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The Brain and
Extraordinary Engine: The Human Brain
n August 2007, Adam Lepak was a rst-year community college student who had spent the previous summer touring
with his high school friends as a “straight edge” rock band. Late to class one morning, he sped along on his motor-
cycle. As he swerved to miss a car that had stopped in his lane, he lost control and crashed into the pavement.
After lying unconscious in a hospital bed for six months, Adam began to regain awareness. As he did, the world
was eerily different to him. Adam was convinced that his family and friends—who had waited patiently by his bed-
side, coaxing him to wake up—were impostors who did not seem to recognize that they were fakes. Adam’s accident
had damaged the regions of his brain responsible for the warm glow of familiarity that comes from recognizing oth-
ers. His brain no longer detected the feeling of “home.” Not only did Adam question the identities of his loved ones,
but he also struggled to recognize that the young man looking back at him in the mirror was himself (Carey, 2009).
Today Adam continues his diffi cult recovery. He has relearned how to walk and talk but has struggled to regain the
feeling of familiarity that provides humans with a sense of self. He has to be reminded—and to remind himself
repeatedly—that he had a motorcycle accident and that the “impostorsaround him are in truth his family and friends.
Adam’s case illuminates the brain’s role in the precious human experiences of having an identity and of feeling
warmth toward others. His experience demonstrates that the brain can potentially repair itself but that such healing
requires hard work and active effort.
The brain is extraordinarily complex. Imagine: This intricate organ that you are reading about is the engine that is
doing the work of learning this material. The brain is also the organ responsible for the research presented here. In
other words, the brain is at once the object of study and the reason we are able to study it.
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The Nervous System // 43
In this chapter, our focus is the nervous system and its command center—the brain. We
will study the essentials of what the brain has come to know about itself, including the
biological foundations of human behavior and the brain’s extraordinary capacity for
adaptation and repair. The chapter concludes with a look at how genetic processes
infl uence who we are as individuals and how we behave.
The nervous system is the body’s electrochemical communication circuitry. The eld
that studies the nervous system is called neuroscience, and the people who study it are
neur o scientists.
The human nervous system is made up of billions of communicating cells,
and it is likely the most intricately organized aggregate of matter on the planet.
A single cubic centimeter of the human brain consists of well over 50 million
nerve cells, each of which communicates with many other nerve cells in
information-processing networks that make the most elaborate computer seem
Characteristics of the Nervous System
The brain and nervous system guide our interactions with the world, moving the body
and directing our adapt a tion to our environment. Several extraordinary characteristics
allow the nervous system to direct our behavior: complexity, integration, adaptability,
and electrochemical transmission.
C O M P L E X I T Y The human brain and nervous system are enormously complex. The
orchestration of all the billions of nerve cells in the brain—to allow you to sing, dance,
write, talk, and think—is an awe-inspiring achievement. As you are reading, your brain
is carrying out a multitude of tasks, including seeing, reading, learning, and breathing.
Extensive assemblies of nerve cells participate in each of these activities, all at once.
I N T E G R A T I O N Neuroscientist Steven Hyman (2001) calls the brain the “great inte-
grator, meaning that the brain does a wonderful job of pulling information together.
Sounds, sights, touch, taste, smells, hearing—the brain integrates all of these as we func-
tion in the world.
The brain and the nervous system have different levels and many different parts. Brain
activity is integrated across these levels through countless interconnections of brain cells
and extensive pathways that link different parts of the brain. Each nerve cell communi-
cates, on average, with 10,000 others, making an astronomical number of connections
(Bloom, Nelson, & Lazerson, 2001). The evidence for these connections is observable,
for example, when a loved one takes your hand. How does your brain know, and tell
you, what has happened? Bundles of interconnected nerve cells relay information about
the sensation in your hand through the nervous system in very orderly fashion, all the
way to the areas of the brain involved in recognizing that someone you love is holding
your hand. Then the brain might send a reply back and prompt your hand to give him
or her a little squeeze.
nervous system
The body’s electrochemical
communication circuitry.
The Nervous System
One cubi c cent i met er
of br ai n ! 5 0 mi l l i on ner ve cel l s.
That s abou t t he si z e of a snack
cube of cheese.
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44 // CHAPTER 2 // The Brain and Behavior
A D A P T A B I L I T Y The world around us is constantly changing. To survive, we must
adapt to new conditions. Our brain and nervous system together serve as our agent
in adapting to the world. Although nerve cells reside in certain brain regions, they
are not xed, unchanging structures. They have a hereditary, biological foun-
dation, but they are constantly adapting to changes in the body and the
T h e t e r m plasticity d e n o t e s t h e b r a i n s s p e c i a l c a p a c i t y f o r c h a n g e . Y o u
might believe that thinking is a mental process, not a physical one. Yet thinking
is a physical event, because your every thought is re ected in physical activity
in the brain. Moreover, the brain can be changed by experience. London cab drivers
who have developed a familiarity with the city show increases in the size of the brain
area thought to be responsible for reading maps (Maguire & others, 2000). Think about
that: When you change the way you think, you are lite r ally changing the brain’s physical
processes and even its shape. Your daily experiences contribute to the wiring or rewiring
of the brain (Nelson, 2011), just as the experiences of those London cab drivers did
(Bavelier & others, 2012).
ELECTROCHEMICAL TRANSMISSION The brain and the nervous system func-
tion essentially as an information-processing system, powered by electrical impulses and
chemical messengers (Emes & Grant, 2012). When an impulse travels down a nerve cell,
or neuron, it does so electrically. When that impulse gets to the end of the line, it com-
municates with the next neuron using chemicals, as we will consider in detail later in
this chapter.
Pathways in the Nervous System
A s w e i n t e r a c t w i t h a n d a d a p t t o t h e w o r l d , t h e b r a i n a n d t h e n e r v o u s s y s -
tem receive and transmit sensory input (like sounds, smells, and avors),
integrate the information received from the environment, and direct the
body’s motor activities. Information ows into the brain through input from
our senses, and the brain makes sense of that information, pulling it together
and giving it meaning. In turn, information moves out of the brain to the
rest of the body, directing all of the physical things we do (Alstermark &
Isa, 2012).
T h e n e r v o u s s y s t e m p o s s e s s e s s p e c i a l i z e d p a t h w a y s t h a t a r e a d a p t e d f o r d i f -
ferent functions. These pathways are made up of afferent nerves, efferent nerves, and
neural networks. Afferent nerves, or sensory nerves, carry information to the brain
and spinal cord. These sensory pathways communicate information
about the external environment (for example, seeing a sunrise) and
internal body processes (for example, feeling tired or hungry) from
sensory receptors to the brain and spinal cord. Efferent nerves,
or motor nerves, carry information out of the brain and spinal
cord—that is, they carry the nervous systems output. These motor
pathways communicate information from the brain and spinal cord
to other areas of the body, including muscles and glands, telling
them to get busy.
Most information processing occurs when information moves
through neural networks. T h e s e n e t w o r k s o f n e r v e c e l l s i n t e -
gratesensory input and motor output (Marchiori & Warglien, 2011;
Wickersham & Feinberg, 2011). For example, as you read your class
notes, the input from your eyes is transmitted to your brain and then
passed through many neural networks, which translate the characters
on the page into neural codes for letters, words, associations, and
meanings. Some of the information is stored in the neural networks,
and, if you read aloud, some is passed on as messages to your lips
The brain’s
special capacity
for change.
afferent nerves
Also called
sensory nerves;
nerves that carry
about the exter-
nal environment
to the brain
and spinal cord
via sensory
efferent nerves
Also called motor
nerves; nerves
that carry infor-
mation out of the
brain and spinal
cord to other
areas of the body.
neural networks
Networks of
nerve cells that
integrate sensory
input and motor
Adapt at i on,
adapt abi l i t y,
Ps y c h ol ogi s t s us e t h es e t er ms
when r ef er r i n g t o t h e abi l i t y t o
function in a changing world.
Af f er ent
ef f er ent
ar e har d t o keep
st r ai ght . I t mi ght be hel pf ul t o
remember that
f f er ent ner ves
r r i ve at t he br ai n and spi nal
cor d, whi l e
f f er ent ner ves
xi t
the brain and spinal cord
f or
af f er ent
ar r i ve
f or
ef f er ent
ex i t .
When we touch or gaze at an object, electrical
charges and chemical messages pulse through our
brain, knitting the cells together into pathways
and networks for processing the information.
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The Nervous System // 45
and tongue. Neural networks make up most of the brain. Working in networks ampli es
the brain’s computing power.
Divisions of the Nervous System
This truly elegant system is highly ordered and organized for effective function. Figure 2.1
shows the two primary divisions of the human nervous system: the central nervous system
and the peripheral nervous system.
The central nervous system (CNS) is made up of the brain and spinal cord. More
than 99 percent of all our nerve cells are located in the CNS. The peripheral nervous
system (PNS) is the network of nerves that connects the brain and spinal cord to other
parts of the body. The functions of the peripheral nervous system are to bring information
to and from the brain and spinal cord and to carry out the commands of the CNS to
execute various muscular and glandular activities.
The peripheral nervous system has two major divisions: the somatic nervous system
and the autonomic nervous system. The somatic nervous system consists of sensory
nerves, whose function is to convey information from the skin and muscles to the CNS
about conditions such as pain and temperature, and motor nerves, whose function is to
tell muscles what to do. The function of the autonomic nervous system is to take mes-
sages to and from the body’s internal organs, monitoring such processes as breathing,
heart rate, and digestion. The autonomic nervous system also is divided into two parts.
central nervous
system (CNS)
The brain and
spinal cord.
nervous system
The network of
nerves that con-
nects the brain
and spinal cord
to other parts
of the body.
somatic nervous system
The body system consisting
of the sensory nerves,
whose function is to convey
information from the skin
and muscles to the CNS
about conditions such as
pain and temperature, and
the motor nerves, whose
function is to tell muscles
what to do.
autonomic nervous system
The body system that takes
messages to and from the
body’s internal organs,
monitoring such processes
as breathing, heart rate,
Human Nervous System
Central Nervous System Peripheral Nervous System
Spinal Cord
Nervous System
Nervous System
Limbic system
Basal ganglia
Cerebral cortex
branch (arouses
the body)
branch (calms
the body)
FIGURE 2.1 Major Divisions of the Human Nervous System The nervous system has two main divisions. One is the central nervous
system ( left ), which comprises the brain and the spinal cord. The nervous system’s other main division is the peripheral nervous system ( right ), which itself
has two parts—the somatic nervous system, which controls sensory and motor neurons, and the autonomic nervous sy s tem, which monitors processes such
as breathing, heart rate, and digestion. These complex systems work together to help us successfully navigate the world.
