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Transcript
C H A P T E R
3
Module 7
Neural and Hormonal
Systems
Module 8
The Brain
M
The Biological Bases
of Behavior
Perhaps you’re wondering what this chapter is doing here. After all, you signed up for
a course in psychology, not biology! In the next two modules, we’ll be covering
material that looks suspiciously as though it belongs in a biology textbook. What’s
going on?
Think of it this way. If your biological being suddenly disappeared, there would be
nothing left. Without a body, there could be no behavior, and without a brain, there
could be no mental processes. You couldn’t play a sport or a musical instrument. You
couldn’t enjoy the taste of a ripe melon or a freshly baked chocolate chip cookie. You
couldn’t solve a problem or fantasize about the upcoming weekend. You could neither
laugh at a joke (a behavior) nor understand the humor behind it (a mental process).
You couldn’t feel anxiety about an upcoming test or fall in love. In a nutshell, if biology
disappeared, so would the stuff of psychology.
It’s possible to study behavior and mental processes from a number of perspectives, including the cognitive perspective, the behavioral perspective, and the socialcultural perspective, and we will do that in other chapters of this book. But now it’s
biology’s turn for the spotlight, and you may be surprised at the insight it provides.
O
D
U
L
E
7
Neural and Hormonal Systems
Neurons: The Building
Blocks of the
Nervous System
Neural Communication
The Structure of
the Nervous System
The Endocrine System
Your body is an incredible organization of functioning systems. Your
skeletal system supports your body, your digestive system extracts
nutrients from food, your immune system wards off disease, your respiratory system allows you to take in oxygen and rid your cells of carbon dioxide, and so on. But the systems that psychologists focus on
are the nervous and endocrine (hormonal) systems, which enable
communication and information processing within our bodies.
Neurons: The Building Blocks
of the Nervous System
What’s the point?
chapter open photo to come
1. What are the primary parts of a
typical neuron, and what functions do
those parts perform?
The nervous system is your body’s electrochemical communication system.
Through it, your brain tells your body parts to move, your face to express emotion, and your internal organs to go about their business.
Your nervous system, in partnership with your sensory systems, also
gathers information so your brain can respond appropriately to
stubbed toes, fire alarms, and the smell of popcorn. Like every other
system in your body, your nervous system is built of cells, and taking
a look at those cells is a good starting point for understanding the
system as a whole.
Module 7 ■ Neural and Hormonal Systems
121
Your brain, spinal cord, and nerves are formed from neurons, the
highly specialized and unique cells of the nervous system. A neuron exists only to perform three tasks:
• To receive information (in the form of electrochemical impulses)
from the neurons that feed into it
• To carry this information down its length
• To pass the information on to the next neurons in line
Figure caption
will be 3 to 5 lines here.
Figure caption will be 3 to 5
lines here. Figure caption
will be 3 to 5 lines here.
Figure title.
neuron
A nerve cell; the
basic building block of the
nervous system.
dendrite
The bushy,
branching extensions of a neuron that receive messages and
conduct impulses toward the
cell body (soma).
soma
The cell body of a
neuron, which contains the
nucleus and other parts that
keep the cell healthy.
axon
The extension of a
neuron through which neural
impulses are sent.
Every behavior, thought, and emotion you’ve ever experienced depends
on the neuron’s remarkable ability to move and process information.
The wonder of it all is that neurons are so limited in function—
their main capability is transmitting an impulse, or “firing.” In some
ways, the guts of modern powerful computers operate in a similar
way. Computers are binary—each electronic
switch (or bit) in a central processor can be either on or off, set to represent either a 1 or a 0.
All of a computer’s extraordinary capabilities—its communication functions, elaborate
games, “number crunching,” mind-dazzling
graphics, and sound—are ultimately accomplished by setting switches in the proper onor-off pattern.
Neurons work in a similar way: They can
“fire” (that is, send an impulse down their
length) or not “fire.” That’s it. The beautiful colors you see in a sunset, the intense emotions you
experienced during your first crush, the memory of your first day of
kindergarten, the taste of pepperoni pizza, the thrill you feel when riding a roller coaster, and the devastating depression so many thousands
suffer from—all emerge from a certain sequence of neurons either firing
or not firing.
