconscious muscle sense to
coordinate voluntary movement.
Sense of Taste (see Fig. 9–2)
1. Chemoreceptors are in taste buds on the tongue;
detect chemicals (foods) in solution (saliva) in the
2. Five basic tastes: sweet, sour, salty, bitter, and
savory; foods stimulate combinations of receptors.
3. Pathway: facial and glossopharyngeal nerves to
taste areas in parietal-temporal lobes.
Sense of Smell (see Fig. 9–2)
1. Chemoreceptors are in upper nasal cavities; several
hundred different ones; detect vaporized chemicals
(many combinations possible).
2. Pathway: olfactory nerves to olfactory bulbs to
olfactory areas in the temporal lobes.
3. Smell contributes greatly to what we call taste.
Hunger and Thirst—visceral (internal)
1. Receptors for hunger: in hypothalamus, detect
changes in GI hormones and nutrient levels in the
blood; hunger is projected to the stomach; adaptation
does occur.
2. Receptors for thirst: in hypothalamus, osmoreceptors
detect changes in body water (water–salt proportions);
thirst is projected to the mouth and
pharynx; adaptation does not occur.
The Eye (see Figs. 9–3 through 9–6)
1. Eyelids and eyelashes keep dust out of eyes; conjunctivae
line the eyelids and cover white of eye.
2. Lacrimal glands produce tears, which flow across
the eyeball to two lacrimal ducts, to lacrimal sac to
nasolacrimal duct to nasal cavity. Tears wash the
anterior eyeball and contain lysozyme to inhibit
bacterial growth.
3. The eyeball is protected by the bony orbit (socket).
4. The six extrinsic muscles move the eyeball; innervated
by the 3rd, 4th, and 6th cranial nerves.
5. Sclera—outermost layer of the eyeball, made of
fibrous connective tissue; anterior portion is the
transparent cornea, the first light-refracting structure.
6. Choroid layer—middle layer of eyeball; dark blue
pigment absorbs light to prevent glare within the
The Senses 217
7. Ciliary body (muscle) and suspensory ligaments—
change shape of lens, which is made of a transparent,
elastic protein and which refracts light.
8. Iris—two sets of smooth muscle fibers regulate
diameter of pupil, that is, how much light strikes
the retina.
9. Retina—innermost layer of eyeball; contains rods
and cones.
• Rods—detect light; abundant toward periphery
of retina.
• Cones—detect color; abundant in center of
• Fovea—in the center of the macula lutea; contains
only cones; area of best color vision.
• Optic disc—no rods or cones; optic nerve
passes through eyeball.
10. Posterior cavity contains vitreous humor (semisolid)
that keeps the retina in place.
11. Anterior cavity contains aqueous humor that
nourishes the lens and cornea; made by capillaries
of the ciliary body, flows through pupil, is reabsorbed
to blood at the canal of Schlemm.
Physiology of Vision
1. Refraction (bending and focusing) pathway of
light: cornea, aqueous humor, lens, vitreous
2. Lens is adjustable; ciliary muscle relaxes for distant
vision, and lens is thin. Ciliary muscle contracts for
near vision, and elastic lens thickens and has
greater refractive power.
3. Light strikes retina and stimulates chemical reactions
in the rods and cones.
4. In rods: rhodopsin breaks down to scotopsin and
retinal (from vitamin A), and an electrical impulse
is generated. In cones: specific wavelengths of light
are absorbed (red, blue, green); chemical reactions
generate nerve impulses.
5. Ganglion neurons from the rods and cones form
the optic nerve, which passes through the eyeball at
the optic disc.
6. Optic chiasma—site of the crossover of medial
fibers of both optic nerves, permitting binocular
7. Visual areas in occipital lobes—each area receives
impulses from both eyes; both areas create one
image from the two slightly different images of
each eye; both areas right the upside-down retinal
The Ear (see Figs. 9–7 through 9–10)
1. Outer ear—auricle or pinna has no real function
for people; ear canal curves forward and down into
temporal bone.
2. Middle ear—eardrum at end of ear canal vibrates
when sound waves strike it. Auditory bones:
malleus, incus, stapes; transmit vibrations to inner
ear at oval window.
• Eustachian tube—extends from middle ear to
nasopharynx; allows air in and out of middle
ear to permit eardrum to vibrate; air pressure
in middle ear should equal atmospheric pressure.
3. Inner ear—bony labyrinth in temporal bone, lined
with membranous labyrinth. Perilymph is fluid
between bone and membrane; endolymph is fluid
within membrane. Membranous structures are the
cochlea, utricle and saccule, and semicircular
4. Cochlea—snail-shell shaped; three internal canals;
cochlear duct contains receptors for hearing: hair
cells in the organ of Corti; these cells contain
endings of the cochlear branch of the 8th cranial
5. Physiology of hearing—sound waves stimulate
vibration of eardrum, malleus, incus, stapes, oval
window of inner ear, perilymph and endolymph of
cochlea, and hair cells of organ of Corti. When hair
cells bend, impulses are generated and carried by
the 8th cranial nerve to the auditory areas in the
temporal lobes. Round window prevents pressure
damage to the hair cells.