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46 // CHAPTER 2 // The Brain and Behavior
The rst part, the sympathetic nervous system, arouses the body to mobilize it for
action and thus is involved in the experience of stress; the second part, the para-
sympathetic nervous system, calms the body.
Stress i s t h e b o d y s r e s p o n s e t o stressors, which are the circumstances
and events that threaten individuals and tax their coping abilities. When we
experience stress, our body readies itself to handle the assault of stress; a
number of physiological changes take place. You certainly know what stress
feels like. Imagine, for example, that you show up for class one morning,
and it looks as if everyone else knows that there is a test that day. You hear
others talking about how much they have studied, and you nervously ask
yourself: “Test? What test?” You start to sweat, and your heart thumps fast
in your chest. Sure enough, the instructor shows up with a stack of exams.
You are about to be tested on material you have not even thought about, much
less studied.
T h e s t r e s s r e s p o n s e b e g i n s w i t h a ght-or- ight reaction, one of the functions
of the sympathetic nervous system. This reaction quickly mobilizes the body’s phys-
iological resources to prepare the organism to deal with threats to survival. Clearly, an
unexpected exam is not literally a threat to your survival, but the human stress response
is such that it can occur in reaction to any threat to personally important motives
(Sapolsky, 2004).
W h e n y o u f e e l y o u r h e a r t p o u n d i n g a n d y o u r h a n d s s w e a t i n g u n d e r s t r e s s , t h o s e
experiences reveal the sympathetic nervous system in action. If you need to run away
from a stressor, the sympathetic nervous system sends blood out to your extremities to
get you ready to take off.
When we undergo stress, we also experience the release of corticosteroids, which are
powerful stress hormones (Smith & others, 2011). Corticosteroids in the brain allow us
to focus our attention on what needs to be done now. For example, in an emergency,
people sometimes report feeling strangely calm and doing just what has to be done,
whether it is calling 911 or applying pressure to a serious cut. Such experiences reveal
the bene ts of corticosteroids for humans in times of emergency (Holsboer & Ising,
2010). Acute stress is the momentary stress that occurs in response to life experiences.
When the stressful situation ends, so does acute stress.
However, we are not in a live-or-die situation most of the time when we experience
stress. Indeed, we can even “stress ourselves out” just by thinking. Chronic stress —that
is, stress that goes on continuously—may lead to persistent autonomic nervous system
arousal (Rohleder, 2012). While the sympathetic nervous system is working to meet
the demands of whatever is stressing us out, the parasympathetic nervous system is
not getting a chance to do its job of maintenance and repair, of digesting food, or of
keeping our organs in good working order. Thus, over time, chronic autonomic nervous
system activity can break down the immune system (Pervanidou &
Chrousos, 2012). Chronic stress is clearly best avoided, although this
objective is easier said than done.
Y e t t h e b r a i n , a n o r g a n t h a t i s i t s e l f p o w e r f u l l y a f f e c t e d b y c h r o n i c
stress, can be our ally in preventing such continuous stress. Consider
that when you face a challenging situation, you can exploit the brain’s
abilities and interpret the experience in a less stressful way. For exam-
ple, you might approach an upcoming exam or an audition for a play
not so much as a stressor but as an opportunity to shine. Many cogni-
tive therapists believe that changing the way people think about their
life opportunities and experiences can help them live less stressfully
(Clark & Beck, 2011; Nay, 2012).
At the beginning of this chapter, you learned how changing the way
you think can produce physical changes in the brain. In light of this
remarkable capacity, it is reasonable to conclude that you can use your
brain’s powers to change how you look at life experiences—and maybe
even deploy the brain as a defense against stress.
s ympathetic
nervous system
The part of the
autonomic ner-
vous system that
arouses the body
to mobilize it for
action and thus
isinvolved in the
experience of
nervous system
The part of the
autonomic ner-
vous system that
calms the body.
The response of
individuals to
and events that
threaten individu-
als and tax their
coping abilities
and that cause
changes to ready
the body to han-
dle the assault of
Sympat het i c,
par asympat het i chow t o
di s t i nguish t hese? Remember
that the sympathetic nervous
syst em f eels sympat hy f or
you when you r e s t r es s ed out ,
and pr ompt s you t o t ake act i on
to reduce the stressor, and
that the parasympathetic
ner vous sy st em hel ps you t o
“rest and digest .
The Nervous System
“It was the classic fight or flight response.
Next time, try flight.”
Used by permission of CartoonStock,
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Neurons // 47
1. The characteristics that allow the ner-
vous system to direct behavior are its
complexity, integration, electrochemical
transmission, and
A. constancy.
B. adaptability.
C. sensitivity.
D. ght-or-fl ight response.
2. Neural networks are networks of nerve
cells that integrate sensory input and
A. the fi ght-or-fl ight response.
B. electrochemical transmission.
C. bodily processes such as heart rate
and digestion.
D. motor output.
3. When you are in danger, the part of the
nervous system that is responsible for
an increase in your heart rate is the
A. central nervous system.
B. peripheral nervous system.
C. sympathetic nervous system.
D. parasympathetic nervous system.
A P P L Y I T ! 4. Shannon and Terrell are
two college students. Shannon is in a con-
stant state of low-level stress. She spends a
lot of time worrying about what might hap-
pen, and she gets herself worked up about
imagined catastrophes. Terrell is more easy-
going, but on his way to class one day he
is in a near-miss traffic accident—at the
moment he sees the truck coming at him,
his body tenses up, his heart races, and he
experiences extreme panic. Which answer
most accurately identifies the individual
who is most likely to catch the cold that is
going around their dorm this semester?
A. Shannon, who is experiencing chronic
B. Terrell, who is experiencing acute stress
C. Shannon, who is experiencing acute
D. Terrell, who is experiencing chronic
W i t h i n e a c h d i v i s i o n o f t h e n e r v o u s s y s t e m , m u c h i s h a p p e n i n g a t t h e c e l l u l a r l e v e l .
Nerve cells, chemicals, and electrical impulses work together to transmit informa-
tion at speeds of up to 330 miles per hour. As a result, information can travel
from your brain to your hands (or vice versa) in just milliseconds (Zoupi,
Savvaki, & Karagogeos, 2011).
There are two types of cells in the nervous system: neurons and glial cells.
Neurons are the nerve cells that handle the information-processing function.
The human brain contains about 100 billion neurons. The average neuron is
a complex structure with as many as 10,000 physical connections with other
cells. Recently, researchers have been especially interested in a special type
of neuron called mirror neurons. Mirror neurons seem to play a role in imitation
and are activated (in primates and humans) when we perform an action but also
when we watch someone else perform that same task (Fontana & others, 2011).
In addition to imitation, these neurons may play a role in empathy and in our under-
standing of others, although their function in this regard is a matter of controversy. To
read more about these mysterious neurons, see Challenge Your Thinking.
Glial cells p r o v i d e s u p p o r t , n u t r i t i o n a l b e n e ts, and other functions in the
nervous system (Cooper, Jones, & Comer, 2011; Selvaraj & others, 2012). Glial
cells keep neurons running smoothly. These cells are not specialized to process
information in the way that neurons are, and there are many more of them in the
nervous system than there are neurons. In fact, for every neuron there are about
10 glial cells.
Specialized Cell Structure
Not all neurons are alike, as they are specialized to handle different information-processing
functions. However, all neurons do have some common characteristics. Most neurons are
created very early in life, but their shape, size, and connections can change throughout the
life span. The way neurons’ function re ects the major characteristic of the nervous system
described at the beginning of the chapter: plasticity. Neurons can and do change.
One of two types
of cells in the
nervous system;
neurons are the
nerve cells that
handle the
glial cells
The second of
two types of cells
in the nervous
system; glial cells
(also called glia)
provide support,
nutritional ben-
efi ts, and other
functions and
keep neurons
That s f ast ! Most of us
wi l l n ever exper i en c e dr i vi ng a c ar
that fast. The supersonic rocket
car t hat hol ds t he wor l d r ecor d
can d r i v e ov er 70 0 mi l es per
hour . I t s Br i t i sh d ev el oper s ar e
shoot ing f or over 1,0 0 0 mi l es
per hour . Now, t he wi sdom of
dr i vi ng a car f ast er t han we can
think is another story . . .
You mi ght t hi nk of gl i al
cel l s as t he pi t cr ew of t he
ner vous sy st em.
Mirror Neurons
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48 // CHAPTER 2 // The Brain and Behavior
irror neurons are active while
we enact a behavior and
when we passively observe
another person performing that behav-
ior (Glenberg, 2011a). That’s a big
deal, because neurons are special-
ized: Motor neurons do not respond to
sensory information, and sensory neu-
rons do not respond to motor informa-
tion. Yet mirror neurons appear to
respond to both kinds of information,
doing and seeing (Gallese & others,
2011). This responsiveness to two
different kinds of input makes mirror
neurons pretty fascinating.
Not fascinated yet? What if you
thought that mirror neurons repre-
sented a kind of mental telepathy? Cognitive psychologist Cecilia
Heyes (2010) describes scientists as “mesmerized” by the way
mirror neurons suggest communication without talking. For exam-
ple, if I see you performing an action, my mirror neurons put my
brain in the very same state you are in, even if you do not speak
aword. Even if I am not intending to imitate your behavior, mirror
neurons in my body will re. My brain somehow seems to “know”
what you are doing—and mentally starts doing it too. In this way,
mirror neurons suggest a direct connection between an observer’s
brain and the person being observed. Such a direct link between
individuals—occurring without language, explanation, or effort—is
exciting to scientists who are interested in understanding human
The discovery of mirror neurons has led to provocative predic-
tions about the role of these neurons in imitation, social cognition
(that is, thinking about oneself and others), empathy, understand-
ing behavior (Iacoboni, 2009), and even autism, a disorder of neu-
ral development characterized by impaired communication and
social interaction (Ramachandran & Oberman, 2006). These pre-
dictions have met with great controversy. Although some scholars
hail mirror neurons as a promising new direction in understanding
the origins of human sociability (Ramachandran, 2008), others
charge that such claims far overstep the evidence (Gernsbacher,
Stevenson, & Schweigart, 2012; Hickok, 2009).