Neurons, like trees and dogs, come in a tremendous variety of
shapes and sizes, but all neurons have similar important structures. Take
a minute now to look at Figure 7.1, which show, these structures in a
motor neuron, a nerve cell that carries messages to muscles and glands. In
this discussion, we will examine neuron parts following the order in
which information travels.
A neuron has endings known as dendrites, which receive information. Dendrites look like branches, and in fact the word dendrite comes
from the Greek word for “tree.” The neuron’s thickest part is the soma,
or cell body. The soma is not responsible for transmitting information;
rather, it contains the cell nucleus and other parts that keep the cell
healthy and functioning properly. Perhaps the most interesting part of
the neuron is the axon, an extension that adds length to the cell. The
122 Part Title ■ Chapter 3 The Biological Bases of Behavior
Figure 7.1 A Typical Motor
neuron’s purpose is to move information from point A to point B, and
the axon creates distance between these points. Axons of neurons in the
brain may be very short, because information doesn’t have to travel very
far between the cells. But in some neurons in the leg, axons extend more
than a meter, making these giant redwoods of the nervous system the
longest cells in your body! Finally, the neuron ends with the axon terminals, which, as you will see, are the points of departure for information making its way to the dendrites of the next neurons in the
sequence. Let’s look more closely at what happens when a neuron “fires.”
Neuron
Neural Communication
The Neural Impulse
2. What roles do the action potential, refractory period,
and resting potential play in generating a neural impulse?
When a neuron “fires,” a tiny electrical charge, called an action potential,
works its way from the dendrites to the axon terminals, much as a bite of
swallowed food makes its way from your mouth to your stomach. This action potential represents the “on” condition of the neuron. Each action potential is followed by a brief recharging phase known as the refractory
period (think of a camera flash that has to recharge before it can be used
again). After the refractory period, the neuron is capable of another action
potential when it is stimulated. When the cell is recharged and ready to
fire again, a resting potential exists. Table 7.1 illustrates these steps.
axon terminal The endpoint
of a neuron, where neurotransmitters are stored.
action potential A neural
impulse; a brief electrical
charge that travels down the
axon of a neuron.
refractory period
The
“recharging phase” when a
neuron, after firing, cannot
generate another action
potential.
Module 7 ■ Neural and Hormonal Systems
123
Communication Between Neurons
TABLE 7.1 THREE PHASES OF COMMUNICATION WITHIN A NEURON
Action potential
The neural impulse created when a
neuron “fires.” The impulse travels from
thedendrites down the axon to the
terminal branches.
Refractory period
The brief instant when a new action
potential cannot be generated because the
neuron is“recharging” after the previous
action potential.
Resting potential
The state of a neuron when it is
“charged” but waiting for the next
action potential tobe generated.
resting potential
The state
of a neuron when it is at rest
and capable of generating an
action potential.
all-or-none principle The
principle stating that if a neuron fires, it always fires at the
same intensity; all action potentials are the same strength.
synapse Tiny, fluid-filled
gap between the axon terminal
of one neuron and the dendrite
of another.
neurotransmitter A chemical messenger that travels
across the synapse from one
neuron to the next and influences whether a neuron will
generate an action potential
(impulse).
3. What role do neurotransmitters play in neural
communication?
So far, we have been discussing how information passes down the
length of a single neuron. But how do messages travel from one neuron
to the next? Amazingly, despite their large numbers, this happens
without any two neurons actually coming in contact with one another!
At every place where an axon terminal of one neuron and the dendrite
of an adjacent neuron meet (and there may be thousands of such places
on any single neuron), a very small, fluid-filled gap called a synapse
exists that action potentials cannot jump. At this point, chemical messengers known as neurotransmitters continue the job and carry the
information across the gap. When an action potential works its way to
the end of a neuron, it causes the release of neurotransmitters from the
axon terminals. The neurotransmitter molecules, which have a distinctive chemical shape, rapidly cross the synapse and fit into receptor sites
on the dendrite of the next neuron (Figure 7.2).