6. Utricle and saccule—membranous sacs in the
vestibule; each contains hair cells that are affected
by gravity. When position of the head changes,
otoliths bend the hair cells, which generate
impulses along the vestibular branch of the 8th cranial
nerve to the cerebellum, midbrain, and cerebrum.
Impulses are interpreted as position of the
head at rest.
7. Semicircular canals—three membranous ovals in
three planes; enlarged base is the ampulla, which
contains hair cells (crista) that are affected by
movement. As body moves, hair cells bend in opposite
direction, generate impulses along vestibular
branch of 8th cranial nerve to cerebellum, midbrain,
and cerebrum. Impulses are interpreted as
movement of the body, changing speed, stopping
or starting.
218 The Senses
Arterial Receptors—in large arteries; detect
changes in blood
1. Aortic arch—curves over top of heart. Aortic sinus
contains pressoreceptors; aortic body contains
chemoreceptors; sensory nerve is vagus (10th
2. Right and left carotid arteries in the neck; carotid
sinus contains pressoreceptors; carotid body contains
chemoreceptors; sensory nerve is the glossopharyngeal
(9th cranial).
3. Pressoreceptors detect changes in blood pressure;
chemoreceptors detect changes in pH or oxygen
and CO2 levels in the blood. This information is
used by the vital centers in the medulla to change
respiration or circulation to maintain normal blood
oxygen and CO2 and normal blood pressure.
The Senses 219
1. State the two general functions of receptors.
Explain the purpose of sensory neurons and sensory
tracts. (p. 198)
2. Name the receptors for the cutaneous senses, and
explain the importance of this information.
(p. 199)
3. Name the receptors for muscle sense and the parts
of the brain concerned with muscle sense.
(p. 200)
4. State what the chemoreceptors for taste and smell
detect. Name the cranial nerve(s) for each of these
senses and the lobe of the cerebrum where each is
felt. (pp. 200, 202)
5. Name the part of the eye with each of the following
functions: (pp. 203–206)
a. Change the shape of the lens
b. Contains the rods and cones
c. Forms the white of the eye
d. Form the optic nerve
e. Keep dust out of eye
f. Changes the size of the pupil
g. Produce tears
h. Absorbs light within the eyeball to prevent glare
6. With respect to vision: (pp. 207, 209)
a. Name the structures and substances that refract
light rays (in order)
b. State what cones detect and what rods detect.
What happens within these receptors when light
strikes them?
c. Name the cranial nerve for vision and the lobe
of the cerebrum that contains the visual area
7. With respect to the ear: (pp. 210–215)
a. Name the parts of the ear that transmit the
vibrations of sound waves (in order)
b. State the location of the receptors for hearing
c. State the location of the receptors that respond
to gravity
d. State the location of the receptors that respond
to motion
e. State the two functions of the 8th cranial nerve
f. Name the lobe of the cerebrum concerned with
g. Name the two parts of the brain concerned with
maintaining balance and equilibrium
8. Name the following: (p. 216)
a. The locations of arterial chemoreceptors, and
state what they detect
b. The locations of arterial pressoreceptors, and
state what they detect
c. The cranial nerves involved in respiratory and
cardiac reflexes, and state the part of the brain
that regulates these vital functions
9. Explain each of the following: adaptation, afterimage,
projection, contrast. (pp. 198–199)
1. Why are the inner ear labyrinths filled with fluid
rather than air? One reason is directly concerned
with hearing, the other with survival.
2. Michael’s summer job in his town was collecting
garbage. At first he thought the garbage and the
truck smelled awful, but by the time he went home
for lunch he decided that he didn’t mind it at all.
Explain what happened, and why his mother did
3. When we are out in very cold weather, why don’t
our eyes freeze shut? Try to think of two reasons.
4. You probably have at one time hit your “funny
bone,” which is really the ulnar nerve where it
crosses the elbow. Such a whack is very painful, and
not just in the elbow but all the way down the forearm
to the ring and little fingers of the hand. This
is referred pain. Explain why it happens, and name
the characteristic of sensations that it illustrates.
5. Albinism is a genetic characteristic in which
melanin is not produced; it may occur in just
about any type of animal. As you probably know,
an albino person will have white skin and hair.
Describe the consequences for the person’s
6. We sometimes hear that blind people have a better
sense of hearing than do sighted people. Do you
think this is really true? Explain. Name two other
senses a blind person may especially depend upon.
7. Look at Question Figure 9–A. In part A, which rectangle
seems wider, the upper one or the lower
one? Measure them, and explain your answer. Part
B shows a Necker cube. Look at the cube and let
your eyes relax. What seems to happen? Why do
you think this happens? Part C has some lines and
some shaded blocks. But what do we see? Explain.
8. Look at Question Figure 9–B. Part A shows a normal
visual field. Parts B, C, and D are the visual
fields of eye disorders you have read about in this
chapter. Try to name each, with a reason for your
220 The Senses
A Normal
Question Figure 9–B

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