We know that mirror neurons are active when we observe some-
one perform an action, but does that indicate understanding for
the action? The answer depends on what we mean by understand-
ing. Of course, mirror neurons, like other neurons, do not “under-
stand.” The idea, rather, is that because of mirror neurons, the
human brain is prepared to imitate. Imitation is an especially im-
portant behavior in a highly social species such as human beings,
and some scholars argue that mirror neurons are a fundamental
factor in imitation (Gallese & others, 2011).
Experience plays a role in mirror
neuron activation. Mirror neuron activity
is higher for behaviors for which we
have expertise and lower for behaviors
we have never performed. Trained
musicians and dancers have different
mirror neuron systems than others,
andtheir mirror neurons re differently
while watching another expert perform
(Calvo-Merino & others, 2006; D’Ausilio
& others, 2006). Mirror neurons do not
respond when we observe a behavior
that is not part of our typical repertoire;
for instance, human mirror neurons do
not activate while we watch a dog bark
(Rizzolatti & Fabbri-Destro, 2010).
These ndings suggest that if mirror
neurons are involved in understanding action, they do not cover
the full range of behaviors we comprehend. If you watch someone
play a violin, having never played one yourself, you still understand
what the person is doing (Hickok, 2009; Hickok & Hauser, 2010).
So, we can understand behavior even without theinvolvement of
mirror neurons. Some researchers question whether mirror neu-
rons play a central role in understanding behavior at all (Brass &
others, 2007; Hickok, 2009; Kilner & Frith, 2008). While acknowl-
edging that there are many ways to gain an understanding of an-
other person’s action, others argue that mirror neurons provide a
unique route—from the inside out (Rizzolatti & Sinigaglia, 2010).
The potential role of mirror neurons in autism is especially
controversial (Glenberg, 2011b). Researchers who champion mirror
neurons as an important evolutionary adaptation suggest that these
neurons may hold the key to illuminating what makes us human
(Ramachandran, 2008). They assert that “broken mirror neurons”
may help to explain the de cits seen in individuals with autism
(Ramachandran & Oberman, 2006). Other researchers strongly criti-
cize this theory, contending that we know far too little about mirror
neurons to claim a central role for them in autism; they point to
mixed results in research linking
mirror neuron function to autism
(Gallese & others, 2011).
Mirror neurons have powerfully
captured neuroscientists’ atten-
tion. This fascination perhaps re-
veals an important truth: that the
mystery of human social behavior
is an immensely compelling area
of scienti c inquiry. The notion
that the key to this mystery might
lie in a set of specialized neurons
is, well, simply mesmerizing.
Do Mirror Neurons Hold the Key to Social Understanding?
What Do You Think?
Why has the discovery of
mirror neurons led to such
excitement and controversy?
What characteristics make
human beings different from
other animals? How would you
study those characteristics?
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Neurons // 49
Every neuron has a cell body, dendrites, and an axon (Figure 2.2). The cell body
contains the nucleus, which directs the manufacture of substances that the neuron needs
for growth and maintenance. Dendrites, treelike bers projecting from a neuron, receive
information and orient it toward the neuron’s cell body. Most nerve cells have numerous
dendrites, which increase their surface area, allowing each neuron to receive input from
many other neurons. The axon is the part of the neuron that carries information away
from the cell body toward other cells. Although extremely thin (1/10,000th of an inch—
a human hair by comparison is 1/1,000 of an inch), axons can be very long, with many
branches. Some extend more than 3 feet—from the top of the brain to the base of the
spinal cord.
Covering all surfaces of neurons, including the dendrites and axons, are very thin
cellular membranes that are much like the surface of a balloon. The neuronal membranes
are semipermeable, meaning that they contain tiny holes, or channels, that allow only
certain substances to pass into and out of the neurons.
A myelin sheath, consisting of a layer of cells containing fat, encases and insulates
most axons. By insulating axons, myelin sheaths speed up transmission of nerve
impulses (Lu & others, 2011). Numerous disorders are associated with problems in
either the creation or the maintenance of this vital insulation. One of them is multiple
sclerosis (MS), a degenerative disease of the nervous system in
which myelin hardens, disrupting the ow of information through
the neurons. Symptoms of MS include blurry and double vision,
tingling sensations throughout the body, and general weakness.
T h e m y e l i n s h e a t h d e v e l o p e d a s t h e n e r v o u s s y s t e m e v o l v e d . A s
brain size increased, it became necessary for information to travel
over longer distances in the nervous system. Axons without myelin
sheaths are not very good conductors of electricity. With the insula-
tion of myelin sheaths, axons transmit electrical impulses and convey
information much more rapidly (Aggarwal & others, 2011). We can
compare the myelin sheath’s development to the evolution of inter-
state highways as cities grew. Highways keep fast-moving, long-
distance traf c from getting snarled by slow local traf c.
cell body
The part of the
neuron that con-
tains the nucleus,
which directs the
manufacture of
substances that
the neuron needs
for growth and
Treelike bers projecting
from a neuron, which receive
information and orient it to-
ward the neuron’s cell body.
The part of the neuron that
carries information away
from the cell body toward
other cells.
myelin sheath
A layer of fat
cells that encases
and insulates
most axons.
Cell body
Myelin sheath
surrounding the axon
Direction of
nerve impulse
Sending Neuron
Receiving Neuron
FIGURE 2.2 The Neuron The drawing shows the parts of a neuron and the connection between one neuron and another. Note the cell body, the
branching of dendrites, and the axon with a myelin sheath.
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50 // CHAPTER 2 // The Brain and Behavior
The Neural Impulse
To transmit information to other neurons, a neuron sends brief electrical impulses (let’s
call them “blips”) through its axon to the next neuron. As you reach to turn this page,
hundreds of such impulses will stream down the axons in your arm to tell your
muscles when to ex and how quickly. By changing the rate of the signals, or
blips, the neuron can vary its message. Those impulses traveling down the axon
are electrical. How does a neuron—a living cell—generate electricity? To
answer this question, we need to examine the axon.
The axon is a tube encased in a membrane. The membrane has hundreds
and thousands of tiny gates in it. These gates are generally closed, but they can
open. We call the membrane semipermeable because uids can sometimes ow
in and out of these gates. Indeed, there are uids both inside and outside the axon.
Floating in those uids are electrically charged particles called ions.
Some of these ions, notably sodium and potassium, carry positive charges. Negatively
charged ions of chlorine and other elements also are present. The membrane surrounding
the axon prevents negative and positive ions from randomly owing into or out of the
cell. The neuron creates electrical signals by moving positive and negative ions back and
forth through its outer membrane. How does the movement of ions across the membrane
occur? Those tiny gates mentioned above, called ion channels, open and close to let the
ions pass into and out of the cell. Normally when the neuron is resting, or not transmit-
ting information, the ion channels are closed, and a slight negative charge is present
along the inside of the cell membrane. On the outside of the cell membrane, the charge
is positive. Because of the difference in charge, the membrane of the resting neuron is
said to be pola r ized, w i t h m o s t n e g a t i v e l y c h a r g e d i o n s o n t h e i n s i d e o f t h e c e l l a n d m o s t
positively charged ions on the outside. This polarization creates a voltage between the
inside and the outside of the axon wall (Figure 2.3). That voltage, called the neuron’s
resting pote ntial, is between " 60 and " 75 millivolts. (A millivolt is 1/1000 of a volt.)
F o r i o n s , i t i s t r u e t h a t o p p o s i t e s a t t r a c t . T h e n e g a t i v e l y c h a r g e d i o n s i n s i d e t h e m e m -
brane and the positively charged ions outside the membrane will rush to each other if
given the chance. Impulses that travel down the neuron do so by opening and closing
ion channels, allowing the ions to ow in and out.
A neuron becomes activated when an incoming impulse—a reaction to, say, a pinprick
or the sight of someone’s face—raises the neuron’s voltage, and the sodium gates at the
base of the axon open brie y. This action allows positively charged sodium ions to ow
resting potential
The stable, negative charge
of an inactive neuron.
The r at e of t he bl i ps
det er mi nes t he i nt ens i t y of
the impulse. So, if you are dying
of suspens e whi l e r eadi ng about
neur al i mpul ses ( and who i sn’ t ? ) ,
the blips are happening faster
as you r us h t o t ur n each page.
0 mV
70 mV
FIGURE 2.3 The Resting
Potential An oscilloscope measures the
difference in electrical potential between two
electrodes. When one electrode is placed
insidean axon at rest and one is placed
outside, the electrical potential inside the cell
is"70 millivolts (mV) relative to the outside.
This potential difference is due to the separation
of positive (#) and negative (") charges along
the membrane.
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Neurons // 51
into the neuron, creating a more positively charged neuron and depolarizing the mem-
brane by decreasing the charge difference between the uids inside and outside the
neuron. Then potassium channels open, and positively charged potassium ions move out
through the neuron’s semipermeable membrane. This out ow returns the neuron to a
negative charge. Then the same process occurs as the next group of channels ips open
brie y. So it goes all the way down the axon, like a long row of cabinet doors opening
and closing in sequence. It is hard to imagine, but this simple system of opening and
closing tiny doors is responsible for the uid movements of a ballet dancer and the y-
ing ngers of a pianist playing a concerto.
The term action potential describes the brief wave of positive electrical charge that
sweeps down the axon (Figure 2.4). An action potential lasts only about 1/1000 of a
second, because the sodium channels can stay open for only a very brief time. They
quickly close again and become reset for the next action potential. When a neuron sends
an action potential, it is commonly said to be ring.