Figure 7.2
Communication Between
Neurons
An interesting fact about how a neuron fires is called the all-or-none
principle, which states that a neuron always fires with the same intensity. It doesn’t matter if there is strong stimulation or weak stimulation
at the cell’s dendrites. As long as there is enough energy to trigger the
cell, it will fire with the same intensity.
One of the best analogies to a neuron and how it fires is, perhaps unfortunately, a toilet. Stop for a moment and think of how a toilet is similar to a neuron. Here are some similarities (perhaps you will be able to
think of more!):
• Like a neuron, a toilet has an action potential. When you flush,
an “impulse” is sent down the sewer pipe.
• Like a neuron, a toilet has a refractory period. There is a short
delay after flushing when the toilet cannot be flushed again because the tank is being refilled.
• Like a neuron, a toilet has a resting potential. The toilet is
“charged” when there is water in the tank and it is capable of
being flushed again.
• Like a neuron, a toilet operates on the all-or-none principle—it
always flushes with the same intensity, no matter how much
force you apply to the handle (providing, of course, you provide
enough force to trigger the mechanism).
124 Part Title ■ Chapter 3 The Biological Bases of Behavior
Module 7 ■ Neural and Hormonal Systems
125
The neurotransmitter molecules can only come to rest in receptor
sites designed to fit their shape, just as a key can only open locks with a
certain configuration. Once in the receptor site, neurotransmitters can
serve two broad functions. Under some circumstances, they have an excitatory effect. This means their arrival makes the receiving neuron
more likely to fire. Other times, neurotransmitters have an inhibitory
effect, which means their arrival makes a neuron less likely to generate
an action potential. The excitatory role is like a green light. It shouts,
“Just do it!” The inhibitory role is like a red light. Its message is “Just
say no!”
There are dozens of neurotransmitters, though so far researchers
have learned the specific functions of only a few (Table 7.2). Neurotransmitters serve different functions, depending not only on the type
of receptor site each locks into, but also on the place where they are released in the brain. (See Psychology Is a Science: Neurotransmitters
and Drugs).
The Neural Chain
4. What are the steps of the neural chain?
excitatory effect
A neurotransmitter effect that makes
it more likely that the receiving neuron will generate an
action potential (impulse).
inhibitory effect
A neurotransmitter effect that makes
it less likely that a receiving
neuron will generate an action
potential (impulse).
receptor cells
Specialized
cells in the sensory systems of
the body that can turn other
kinds of energy into action
potentials (impulses) that the
nervous system can process.
The neural chain describes the path information follows as it is
processed by the nervous system. To understand it, consider the example of playing your favorite radio station through your stereo system.
What is necessary for this task? First, information must be available
from the station. Whenever the station is broadcasting, its radio waves
are present in the room, but people are not equipped to intercept and
interpret these waves directly. That’s why we need the stereo system!
The stereo’s antenna picks up the radio waves and sends them as an electronic message along a wire to the radio receiver. The receiver must
process this information by amplifying and filtering it. Then the information is sent to the speakers, again via a wire. Finally, the speakers vibrate to create the sound of your favorite new hit. The stereo goes
through this process of receiving, processing, and outputting information continuously.
Your nervous system also specializes in receiving and processing information, and it contains functional components similar to those that
make up your stereo system. First, you need to gather information from
your environment. Your “antennae” are the receptor cells of your various sensory systems. These amazing cells have the ability to take other
kinds of energy and put them in the form of neural impulses your brain
can understand. Your eyes, for example, have receptor cells that take
light energy and turn it into nerve impulses. Your ears have similar cells
126 Part Title ■ Chapter 3 The Biological Bases of Behavior
that process sound energy, and elsewhere in your body other such cells
process smells, tastes, and touch into nerve impulses. Without these receptor cells, your brain would be helpless. By itself, your brain cannot
detect light, or sound, or smell. Just as you need your stereo to turn
radio waves into something meaningful, your brain needs your senses
and their receptor cells to gather and transform information into a form
your brain can understand.
The sense organs are not actually located in the brain, so your neural
system must literally move the information your receptor cells pull in.
This movement occurs as billions of neurotransmitter molecules pass
messages among millions of neurons—from your fingertips, your eyeballs, your ears, your nose, and your mouth to the proper area of the
brain for processing. As a stereo uses metal wires, your body uses living
wires known as nerves, constructed of individual neurons. Those that
connect the sense organs to the brain and spinal cord are sensory
nerves. Without them, your brain would be no more effective than
your stereo receiver would be if somebody cut the wire bringing information from the antenna.
The brain, like a stereo receiver, is the real powerhouse of the system, processing the constant, massive barrage of sensory data flowing in
from the sensory nerves. Your brain must process information about
what you see, hear, taste, smell, and feel throughout your body, if only
to ignore much of the information as probably insignificant. It is the
brain’s responsibility to deal with it all and make appropriate decisions,
just as your stereo receiver properly filters and amplifies an incoming
radio signal. The billions of neurons that do this processing in your
brain and spinal cord are called interneurons.
Many times, your brain determines that some action is necessary to
deal with incoming information. If your brain detects a ball moving toward your head, you need to either catch the ball or duck to avoid getting hit. If your brain detects a question asked by your teacher, you need
to decide on an appropriate answer and say it. If your brain detects that
you’re overheating, you need to begin sweating. The point is that while
the brain can determine a course of action on its own (such as speaking or
sweating), it cannot actually do these things. To trigger actions the
brain must get word to the body’s muscles and glands, just as your
stereo system has to convey the processed sound signal from the receiver
to the speakers. Your stereo uses more wires for this purpose. Similarly,
your nervous system uses motor nerves to carry information away from
your brain to the parts of the body that can take action. Without motor
nerves and the muscles and glands they attach to, your brain could not
accomplish anything. (Your stereo wouldn’t be much good without
speakers, would it?)
sensory nerves Nerves that
carry information to the central nervous system.
interneurons
Nerve cells in
the brain and spinal cord responsible for processing information related to sensory
input and motor output.
motor nerves
Nerves that
carry information from the
central nervous system.
Module 7 ■ Neural and Hormonal Systems
127
Neurotransmitters and Drugs
The synapse is where it’s at when it comes to the
effects of many types of drugs. Let’s take a quick
look at the role of a few neurotransmitters, and see
what happens when outside chemicals are added to
the mix.
One neurotransmitter, acetylcholine (ACh), enables both memory and movement. ACh is present
in every synapse of motor nerves and at the final
connection between the last neuron and muscle
Figure 7.3
Agonists and Antagonists
fiber. Chemical substances can disrupt the normal
effects of ACh, however. Some native tribes in
South America use such a substance, a poison called
curare, to coat the tips of the darts they use in their
blowguns. When these darts strie an animal, the
result is paralysis. Why? Because the curare molecules fill the receptor sites on dendrites that normally receive ACh, but the curare molecules do not
stimulate an action potential in the receiving neu-
ron the way ACh would. Thus, since ACh is effectively blocked from doing its job, movement ceases. Substances such as curare that block the effects
of a neurotransmitter are called antagonists.
Like curare, black widow spider venom also interacts with ACh, but not in the same way curare
does. The venom fills the ACh receptor sites, but
its chemical structure is so similar to ACh that it
mimics ACh’s effect on the receiving neuron. So
now two substances, ACh and spider venom, are
doing the same thing. The result is excessive and
uncontrollable movement, in the form of convulsions. The spider venom is called an agonist because it enhances the effect of a neurotransmitter.
Figure 7.3 illustrates how antagonists and agonists
interact with neurotransmitters.
Another neurotransmitter with interesting effects is dopamine. Schizophrenia, a serious illness
that disrupts a person’s sense of reality, is associated with high levels of dopamine. Drugs commonly
prescribed for this illness alleviate some of the
symptoms of schizophrenia by blocking the action
of dopamine at the synapse. These drugs are
dopamine antagonists. Another disorder, depression, is associated with low levels of the neurotransmitter serotonin. Some medications, the
most famous of which is Prozac, work to reduce
depression by enhancing the availability of serotonin at the synapse. Prozac, therefore, is a serotonin agonist. Table 7.2 summarizes the effects of
serotonin and some other neurotransmitters.