T h e a c t i o n p o t e n t i a l a b i d e s b y t h e all-or-nothing principle: Once the electrical
impulse reaches a certain level of intensity, called its threshold, it res and moves all
the way down the axon without losing any of its intensity. The impulse traveling down
an axon can be compared to the burning fuse of a recracker. Whether you use a match
or blowtorch to light the fuse, once the fuse has been lit, the spark travels quickly and
with the same intensity down the fuse.
Synapses and Neurotransmitters
T h e m o v e m e n t o f a n i m p u l s e d o w n a n a x o n m a y b e c o m p a r e d t o a c r o w d s w a v e
motion in a stadium. With the wave, there is a problem, however—the aisles. How does
the wave get across the aisle? Similarly, neurons do not touch each other directly, and
electricity cannot travel over the space between them. Yet somehow neurons manage to
communicate. This is where the chemical part of electrochemical transmission comes
in. Neurons communicate with each other through chemicals that carry messages across
the space. This connection between one neuron and another is one of the most intrigu-
ing and highly researched areas of contemporary neuroscience (Emes & Grant, 2012).
Figure 2.5 gives an overview of how this connection between neurons takes place.
S Y N A P T I C T R A N S M I S S I O N Synapses a r e t i n y s p a c e s b e t w e e n n e u r o n s ; t h e
gap between neurons is referred to as a syna p tic gap. Most synapses lie between the
axon of one neuron and the dendrites or cell body of another neuron (Turrigiano, 2011).
action potential
The brief wave of positive
electrical charge that
sweeps down the axon.
all-or-nothing principle
The principle that once the
electrical impulse reaches
acertain level of intensity
(its threshold ), it fi res and
moves all the way down the
axon without losing any
Tiny spaces between neu-
rons; the gaps between
neurons are referred to as
synaptic gaps.
0 mV
40 mV
70 mV
0 mV
40 mV
70 mV
Upswing Downswing
0 mV
40 mV
70 mV
of impulse
Action potential generated by an
impulse within a neuron
Movement of sodium (Na
) and potassium (K
ions responsible for the action potential
FIGURE 2.4 The Action Potential
An action potential is a brief wave of positive
electrical charge that sweeps down the axon as
the sodium channels in the axon membrane
open and close. ( a ) The action potential causes
a change in electrical potential as it moves
along the axon. ( b ) The movements of sodium
ions (Na
) and potassium ions (K
) into and
out of the axon cause the electrical changes.
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52 // CHAPTER 2 // The Brain and Behavior
Before an impulse can cross the synaptic gap, it must be converted into a chemical
Each axon branches out into numerous bers that end in structures called terminal
bu t tons. Stored in very tiny synaptic vesicles (sacs) within the terminal buttons are chem-
ical substances called neurotransmitters. As their name suggests, neurotransmitters
transmit, or carry, information across the synaptic gap to the next neuron. When a nerve
impulse reaches the terminal button, it triggers the release of neurotransmitter molecules
from the synaptic vesicles (Liu & others, 2011). The neurotransmitter molecules ood
the synaptic gap. Their movements are random, but some of them bump into receptor
sites in the next neuron.
The neurotransmitters are like pieces of a puzzle, and the receptor sites on the next
neuron are differently shaped spaces. If the shape of the receptor site corresponds to the
shape of the neurotransmitter molecule, the neurotransmitter acts like a key to open the
Chemical substances that
are stored in very tiny sacs
within the terminal buttons
and involved in transmitting
information across a synap-
tic gap to the next neuron.
FIGURE 2.5 How Synapses and Neurotransmitters Work ( A ) The axon of the presynaptic (sending) neuron meets dendrites of the
postsynaptic (receiving) neuron. ( B ) This is an enlargement of one synapse, showing the synaptic gap between the two neurons, the terminal button, and the
synaptic vesicles containing a neurotransmitter. ( C ) This is an enlargement of the receptor site. Note how the neurotransmitter opens the channel on the
receptor site, triggering the neuron to re.
Direction of
nerve impulse
Axon of sending
Terminal button
Synaptic vesicle
Synaptic gap
Dendrite of
receiving neuron
Synaptic vesicle releases
Neurotransmitters attach at
the receptor binding site;
channel o
Receptor with
binding site
In the terminal button, the impulse triggers the release
of neurotransmitters into the synaptic gap.
At a receptor site on the dendrite of the receiving
neuron, the neurotransmitter causes channels to
open and creates an action potential.
The neural impulse travels down the
axon toward dendrites of the next neuron.
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Neurons // 53
receptor site, so that the neuron can receive the signals coming from
the previous neuron. After delivering its message, some of the neu-
rotransmitter is used up in the production of energy, and some of it is
reabsorbed by the axon that released it to await the next neural
impulse. This reabsorption is termed reuptake. Essentially, a message
in the brain is delivered across the synapse by a neurotransmitter,
which pours out of the terminal button just as the message approaches
the synapse.
N E U R O C H E M I C A L M E S S E N G E R S T h e r e a r e m a n y d i f f e r -
ent neurotransmitters. Each plays a speci c role and functions in a
speci c pathway. Whereas some neurotransmitters stimulate or excite
neurons to re, others can inhibit neurons from ring (Ellender & others,
2011). Some neurotransmitters are both excitatory and inhibitory.
A s t h e n e u r o t r a n s m i t t e r m o v e s a c r o s s the synaptic gap to the receiv-
ing neuron, its molecules might spread out, or they might be con ned
to a small space. The molecules might come in rapid sequence or might
be spaced out. The receiving neuron integrates this information before
reacting to it.
Neurotransmitters t into the receptor sites like keys in keyholes. Other substances,
such as drugs, can sometimes t into those receptor sites as well, producing a variety of
effects. Similarly, many animal venoms, such as that of the black widow spider, are
neurotransmitter-like substances that act by disturbing neurotransmission.
Most neurons secrete only one type of neurotransmitter, but often many different
neurons are simultaneously secreting different neurotransmitters into the synaptic gaps
of a single neuron. At any given time, a neuron is receiving a mixture of messages
from the neurotransmitters. At its receptor sites, the chemical molecules bind to the
membrane and either excite the neuron, bringing it closer to the threshold at which it
will re, or inhibit the neuron from ring. Usually the binding of an excitatory neu-
rotransmitter from one neuron will not be enough to trigger an action potential in the
receiving neuron. Triggering an action potential often takes a number of neurons send-
ing excitatory messages simultaneously or fewer neurons sending rapid- re excitatory
R e s e a r c h e r s h a v e i d e n t i ed more than 100 neurotransmitters in the brain alone,
each with a unique chemical makeup. The rapidly growing list is likely to increase
beyond 100 (G. B. Johnson, 2012). In organisms ranging from snails to whales,
neuroscientists have found the same neurotransmitter molecules that our own
brains use. To get a better sense of what neurotransmitters do, let’s consider
seven that have major effects on behavior.
Acetylcholin e Acetylcholine (ACh) usually stimulates the ring of neurons
and is involved in the action of muscles, learning, and memory (Kalmbach,
Hedrick, & Waters, 2012). ACh is found throughout the central and peripheral nervous
systems. The venom of the black widow spider causes ACh to gush out of the synapses
between the spinal cord and skeletal muscles, producing violent spasms.
Individuals with Alzheimer disease, a degenerative brain disorder that involves a decline
in memory, have an acetylcholine de ciency (Griguoli & Cherubini, 2012). Some of the
drugs that alleviate the symptoms of Alzheimer disease do so by compensating for the loss
of the brain’s supply of acetylcholine.
G A B A GABA (gamma aminobutyric acid) i s f o u n d t h r o u g h o u t t h e c e n t r a l
nervous system. It is believed to be the neurotransmitter in as many as one-third
of the brain’s synapses. GABA is important in the brain because it keeps many
neurons from ring (Richter & others, 2012). In this way, it helps to control the
precision of the signal being carried from one neuron to the next. Low levels of
The neurotransmitter-like venom of the black
widow spider does its harm by disturbing
Bot ox i nj ect i ons cont ai n
bot ul i n, a poi s on that, by
des t r oyi ng ACh, bl ocks t he
recipients facial muscles from
mo v i n g . Wr i n k l e s , a s w e l l a s ma n y
genui ne f aci al ex pr es s i ons , ar e
thereby prevented.
You can t hi nk of
GA BA as t he br ai n s br ak e pedal .
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54 // CHAPTER 2 // The Brain and Behavior
GABA are linked with anxiety. Antianxiety drugs increase the inhibiting
effects of GABA.
Norepinephrine Norepinephrine inhibits the ring of neurons
in the central nervous system, but it excites the heart muscle, intes-
tines, and urogenital tract. Stress stimulates the release of norepi-
nephrine (Wong & others, 2012). This neurotransmitter also helps to
control alertness. Too little norepinephrine is associated with depres-
sion, and too much triggers agitated, manic states. For example,
amphetamines and cocaine cause hyperactive, manic states of behav-
ior by rapidly increasing brain levels of norepinephrine (Janak,
Bowers, & Corbit, 2012).
Recall from the beginning of the chapter that one of the most
important characteristics of the brain and nervous system is integra-
tion. In the case of neurotransmitters, they may work in teams of two
or more. For example, norepinephrine works with acetylcholine to
regulate states of sleep and wakefulness.
Dopamine Dopamine helps to control voluntary movement and
affects sleep, mood, attention, learning, and the ability to recognize rewards in the envi-
ronment (Meyer, 2012). Dopamine is related to the personality trait of extraversion (being
outgoing and gregarious), as we will see in Chapter 10. Stimulant drugs such as cocaine
and amphetamines produce excitement, alertness, elevated mood, decreased fatigue, and
sometimes increased motor activity mainly by activating dopamine receptors (Perez-Costas,
Melendez-Ferro, & Roberts, 2010).
L o w l e v e l s o f d o p a m i n e a r e a s s o c i a t e d w i t h P a r k i n s o n d i s e a s e , i n w h i c h p h y s i c a l
movements deteriorate (Berthet & others, 2012). High levels of dopamine are associated
with schizophrenia (Eriksen, Jorgensen, & Gether, 2010), a severe psychological disorder
that we will examine in Chapter 12.