Prescribed medications are not the only substances that exert their effects at the synapse. All
mind-altering chemicals, ranging from caffeine to
cocaine, operate by influencing neurotransmission.
Many of these relationships between drugs and
neurochemistry are very complicated. A single
drug, such as alcohol, might influence several different neurotransmitters in different ways. Research
on neurotransmitters is always in progress and
brings fascinating and important results.
TABLE 7.2 EXAMPLES OF NEUROTRANSMITTER
FUNCTIONS
Acetylcholine (ACh)
128 Part Title ■ Chapter 3 The Biological Bases of Behavior
• Muscle action
• Learning
• Memory
• ACh-producing neurons have deteriorated
in people with Alzheimer’s disease.
Dopamine
• Learning
• Attention
• Emotion
• Excess dopamine activity is associated with
schizophrenia.
Serotonin
• Hunger
• Sleep
• Arousal
• Mood
• Low levels of dopamine are associated
with depression.
acetylcholine
[ah-seat-elKO-leen] A neurotransmitter
that triggers muscle contraction and affects learning and
memory.
antagonist
A drug that
blocks the effect of a neurotransmitter.
agonist
A drug that boosts
the effect of a neurotransmitter.
dopamine
A neurotransmitter that affects learning, attention, and emotion; excess
dopamine activity is associated
with schizophrenia.
serotonin [sare-oh-TON-in]
A neurotransmitter that affects
hunger, sleep, arousal, and
mood; serotonin appears in
lower than normal levels in
depressed persons.
Module 7 ■ Neural and Hormonal Systems
129
The central nervous system (CNS) includes the brain and the
spinal cord, so important to the nervous system that they are both encased in bone for protection. The brain is the location where most information processing takes place, and the spinal cord is the main pathway
information follows as it enters and leaves the brain. In shape, the spinal
cord tapers from about the thickness of a broomstick where it joins the
brain to the diameter of a pencil at the base of the back. The interneurons that make up the CNS are responsible for processing information.
The peripheral nervous system (PNS) contains all the nerves that
feed into and branch out from the brain, and more often, the spinal
cord. The word peripheral means “outer region” (perhaps you’ve heard
the phrase “peripheral vision,” which refers to your ability to see things
that are on the outer regions of your visual field). The PNS divides into
two subsystems:
Figure 7.4 A Neural Chain
• The somatic nervous system contains the motor nerves you use
central nervous system (CNS)
The brain and the spinal cord.
peripheral nervous system
(PNS) The sensory and motor
nerves that connect the central
nervous system to the rest of
the body.
somatic nervous system
The division of the peripheral
nervous system that controls
the body’s skeletal muscles.
autonomic [aw-tuh-NAHMik] nervous system The division of the peripheral nervous
system that controls the
glands and muscles of the internal organs. Its subdivisions
are the sympathetic (arousing)
division and the parasympathetic (calming) division.
Figure 7.4 shows a neural chain so basic that the initial action is determined by the spinal cord without the involvement of the brain. In
this case, the response to the heat from the flame is a simple reflex. To
react quickly to a dangerous situation, an interneuron in the spinal cord
sends the command to withdraw the finger even before other interneurons relay the information to your brain!
to activate muscles voluntarily. You develop the idea to walk
across a room using your central nervous system, but you rely on
the motor and sensory nerves of your somatic nervous system to
carry the CNS’s commands to the muscles of your legs and to get
feedback about what your legs are actually doing.
• The second component of the PNS is the autonomic nervous
system, which monitors the automatic functions of your body.
Your autonomic nervous system controls your breathing, blood
pressure, and digestive processes.
Did you notice this was the
second time in this module
we have a word built from
the Greek root soma, which
means “body”?
Figure 7.5 Divisions of the
Nervous System
The Structure of the Nervous System
5. What are the various divisions of the nervous
system, and what is the function of each of these
subsystems?
So far we’ve examined the nervous system by zooming in on its smaller
pieces, sensory and motor nerves made up of tiny neurons sending their
neurotransmitters to one another. Now it’s time to take a step back for a
broader view of the whole communication system in which these pieces
function.
One good way to understand the nervous system is to study its
major divisions, which you can see in Figure 7.5. The nervous system
has two major components, the central nervous system and the peripheral nervous system.
130 Part Title ■ Chapter 3 The Biological Bases of Behavior
Module 7 ■ Neural and Hormonal Systems
131
sympathetic division The
part of the autonomic nervous
system that arouses the body
to deal with perceived threats.
parasympathetic division
The part of the autonomic
nervous system that calms the
body.
A final split is within the autonomic nervous system, which has both a
sympathetic division and a parasympathetic division (Figure 7.6). These two divisions work together in a masterful example of checks and balances—it’s
not just our government that relies on this principle! The sympathetic division is in charge of arousal; it controls a number of responses, collectively
referred to as the fight-or-flight response, that prepare you to deal with threats
or challenges. If you hear footsteps closing in behind you late at night on a
deserted sidewalk, if a teacher announces a pop quiz at the beginning of
class, if you’re about to make a nervous call to a potential dating partner
you’ve never phoned before, your sympathetic nervous system will kick in.
The parasympathetic division of the autonomic nervous system opposes the sympathetic division and generates responses that calm you
down. The sympathetic division may send your blood pressure higher
when your parent catches you coming in after your curfew; your parasympathetic division brings your blood pressure back down to normal when
your parent responds calmly to your explanation of car trouble.
The Endocrine System
endocrine
[EN-duh-krin]
system One of the body’s two
6. How does the way the endocrine system
communicates differ from the way the nervous system
communicates?
Your body has another system for communicating information. This
system, slower to awaken and slower to shut down than the nervous system, is the endocrine system. It is made up of the endocrine glands,
which produce hormones, chemical substances that circulate throughout the body in the blood. Hormones and neurotransmitters are similar
in function: Both carry messages, and both communicate by locking
into receptor sites.
Figure 7.7 illustrates the major endocrine glands. The most important is the pituitary gland, so crucial it is sometimes referred to
as the “master gland.” The pea-sized pituitary is located at the base of
the brain, and it actually connects to a brain part called the hypothalamus through tissue that is part glandular and part neural. This connection illustrates the close relationship between the nervous and
communication systems; a set
of glands that produce hormones, chemical messengers
that circulate in the blood.
hormone Chemical messengers produced by the endocrine glands and circulated in
the blood.
pituitary gland
The endocrine system’s highly influential “master gland” that, in
conjunction with the brain,
controls the other endocrine
glands.
Figure 7.6 The Sympathetic
and Parasympathetic
Divisions of the Autonomic
Nervous System The sympa-
thetic division arouses us and
expends energy. The parasympathetic division calms us
and conserves energy.
132 Part Title ■ Chapter 3 The Biological Bases of Behavior
Figure 7.7 Major Glands of
the Endocrine System
Module 7 ■ Neural and Hormonal Systems
133
thyroid gland Endocrine
gland that helps regulate
energy level in the body.
adrenal glands
Endocrine
glands that help to arouse the
body in times of stress.
endocrine systems. The brain may call on the pituitary to release hormones that stimulate or inhibit the release of other hormones from
other endocrine glands. Can you see why the pituitary is called the
master gland?
It is also true that the brain monitors the levels of hormones circulating in the blood, and may be influenced by their levels. Hunger, for
example, is a response to a complex interaction of the neural and endocrine systems. The hypothalamus and pituitary work together to
monitor and control the levels of glucose (blood sugar that your cells
use for fuel) and insulin (a hormone the pancreas gland secretes, which
allows the cells to use the available glucose) and thus determine, after
considering a host of other factors, how hungry you are at any given
moment. The important pituitary also releases hormones related to
physical growth and pregnancy.
Other endocrine glands include the thyroid, the adrenals, and the
sex glands (or gonads). The thyroid gland, located in the neck, helps
to regulate energy level. The adrenal glands, which perch atop the
kidneys, release epinephrine and norepinephrine (also called adrenaline and
noradrenaline). These substances enhance strength and endurance in
emergency situations. The sex glands—ovaries in females and testes in
males—release hormones that influence emotion and physical development. The primary male hormone is testosterone and the primary female
hormone is estrogen, but both males and females have each hormone
present in their systems.