Serotonin Serotonin is involved in the regulation of sleep, mood, attention, and learn-
ing. In regulating states of sleep and wakefulness, it teams with acetylcholine and nor-
epinephrine. Lowered levels of serotonin are associated with depression (Karg & Sen,
2012). The antidepressant drug Prozac works by slowing down the reuptake of serotonin
into terminal buttons, thereby increasing brain levels of serotonin (Little, Zhang, & Cook,
2006). Figure 2.6 shows the brain pathways for serotonin. There are 15 known types of
serotonin receptors in the brain (Hoyer, Hannon, & Martin, 2002), and each type of
antidepressant drug has its effects on different receptors.
Endorphins Endorphins are natural opiates that mainly stimulate the ring of neu-
rons. Endorphins shield the body from pain and elevate feelings of pleasure. A long-
distance runner, a woman giving birth, and a person in shock after a car wreck
all have elevated levels of endorphins (Mahler & others, 2009).
As early as the fourth century b.c.e. , the Greeks used wild poppies to induce
euphoria. More than 2,000 years later, the magical formula behind opium’s addic-
tive action was nally discovered. In the early 1970s, scientists found that opium
plugs into a sophisticated system of natural opiates that lie deep within the brain’s
pathways (Pert, 1999; Pert & Snyder, 1973). Morphine (the most important nar-
cotic of opium) mimics the action of endorphins by stimulating receptors in the
brain involved with pleasure and pain (Vetter & others, 2006).
Oxytocin Oxytocin is a hormone and neurotransmitter that plays an important
role in the experience of love and social bonding. A powerful surge of oxytocin
is released in mothers who have just given birth, and oxytocin is related to the
FIGURE 2.6 Serotonin Pathways
E a c h o f t h e n e u r o t r a n s m i t t e r s in the brain has
speci c pathways in which it functions. Shown
here are the pathways for serotonin.
Research has linked the hormone
oxytocin to bonding between
parents and their newborn.
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Neurons // 55
onset of lactation and breast feeding (Vrachnis & others, 2012). Oxytocin, however, is
not only involved in a mother’s ability to provide nourishment for her baby (Carter &
others, 2007). It is also a factor in the experience of parents who nd themselves “in
love at rst sight” with their newborn (Young, 2009).
Oxytocin is released as part of the sexual orgasm and is thought to play a role in the
human tendency to feel pleasure during orgasm and to form emotional bonds with roman-
tic partners (Magon & Kaira, 2011). Provocative research has related oxytocin to the
way that women respond to stress. According to Shelley Taylor (2001, 2007, 2011a,
2011b), women under stress do not experience the classic ght-or- ight response—rather,
the in ux of oxytocin suggests that women may seek bonds with others when under
stress. Taylor refers to this response as “tend and befriend.
D R U G S A N D N E U R O T R A N S M I T T E R S Most drugs that in uence behavior do
so mainly by interfering with the work of neurotransmitters (Hart, Ksir, & Ray, 2011).
Drugs can mimic or increase the effects of a neurotransmitter, or they can block those
effects. An agonist is a drug that mimics or increases a neurotransmitter’s effects. For
example, the drug morphine mimics the actions of endorphins by stimulating receptors
in the brain and spinal cord associated with pleasure and pain. An antagonist is a drug
that blocks a neurotransmitter’s effects. For example, drugs used to treat schizophrenia
interfere with the activity of dopamine.
Neural Networks
So far, we have focused mainly on how a single neuron functions and on how a
nerve impulse travels from one neuron to another. Now let’s look at how large
numbers of neurons work together to integrate incoming information and coor-
dinate outgoing information. Figure 2.7 shows a simpli ed drawing of a neural
network, or pathway. This diagram gives you an idea of how the activity of one
neuron is linked with that of many others.
S o m e n e u r o n s h a v e s h o r t a x o n s a n d c o m m u n i c a t e w i t h o t h e r , n e a r b y n e u -
rons. Other neurons have long axons and communicate with circuits of neu-
rons some distance away. These neural networks are not static (Fietta &
Fietta, 2011). They can be altered through changes in the strength of synaptic
connections. Any piece of information, such as a name, might be embedded in
hundreds or even thousands of connections between neurons (Wickersham &
Feinberg, 2012). In this way, human activities such as being attentive, memoriz-
ing, and thinking are distributed over a wide range of connected neurons. The strength
of these connected neurons determines how well you remember the information
(Goldman, 2009).
agon ist
A drug that mimics or in-
creases a neurotransmitter’s
A drug that
blocks a neu-
FIGURE 2.7 An Example of a
Neural Network Inputs (information from
the environment and from sensory receptors,
such as the details of a person’s face) become
embedded in extensive connections between
neurons in the brain. This embedding process
leads to outputs such as remembering the
person’s face.
Ther e’ s a bi g
hi nt her e f or how t o st udy
successf ul l y. When your goal i s t o
remember something, the best
way i s t o bui l d a neur al n et wor k.
That means mak i ng connect i ons
be t we en the material and other
things in your lifeexperiences,
family, everyday habits. Actively
engagi ng wi t h t he mat er i al wi l l
cr eat e neur al n et wor k s t o
hel p y ou r emember .
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56 // CHAPTER 2 // The Brain and Behavior
1. The part of the neuron that carries
information away from the cell body
toward other cells is the
A. dendrite.
B. synapse.
C. nucleus.
D. axon.
2. The law stating that once the electrical
impulse reaches its threshold, it fires
and moves down the axon without
losing intensity is called
A. neurotransmission.
B. the action potential.
C. the neural impulse.
D. the all-or-nothing principle.
3. The chemical substances that carry
information across the synaptic gap
tothe next neuron are called
A. neurotransmitters.
B. synapses.
C. endorphins.
D. hormones.
APPLY IT! 4. Many years ago some
researchers found that when people were
experiencing stressful, threatening
circumstances—in this case, getting a
painful electrical shock—they did not
“fight” and they did not “flee.” Instead,
they asked for a friend to sit by them
during the shocks. Which of the following
helps to explain this “misery loves
company” effect?
A. The participants were all men.
B. The participants were all women.
C. The participants had faulty autonomic
nervous systems.
D. The participants had serious psychologi-
cal disorders.
Of course, the human body’s extensive networks of neurons are not visible to the naked
eye. Fortunately technology is available to help neuroscientists form pictures of the
structure and organization of neurons and the larger structures they make up without
harming the organism being studied. This section explores techniques that scientists use
in brain research and discusses what these tools reveal about the brain’s structures and
functions. We pay special attention to the cerebral cortex, the region of the brain that is
most relevant to the topics in this book.
How Researchers Study the Brain and
Nervous System
Early knowledge of the human brain came mostly from studies of individuals who had
suffered brain damage from injury or disease or who had brain surgery to relieve another
condition. Modern discoveries have relied largely on technology that enables researchers
to “look inside” the brain while it is at work. Let’s examine some of these innovative
B R A I N L E S I O N I N G Brain lesioning is an abnormal disruption in the tissue of the
brain resulting from injury or disease. In a lab setting, neuroscientists produce lesions in
laboratory animals to determine the effects on the animal’s behavior (Hosp & others,
2011). They create the lesions by surgically removing brain tissue, destroying tissue with
a laser, or eliminating tissue by injecting it with a drug (Ho & others, 2011). Examining
the person or animal that has the lesion gives the researchers a sense of the function of
the part of the brain that has been damaged.
E L E C T R I C A L R E C O R D I N G T h e electroencephalograph (EEG) records the
brain’s electrical activity. Electrodes placed on the scalp detect brain-wave activity, which
is recorded on a chart known as an electroencephalogram (Figure 2.8). This device can
assess brain damage, epilepsy (a condition that produces seizures, caused by abnormal
electrical surges in the brain), and other problems (Rosenthal, 2012). Paul Ekman,
Structures of the Brain
and Their Functions
Brain Structures
and Functions
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Structures of the Brain and Their Functions // 57
Richard Davidson, and Wallace Friesen (1990) measured EEG activity
during emotional experiences provoked by lm clips. Individuals in
this study watched amusing lm clips (such as a puppy playing with
owers, and monkeys taking a bath) as well as clips likely to provoke
fear or disgust (a leg amputation and a third-degree burn victim). How
does the brain respond to such stimuli? The researchers found that
while watching the amusing clips, people tended to exhibit more left
than right prefrontal activity, as shown in EEGs. In contrast, when the
participants viewed the fear-provoking lms, the right prefrontal area
was generally more active than the left.
Do these differences generalize to overall differences in feelings of
happiness? They just might. Heather Urry and her colleagues (2004)
found that individuals who have relatively more left than right pre-
frontal activity (what is called prefrontal asymmetry ) tend to rate
themselves higher on a number of measures of well-being, including
self-acceptance, positive relations with others, purpose in life, and life
Not every recording of brain activity is made with surface elec-
trodes that are attached to the scalp. In single-unit recording, which
provides information about a single neuron’s electrical activity, a thin
probe is inserted in or near an individual neuron. The probe transmits
the neuron’s electrical activity to an ampli er so that researchers can
“see” the activity.
B R A I N I M A G I N G F o r y e a r s , m e d i c a l p r a c t i t i o n e r s h a v e u s e d
Xrays to reveal damage inside and outside the body, both in the brain
and in other locations. A single X ray of the brain is hard to interpret,
however, because it shows a two-dimensional image of the three-
dimensional interior of the brain. An improved technique, co m puterized axial tomography
(CAT scan or CT scan), produces a three-dimensional image obtained from X rays of
the head that are assembled into a composite image by a computer. The CT scan provides
valuable information about the location and extent of damage involving stroke, language
disorder, or loss of memory (Pasi, Poggesi, & Pantoni, 2011).
Positron-emission tomography (PET scan) is based on metabolic changes in the brain
related to activity. PET measures the amount of glucose in various areas of the brain and
then sends this information to a computer for analysis. Neurons use glucose for energy,
so glucose levels vary with the levels of activity throughout the brain. Tracing the
amounts of glucose generates a picture of activity levels throughout the brain.
An interesting application of the PET technique is the work of Stephen Kosslyn and
colleagues (1996) on mental imagery, the brain’s ability to create perceptual states in the
absence of external stimuli. For instance, if you were to think of your favorite song right
now, you could “hear” it in your mind’s ear; or if you re ected on your mother’s face, you
could probably “see” it in your mind’s eye. Research using PET scans has shown that often
the same area of the brain—a location called Area 17—is activated when we think of see-
ing something as when we are actually seeing it. However, Area 17 is not always activated
for all of us when we imagine a visual image. Kosslyn and his colleagues asked
their participants to visualize a letter in the alphabet and then asked those indi-
viduals to answer some yes or no questions about the letter. For instance, a
person might be thinking of the letter C and have to answer the question “Does
it have curvy lines?” The answer would be yes. If the person was thinking of
F, the answer would be no. The fascinating result of this work was that individu-
als who showed brain activation on the PET scan in Area 17 while engaged in the
visualization task answered the questions faster than those who were not using Area 17.
A n o t h e r t e c h n i q u e , magnetic resonance imaging (MRI), involves creating a magnetic
eld around a person’s body and using radio waves to construct images of the person’s
tissues and biochemical activities. The magnetic eld of the magnet used to create an
FIGURE 2.8 An EEG Recording
The electroencephalograph (EEG) is widely used
insleep research. It has led to some major
breakthroughs in understanding sleep by showing
how the brain’s electrical activity changes during
sleep. Do you know anyone who has experienced
a stroke or brain-damaging head injury? These
experiences create lesioned areas in the brain.
So, al t hough human
br ai ns ar e si mi l ar t o one anot her
in some ways, in ot her ways, all
br ai ns ar e uni que.
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58 // CHAPTER 2 // The Brain and Behavior
MRI image is over 50,000 times more powerful than the earth’s magnetic eld (Parry
& Matthews, 2002). MRI generates very clear pictures of the brain’s interior, does not
require injecting the brain with a substance, and (unlike Xrays) does not pose a prob-
lem of radiation overexposure (Nyberg, 2004). Getting an MRI scan involves lying still
in a large metal barrel-like tunnel. MRI scans provide an excellent picture of the archi-
tecture of the brain and allow us to see if and how experience affects brain structure.
In one MRI study, Katrin Amunts and colleagues (1997) documented a link between
the number of years a person has practiced musical skills (playing the piano, for exam-
ple) and the size of the brain region that is responsible for controlling hand movements.
Recently, MRI has been applied to the emerging neuroscience of personality. Person-
ality psychology (see Chapter 10) focuses on the ways individuals differ from one another
on dimensions such as extraversion and emotional stability. To read about work in this
area, check out the Intersection.
Although MRI reveals considerable information about brain structure, it cannot portray
brain function. Other techniques, however, can serve as a window on the brain in action
(Sperling, 2011). The newest such method, functional magnetic resonance imaging, or
fMRI, allows scientists literally to see what is happening in the brain while it is working
(Figure 2.9). Like the PET scan, fMRI rests on the idea that mental activity is associated
with changes in the brain. While PET is about the use of glucose as fuel for thinking, fMRI
exploits changes in blood oxygen that occur in association with brain activity. When part
pon meeting someone for
the rst time, probably one
of the things you notice is
whether the person is kind,
warm, and genuine (think of the nic-
est person you know) or hostile, cold,
mean, and antagonistic (think of
Simon Cowell from The X Factor and
America n Idol ). Personality psycholo-
gists refer to “niceness” as agree-
ableness , a trait that re ects the
tendency to be kind, altruistic, and
compassionate. Agreeable people
are cooperative rather than competi-
tive, and polite rather than rude.
When measured using questionnaire items such as “I try to be
courteous to everyone I meet” and “Most people I know like
me,agreeableness is associated with many positive personal
qualities, including altruism, honesty, and kindness (Graziano &
Tobin, 2009; Hall & others, 2010; MacDonald, Bore, & Munro,
2008). Might particular brain differences be associated with
Colin DeYoung and his colleagues (2010) investigated this
question. They asked 116 adults to complete questionnaires
measuring aspects of their personalities, including agreeable-
ness, and then used MRI scans
to examine whether personality
characteristics were related to struc-
tural differences in the brain. In this
study, the researchers speci cally
looked at the correlation between per-
sonality traits and volume in various
brain areas. They found that agree-
ableness was associated with greater
volume in the fusiform face area , a
dime-size location on the right hemi-
sphere (just above the ear) that
is believed to play a role in face
recognition (Wiese & others, 2012).
Agreeableness was also related to
greater volume in the posterior cingulate cortex, a brain area
associated with empathy and understanding other people’s beliefs
(Saxe & Powell, 2006).
What does this research tell us about personality and brain?
Does a persons brain structure make that individual nicer?
Remember that the brain is in uenced by experience. Just as cab
drivers developed different connections
in their brains after learning London’s
map, agreeable people might be creat-
ing nicer brains by behaving kindly.
Neuroscience and Personality: Are Some
Brains Nicer Than Others?
What kind of brain are
you creating in your typical
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Structures of the Brain and Their Functions // 59
of the brain is working, oxygenated blood rushes into the area. This
oxygen, however, is more than is needed. In a sense, fMRI is based
on the fact that thinking is like running sprints. When you run the
100-yard dash, blood rushes to the muscles in your legs, carrying
oxygen. Right after you stop, you might feel a tightness in your leg,
because the oxygen has not all been used. Similarly, if an area of the
brain is hard at work—for example, solving a math problem—the
increased activity leads to a surplus of oxygenated blood. This “extra
oxygen allows the brain activity to be imaged.
Getting an fMRI involves reclining in the same large metal bar-
rel as does an MRI, but in the case of fMRI, the person is active—
listening to audio signals sent by the researcher through headphones
or watching visual images that are presented on a screen mounted
overhead. Pictures of the brain are taken, both while the brain is at
rest and while it is engaging in an activity such as listening to music,
looking at a picture, or making a decision. By comparing the at-rest
picture to the active picture, fMRI reveals what speci c brain activ-
ity is associated with the mental experience being studied. fMRI
technology is one of the most exciting methodological advances to
hit psychology in a long time.
Note that saying that fMRI tells us about the brain activity asso-
ciated with a mental experience is a correlational statement. As we
saw in Chapter 1, correlations point to the association between vari-
ables, not to the potential causal link between them. Although, for example, identifying
a picture as a cat may relate to activation in a particular brain area, we do not know if
recognizing the cat caused the brain activity (Dien, 2009).
A n a d d i t i o n a l m e t h o d f o r s t u d y i n g b r a i n f u n c t i o n i n g , a n d o n e t h a t does allow
for causal inferences, is transcranial magnetic stimulation (TMS) (Lepage &
Theoret, 2010). First introduced in 1985 (Barker, Jalinous, & Freeston, 1985),
TMS is often combined with brain-imaging techniques to establish causal links
between brain activity and behavior, to examine neuronal functioning following
brain-injuring events such as accidents and strokes, and even to treat some neu-
rological and psychological disorders.
In the TMS procedure, magnetic coils are placed over the person’s head and directed
at a particular brain area. TMS uses a rapidly changing magnetic eld to induce brief
electric current pulses in the brain, and these pulses trigger action potentials in neurons
(Siebner & others, 2009). Immediately following this burst of action potentials, activity
in the targeted brain area is inhibited, causing what is known as a virtual lesion.
Completely painless, this technique, when used with brain imaging, allows
scientists to examine the role of various brain regions. If a brain region is
associated with a behavior, as demonstrated using fMRI or PET, then the
temporary disruption of processing in that area should disrupt that behavior
as well. So, for instance, if researchers were doing a study involving the cat
recognition example described above, they might use TMS to disrupt the brain area
that was associated with cat recognition and see whether the study’s participants are
temporarily unable to identify a picture of the feline.
How the Brain Is Organized
As a human embryo develops inside its mother’s womb, the nervous system begins form-
ing as a long, hollow tube on the embryo’s back. At 3 weeks or so after conception, cells
making up the tube differentiate into a mass of neurons, most of which then develop into
three major regions of the brain: the hindbrain, which is adjacent to the top part of the
spinal cord; the midbrain, which rises above the hindbrain; and the forebrain, which is
the uppermost region of the brain (Figure 2.10).
FIGURE 2.9 Functional Magnetic
Resonance Imaging (fMRI) Through fMRI,
scientists can literally see what areas of the brain
are active during a task by monitoring oxygenated
blood levels.
Sor r y, l ef t i es! Most
fMRI studies include only right-
handed peopl e. As we wi l l see l at er ,
handedness can i nf l uenc e br ai n
st r uct ur e.
I t sounds ki nda scar y, huh?
But i t s not . TMS i s al s o used
to treat some psychological
di s or der s.
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60 // CHAPTER 2 // The Brain and Behavior
Spinal cord
Development of the
Nervous System The
photograph shows the
primitive tubular appearance
of the nervous system at
6 weeks in the human embryo.
The drawing shows the major
brain regions and spinal cord
as they appear early in the
development of a human
Pituitary gland
Governs sleep and
Involved in memory
Spinal cord
Involved in fear and the
discrimination of objects
necessary for organism’s
Cerebral cortex
Extensive, wrinkled outer layer
of the forebrain; governs higher
brain functions, such as thinking,
learning, and consciousness
Governs eating, drinking,
and sex; plays a role in
emotion and stress
Rounded structure involved in
motor coordination
Reticular formation
Diffuse collection of neurons
involved in arousal and
stereotyped patterns, such
as walking
Medulla (green)
Governs breathing and
Relays information between
lower and higher brain centers
FIGURE 2.11 Structure and Regions in the Human Brain To get a feel for where these structures are in your own brain, use the eye
(pictured on the left of the gure) as a landmark. Note that structures such as the thalamus, hypothalamus, amygdala, pituitary gland, pons, and reticular
formation reside deep within the brain.
H I N D B R A I N The hindbrain, located at the skull’s rear, is the lowest portion of the
brain. The three main parts of the hindbrain are the medulla, cerebellum, and pons.