I’m seated at my desk right now, working on a computer that will
process e-mail, connect to the Internet, and fax with a click of the
mouse. It does this through a cable modem, which is also the source of
the television programming I can access with the remote control that
sits next to the telephone I used to talk to my son, 90 miles away, a few
minutes ago. Also on the desk is a stack of bills, delivered via the U.S.
Postal Service. Later this afternoon I will pay them—electronically—by
using the computer to send instructions to my credit union. While I depend on these methods of communication to function effectively, my
body would be unable to function without the communication capabilities of the nervous and endocrine systems. They are our personal information highways.
134 Part Title ■ Chapter 3 The Biological Bases of Behavior
R
E
V
I
E
W
Module 7: Neural and Hormonal Systems
What’s the Point?
Neurons: The Building Blocks
of the Nervous System
1. What are the primary parts of a typical
neuron, and what functions do those parts
perform?
Neurons are cells that are unique to the
nervous system. Their primary parts are the
dendrites, soma, axon, and axon terminals.
Dendrites are bushy endings that receive
information from other cells. The soma is
the cell body, which contains the nucleus
and other parts that maintain the cell’s
health. The axon ends in a number of axon
terminals, which are the points at which
messages leave the neuron for transmission
to dendrites on other cells.
Neural Communication
2. What roles do the action potential, refractory
period, and resting potential play in generating
a neural impulse?
Neurons, like guns, can either fire or not
fire, and they always fire with the same
intensity. When a neuron fires, the action
potential—a tiny electrical charge—
works its way down the axon to the axon
terminals. Following this firing, the neuron requires a brief period of recharging—the refractory period. A neuron
that has recharged but has not yet fired
again is in a state known as the resting
potential.
3. What role do neurotransmitters play in neural
communication?
The transfer of messages from one neuron
to another is an electrochemical process.
Within a neuron, most of the action is electrical. But when messages pass from one
neuron to another, they take the form of
chemical substances that travel across the
synapse, the tiny gap between two neurons.
These substances are called neurotransmitters, and they have unique shapes that fit
like keys into sites on the dendrites of the
cells receiving the messages. Once in place
in the site, neurotransmitters either encourage the receiving cell to fire (excitatory
effect), or they help to repress firing in the
receiving cell (inhibitory effect).
4. What are the steps of the neural chain?
A neural chain is the sequence of events that
take place when your nervous system gathers
information, translates it into a form your
brain can process, moves it to the brain for
processing, and enables your body to take
any necessary actions. In the first step, receptor cells in your sensory systems gather information and turn it into nerve impulses. In
the second step, neurons and neurotransmitters move the impulses along sensory nerves
to appropriate areas of your brain. In the
third step, interneurons in your brain and
spinal cord process the information and determine whether an action is necessary. In the
fourth and final step, the brain uses motor
nerves to transmit information to parts of
your body that can react appropriately.
Module 7 ■ Neural and Hormonal Systems
135
The Structure of the Nervous System
5. What are the divisions of the nervous system, and
what is the function of each of these subsystems?
The two major divisions of the nervous system are the central nervous system (CNS)
and the peripheral nervous system (PNS).
The CNS consists of the brain, where most
information processing takes place, and the
spinal cord, the main pathway information
follows into and out of the brain. The PNS
contains all nerves that connect the CNS to
the rest of the body. The PNS is further divided into the somatic nervous system
(which controls voluntary movements of the
body’s skeletal muscles) and the autonomic
nervous system (which controls the self-regulated action of internal organs and glands).
The autonomic nervous system in turn has
two more subdivisions: the sympathetic division, which arouses us and expends energy, and the parasympathetic division,
which calms us and conserves energy.
The Endocrine System
6. How does the way the endocrine system communicates differ from the way the nervous system
communicates?
The endocrine system comprises all the
glands that produce hormones. The major
endocrine glands are the pituitary, thyroid, adrenals, pancreas, and sex glands
(ovaries in females and testes in males).
Although both the neural system and the
endocrine system carry messages and
communicate by locking chemicals into
receptor sites, the endocrine system communicates more slowly than the nervous
system. Another difference is that the
neural system’s chemical messengers are
neurotransmitters, which transmit information along nerves, whereas the endocrine system’s chemical messengers are
hormones, which travel through the
bloodstream.