Figure 2.11 locates these structures.
The medulla begins where the spinal cord enters the skull. This structure controls
many vital functions, such as breathing and heart rate. It also regulates our re exes.
Located at the skull’s rear,
the lowest portion of the
brain, consisting of the me-
dulla, cerebellum, and pons.
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Structures of the Brain and Their Functions // 61
The cerebellum extends from the rear of the hindbrain, just above the medulla. It
consists of two rounded structures thought to play important roles in motor coordination
(Manto & others, 2012). Leg and arm movements are coordinated by the cerebellum, for
example. When we play golf, practice the piano, or learn a new dance, the cerebellum
is hard at work. If another portion of the brain commands us to write the number 7, it
is the cerebellum that integrates the muscular activities required to do so. Damage to the
cerebellum impairs the performance of coordinated movements. When this damage
occurs, people’s movements become awkward and jerky. Extensive damage to the cer-
ebellum makes it impossible even to stand up.
T h e pons i s a b r i d g e i n t h e h i n d b r a i n t h a t c o n n e c t s t h e c e r e b e l l u m a n d t h e b r a i n
stem. It contains several clusters of bers involved in sleep and arousal (Espana &
Scammell, 2011).
A r e g i o n c a l l e d t h e brain stem includes much of the hindbrain (it does not include the
cerebellum) and the midbrain (which we examine below) and gets its name because it looks
like a stem. Embedded deep within the brain, the brain stem connects with the spinal cord
at its lower end and then extends upward to encase the reticular formation in the midbrain.
The most ancient part of the brain, the brain stem evolved more than 500 million years
ago (Carter, 1998). Clumps of cells in the brain stem determine alertness and regulate basic
survival functions such as breathing, heartbeat, and blood pressure (Yeomans, 2012).
M I D B R A I N The midbrain, located between the hindbrain and forebrain, is an area
in which many nerve- ber systems ascend and descend to connect the higher and lower
portions of the brain (Ishikawa & others, 2012). In particular, the midbrain relays infor-
mation between the brain and the eyes and ears. The ability to attend to an object visu-
ally, for example, is linked to one bundle of neurons in the midbrain. Parkinson disease,
a deterioration of movement that produces rigidity and tremors, damages a section near
the bottom of the midbrain.
Two systems in the midbrain are of special interest. One is the reticular formation
(see Figure 2.11), a diffuse collection of neurons involved in stereotyped patterns of
behavior such as walking, sleeping, and turning to attend to a sudden noise. The other
system consists of small groups of neurons that use the neurotransmitters serotonin,
dopamine, and norepinephrine. Although these groups contain relatively few cells, they
send their axons to a remarkable variety of brain regions, an operation that perhaps
explains their involvement in complex, integrative functions.
F O R E B R A I N You try to understand what all of these terms and parts of the brain
mean. You talk with friends and plan a party for this weekend. You remember that it has
been 6 months since you went to the dentist. You are con dent you will do well on the
next exam in this course. All of these experiences and millions more would not be pos-
sible without the forebrain, the brain’s largest division and its most fo r ward part.
Before we explore the structures and function of the forebrain, let’s stop for a moment
and examine how the brain evolved. The brains of the earliest vertebrates were smaller and
simpler than those of later animals. Genetic changes during the evolutionary process were
responsible for the development of more complex brains with more parts and more inter-
connections (Raven & others, 2011). Figure 2.12 compares the brains of a rat, cat, chim-
panzee, and human. In both the chimpanzee’s brain and (especially) the human’s brain, the
hindbrain and midbrain structures are covered by a forebrain structure called the cerebral
cortex. The human hindbrain and midbrain are similar to those of other animals, so it is
the relative size of the forebrain that mainly differentiates the human brain from the brains
of animals such as rats, cats, and chimps. The human forebrain’s most important structures
are the limbic system, thalamus, basal ganglia, hypothalamus, and cerebral cortex.
Limbic System The limbic system, a loosely connected network of structures under
the cerebral cortex, is important in both memory and emotion (LeDoux, 2012). Its two
principal structures are the amygdala and the hippocampus (see Figure 2.11).
The amygdala is an almond-shaped structure located inside the brain toward the base.
In fact, there is an amygdala on each side of the brain. The amygdala is involved in the
brain stem
The stemlike brain area that
includes much of the hind-
brain (excluding the cerebel-
lum) and the midbrain;
connects with the spinal
cord at its lower end and
then extends upward to en-
case the reticular formation
in the midbrain.
Located between
the hindbrain
and forebrain,
anarea in which
many nerve-fi ber
systems ascend
and descend to
connect the
higher and lower
portions of the
brain; in particu-
lar, the midbrain
relays informa-
tion between the
brain and the
eyes and ears.
reticular formation
A system in the midbrain
comprising a diffuse collec-
tion of neurons involved in
stereotyped patterns of
behavior such as walking,
sleeping, and turning to
attend to a sudden noise.
The brain’s largest division
and its most forward part.
limbic system
A loosely connected network
of structures under the cere-
bral cortex, important in
both memory and emotion.
Its two principal structures
are the amygdala and the
An almond-
shaped structure
within the base
of the temporal
lobe that is in-
volved in the dis-
crimination of
objects that are
necessary for the
organism’s sur-
vival, such as
appropriate food,
mates, and social
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62 // CHAPTER 2 // The Brain and Behavior
discrimination of objects that are necessary for the organism’s survival, such as appropri-
ate food, mates, and social rivals. Neurons in the amygdala often re selectively at
the sight of such stimuli, and lesions in the amygdala can cause animals to engage
in inappropriate behavior such as attempting to eat, ght, or even mate with an
object like a chair. In this book, you will encounter the amygdala whenever
we investigate intense emotions such as fear and rage (Toyoda & others, 2011).
The amygdala also is involved in emotional awareness and expression through
its many connections with a variety of brain areas (Amano & others, 2011).
The hippocampus has a special role in the storage of memories (Bastin &
others, 2012). Individuals who suffer extensive hippocampal damage cannot retain
any new conscious memories after the damage. It is fairly certain, though, that memories
are not stored “in” the limbic system. Instead, the limbic system seems to determine
what parts of the information passing through the cortex should be “printed” into dura-
ble, lasting neural traces in the cortex.
Thalamus T h e thalamus i s a f o r e b r a i n s t r u c t u r e t h a t s i t s a t t h e t o p o f t h e b r a i n
stem in the central core of the brain (see Figure 2.11). It serves as an essential relay
station, functioning much like a server in a computer network. That is, an important
function of the thalamus is to sort information and send it to the appropriate places in
the forebrain for further integration and interpretation (Jia, Goldstein, & Harrison,
2009). For example, one area of the thalamus receives information from the cerebellum
and projects it to the motor area of the cerebral cortex. Indeed, most neural input to the
cerebral cortex goes through the thalamus. Whereas one area of the thalamus works to
orient information from the sense receptors (hearing, seeing, and so on), another region
seems to be involved in sleep and wakefulness, having ties with the reticular formation.
Basal Ganglia Above the thalamus and under the cerebral cortex lie large clusters,
or ganglia, of neurons called basal ganglia. T h e b a s a l g a n g l i a w o r k w i t h t h e c e r e b e l l u m
and the cerebral cortex to control and coordinate voluntary movements. Basal ganglia
enable people to engage in habitual behaviors such as riding a bicycle and typing a text
message. Individuals with damage to basal ganglia suffer from either unwanted move-
ment, such as constant writhing or jerking of limbs, or too little movement, as in the
The structure in
the limbic system
that has a special
role in the stor-
age of memories.
The forebrain structure that
sits at the top of the brain
stem in the brain’s central
core and serves as an im-
portant relay station.
basal ganglia
Large neuron clusters located
above the thalamus and un-
der the cerebral cortex that
work with the cerebellum
and the cerebral cortex to
control and coordinate vol-
untary movements.
Cerebral cortex
Brain stem
Cerebral cortex
Brain stem
Brain stem
Cerebral cortex
FIGURE 2.12 The Brain in Different Species This gure compares the brain of a rat, a cat, a chimpanzee, and a human being. As you
examine the illustrations, remember that each organism’s brain is adapted to meet different environmental challenges. > What structures are similar
across the species? > Why do you think there are some common features, and what does this commonality tell us about these brain structures?
> Why don’t rats have a large cerebral cortex? > How might life be different for a rat or a cat with a human brain?
Our amygdal ae
respond automatically to
st i mul i t o cut e puppi es, scar y
dogs, and at t r act i ve pot ent i al
romant ic part nerswit hout
our e ver not i ci ng .
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Structures of the Brain and Their Functions // 63
slow and deliberate movements of people with Parkinson disease (Lenglet & others,
2012; Starkstein, 2012).
Hypothalamus The hypothalamus, a small forebrain structure just below the thal-
amus, monitors three pleasurable activities—eating, drinking, and sex—as well as emo-
tion, stress, and reward (see Figure 2.11 for the location of the hypothalamus). As we
will see later, the hypothalamus also helps direct the endocrine system.
P e r h a p s t h e b e s t w a y t o d e s c r i b e t h e f u n c t i o n o f t h e h y p o t h a l a m u s i s a s a r e g u l a t o r
of the body’s internal state. It is sensitive to changes in the blood and neural input, and
it responds by in uencing the secretion of hormones and neural outputs. For example,
if the temperature of circulating blood near the hypothalamus is increased by just 1 or
2 degrees, certain cells in the hypothalamus start increasing their rate of ring. As
a result, a chain of events is set in motion. Increased circulation through the skin
and sweat glands occurs immediately to release this heat from the body. The
cooled blood circulating to the hypothalamus slows down the activity of some
of the neurons there, stopping the process when the temperature is just right—
37.1 degrees Celsius (98.6 degrees Fahrenheit). These temperature-sensitive
neurons function like a nely tuned thermostat in maintaining the body in a
balanced state.
T h e h y p o t h a l a m u s a l s o i s i n v o l v e d i n e m o t i o n a l s t a t e s , p l a y i n g a n i m p o r -
tant role as an integrative location for handling stress. Much of this integration
is accomplished through the hypothalamus’s action on the pituitary gland, an
important endocrine gland located just below it (Dorn & Biro, 2011).