Key Terms
neuron
excitatory effect
central nervous system (CNS)
dendrite
inhibitory effect
peripheral nervous system (PNS)
soma
acetylcholine
somatic nervous system
axon
antagonist
autonomic nervous system
axon terminal
agonist
sympathetic division
action potential
dopamine
parasympathetic division
refractory period
serotonin
endocrine system
resting potential
receptor cells
hormone
all-or-none principle
sensory nerves
pituitary gland
synapse
interneurons
thyroid gland
neurotransmitter
motor nerves
adrenal gland
136 Part Title ■ Chapter 3 The Biological Bases of Behavior
Self-Test
Multiple-Choice Questions: Choose the best
answer for each of the following questions:
1. The nervous system is
a. an electrical communication system.
b. a chemical communication system.
c. a hormonal communication system.
d. an electrochemical communication
system.
2. The all-or-none principle states that
a. a neuron always fires with the same intensity; neurons either fire or they don’t
fire.
b. neurotransmitters are found in all
neural chains but not in the hormonal
system.
c. all live humans have firing neurons, but
no dead humans have firing neurons.
d. all the brain can do is determine a
course of action; none of our behaviors
are performed directly by the brain.
3. The brief recharging period when a neuron
cannot fire is known as the
a. action potential.
b. refractory period.
c. resting potential.
d. neural impulse.
4. The chemical messengers in the neural
system are
a. neurotransmitters.
b. hormones.
c. agonists.
d. antagonists.
5. In the nervous system, __________ pick
up information (about images, smells,
tastes, and so on) from the world around us
and transform that information into neural
impulses that your brain can understand.
a. dendrites
c. receptor cells
b. axon terminals d. interneurons
137 Part Title ■ Chapter 3 The Biological Bases of Behavior
6. The two major components of the nervous
system are
a. the somatic nervous system and the
autonomic nervous system.
b. the central nervous system and the
peripheral nervous system.
c. the sympathetic nervous system and the
parasympathetic nervous system.
d. the hormonal system and the endocrine
system.
7. The __________ gland is called the “master gland” because it has a close connection
with the brain and helps to monitor and
control other glands.
c. thyroid
a. adrenal
b. hypothalamus d. pituitary
8. Matching Terms and Definitions: For
each definition, choose the best matching
term from the list that follows.
Definition
a. The tiny, fluid-filled gap between the
axon terminal of one neuron and the
dendrite of another.
b. The part of the autonomic nervous
system that arouses the body to deal
with perceived threats.
c. Nerves that carry information TO the
central nervous system.
d. The division of the peripheral nervous
system that controls the body’s skeletal
muscles.
e. Bushy, branching extensions of a neuron that receive messages and conduct
impulses toward the cell body.
f. The part of the autonomic nervous
system that calms the body.
g. The division of the peripheral nervous
system that controls the glands and
muscles of the internal organs.
Module 7 ■ Neural and Hormonal Systems
137
h. Nerves that carry information FROM
the central nervous system.
i. The cell body of a neuron, which contains the nucleus and other parts that
keep the cell healthy.
j. The state of a neuron when it has
recharged and is able to fire.
Term
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
dendrite
soma
resting potential
refractory period
synapse
sensory nerves
motor nerves
interneurons
somatic nervous system
autonomic nervous system
sympathetic division
parasympathetic division
138 Part Title ■ Chapter 3 The Biological Bases of Behavior
Fill-in-the-Blank Questions
9. __________ are specialized cells found in
the brain, spinal cord, and nerves.
10. The __________ __________ is a brief
electrical charge that travels down the axon
of a neuron.
11. A neurotransmitter that exerts an
__________ effect makes the receiving
neuron MORE likely to fire; a neurotransmitter that exerts an __________
effect makes the receiving neuron LESS
likely to fire.
Brief Essay Question
12. How exactly would a neural message travel
from one neuron to the next in its path
through the body? Name the parts of the
neuron that the message would travel
through, and be sure to follow the correct
order. Then discuss what happens at the
synapse.