I f c e r t a i n a r e a s o f t h e h y p o t h a l a m u s a r e e l e c t r i c a l l y s t i m u l a t e d , a f e e l i n g o f
pleasure results. In a classic experiment, James Olds and Peter Milner (1954)
implanted an electrode in the hypothalamus of a rat’s brain. When the rat ran to a
corner of an enclosed area, a mild electric current was delivered to its hypothalamus.
The researchers thought the electric current
would cause the rat to avoid the corner. Much
to their surprise, the rat kept returning to the
corner. Olds and Milner believed they had dis-
covered a pleasure center in the hypothalamus.
Olds (1958) conducted further experiments and
found that rats would press bars until they
dropped over from exhaustion just to continue
to receive a mild electric shock to their hypo-
thalamus. One rat pressed a bar more than
2,000 times an hour for a period of 24 hours
to receive the stimulation to its hypothalamus
(Figure2.13). Today researchers agree that the
hypothalamus is involved in pleasurable feel-
ings but that other areas of the brain, such as
the limbic system and a bundle of bers in the
forebrain, are also important in the link between
the brain and pleasure.
T h e O l d s s t u d i e s h a v e i m p l i c a t i o n s f o r d r u g
addiction. The rat pressed the bar mainly
because this action produced a positive,
rewarding effect (pleasure), not because it
wanted to avoid or escape a negative effect
(pain). Cocaine users talk about the drug’s
ability to heighten pleasure in food, sex, and a
variety of activities, highlighting the reward
aspects of the drug (Kalivas, 2007). We will
look into the effects of drugs on the brain’s
reward centers in Chapter 5.
A small forebrain
structure, located
just below the
thalamus, that
monitors three
eating, drinking,
and sex—as well
as emotion,
stress, and
FIGURE 2.13 Results of the Experiment on the Role of
the Hypothalamus in Pleasure The graphed results for one rat show
that it pressed the bar more than 2,000 times an hour for a period of 24
hours to receive stimulation to its hypothalamus.
6:00 P.M. 6:00 A.M.
Cumulative bar presses
Pl easur e cent er
receptors can become inactive
af t er t he us e of dr ugs s uch as
Ec s t as y an d me t h a mp h e t a mi n e .
The damagi ng ef f ect s of t hese dr ugs
on t he br ai n s r ewar d syst em ar e
what sedu c e i ndi vi d ual s i n t o a
hopel ess pur sui t of t he same
feelings they had during
their first highbut they
wi l l n ever f eel t hat
hi gh agai n.
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64 // CHAPTER 2 // The Brain and Behavior
The Cerebral Cortex
The cerebral cortex is part of the forebrain and is the most recently
developed part of the brain in the evolutionary scheme. The word cortex
means “bark” (as in the bark of a tree) in Latin, and the cerebral cortex
is in fact the outer layer of the brain. It is in the cerebral cortex that the
most complex mental functions, such as thinking and planning, take
The neocortex (or “new bark”) is the outermost part of the cerebral
cortex. In humans, this area makes up 80 percent of the cortex (compared
with just 30 to 40 percent in most other mammals). The size of the
neocortex in mammals is strongly related to the size of the social group
in which the organisms live. Some scientists theorize that this part of
the human brain, which is responsible for high-level thinking, evolved
so that we could understand one another (Dunbar & Schultz, 2007).
The neural tissue that makes up the cerebral cortex covers the lower
portions of the brain like a sheet that is laid over the brain’s surface. In
humans the cerebral cortex is greatly convoluted with lots of grooves
and bulges, and these considerably enlarge its surface area (compared
with a brain with a smooth surface). The cerebral cortex is highly con-
nected with other parts of the brain (Mesulam, 2012). Millions of axons connect the
neurons of the cerebral cortex with those located elsewhere in the brain.
L O B E S The wrinkled surface of the cerebral cortex is divided into two halves called
hemispheres (Figure 2.14). Each hemisphere is subdivided into four regions, or lobes
occipital, temporal, frontal, and parietal (Figure 2.15).
T h e occipital lobes, l o c a t e d a t t h e b a c k o f t h e h e a d , r e s p o n d t o v i s u a l s t i m u l i . C o n n e c -
tions among various areas of the occipital lobes allow for the processing of information
about such aspects of visual stimuli as their color, shape, and motion. A person can have
perfectly functioning eyes, but the eyes only detect and transport information. That informa-
tion must be interpreted in the occipital lobes for the viewer to “see it.A stroke or a wound
in an occipital lobe can cause blindness or wipe out a portion of the person’s visual eld.
cerebral cortex
Part of the fore-
brain, the outer
layer of the brain,
responsible for
the most complex
mental functions,
such as thinking
and planning.
The outermost
part of the cere-
bral cortex, mak-
ing up 80 percent
of the human
brain’s cortex.
occipital lobes
Structures lo-
cated at the back
of the head that
respond to visual
FIGURE 2.14 The Human
Brain’s Hemispheres The two halves
(hemispheres) of the human brain can be
seen clearly in this photograph.
Frontal lobe
Temporal lobe
Parietal lobe
Sensory association cortex
Visual association
Motor cortex
Auditory cortex
(mostly hidden from view)
association cortex
Motor association
Visual cortex
Somatosensory cortex
Lo be s o f th e B ra in Fu n ctio n a l Re g ion s Within the Lo b e s
FIGURE 2.15 The Cerebral Cortex’s Lobes and Association Areas The cerebral cortex ( left ) is roughly divided into four lobes:
occipital, temporal, frontal, and parietal. The cerebral cortex ( right ) also consists of the motor cortex and somatosensory cortex. Further, the cerebral cortex
includes association areas, such as the visual association cortex, auditory association cortex, and sensory association cortex.
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Structures of the Brain and Their Functions // 65
The temporal lobes, the part of the cerebral cortex just above the ears, are
involved in hearing, language processing, and memory. The temporal lobes
have a number of connections to the limbic system. For this reason, people
with damage to the temporal lobes cannot le experiences into long-term
T h e frontal lobes, the portion of the cerebral cortex behind the forehead, are
involved in personality, intelligence, and the control of voluntary muscles. A fascinat-
ing case study illustrates how damage to the frontal lobes can signi cantly alter per-
sonality. Phineas T. Gage, a 25-year-old foreman who worked for the Rutland and
Burlington Railroad, was the victim of a terrible accident in 1848. Phineas and several
co-workers were using blasting powder to construct a roadbed. The crew drilled holes
in the rock and gravel, poured in the blasting powder, and then tamped down the
powder with an iron rod. While Phineas was still tamping it down, the powder exploded,
driving the iron rod up through the left side of his face and out through the top of his
head. Although the wound in his skull healed in a matter of weeks, Phineas had become
a different person. Previously he had been a mild-mannered, hardworking, emotionally
calm individual, well liked by all who knew him. Afterward he was obstinate, moody,
irresponsible, sel sh, and incapable of participating in any planned activities. Damage
to the frontal lobe area of his brain had dramatically altered Phineass personality.
Without intact frontal lobes, humans are emotionally shallow, distractible, listless, and
so insensitive to social contexts that they may belch with abandon at dinner parties.
Individuals with frontal lobe damage become so distracted by irrelevant stimuli that they
often cannot carry out some basic directions. In one such case, an individual, when asked
to light a candle, struck a match correctly, but instead of lighting the candle, he put it
in his mouth and acted as if he were smoking it (Luria, 1973).
The frontal lobes of humans are especially large when compared with those of other
animals. For example, the frontal cortex of rats barely exists; in cats, it occupies a paltry
3.5 percent of the cerebral cortex; in chimpanzees, 17 percent; and in humans, approxi-
mately 30 percent.
An important part of the frontal lobes is the prefrontal cortex, which is at the front
of the motor cortex (see Figure 2.15) and is involved in higher cognitive functions such
as planning, reasoning, and self-control (Teffer & Semendeferi, 2012). Some neurosci-
entists refer to the prefrontal cortex as an executive control system because of its role in
monitoring and organizing thinking (Carlson, 2011; Diamond, 2013).
T h e parietal lobes, located at the top and toward the rear of the head, are involved
in registering spatial location, attention, and motor control (Ptak &
Schnider, 2011). Thus, the parietal lobes are at work when you are judg-
ing how far you have to throw a ball to get it to someone else, when
you shift your attention from one activity to another (turn your attention
away from the TV to a noise outside), and when you turn the pages of
this book. The brilliant physicist Albert Einstein said that his reasoning
often was best when he imagined objects in space. It turns out that his
parietal lobes were 15 percent larger than average (Witelson, Kigar, &
Harvey, 1999).
A word of caution is in order about going too far in localizing function
within a particular lobe. Although this discussion has attributed speci c
functions to a particular lobe (such as vision in the occipital lobe), there
are considerable integration and connection between any two or more lobes
and between lobes and other parts of the brain.
S O M A T O S E N S O R Y C O R T E X A N D M O T O R C O R T E X Two
other important regions of the cerebral cortex are the somatosensory cortex
and the motor cortex (see Figure 2.15). The somatosensory cortex pro-
cesses information about body sensations. It is located at the front of the
parietal lobes. The motor cortex, at the rear of the frontal lobes, processes
information about voluntary movement.
temporal lobes
Structures in the
cerebral cortex
that are located
just above the
ears and are
involved in hear-
ing, language
processing, and
frontal lobes
The portion of the
cerebral cortex
behind the fore-
head, involved in
personality, intel-
ligence, and the
control of volun-
tary muscles.
prefrontal cortex
An important part of the
frontal lobes that is involved
in higher cognitive functions
such as planning, reasoning,
and self-control.
parietal lobes
Structures at the
top and toward
the rear of the
head that are
involved in regis-
tering spatial lo-
cation, attention,
and motor
A region in the
cerebral cortex
that processes
about body sen-
sations, located
at the front of the
parietal lobes.
motor cortex
A region in the
cerebral cortex
that processes
about voluntary
movement, lo-
cated just behind
the frontal lobes.
The t empor al l obes i ncl ude
the fusiform face ar ea, descr i bed
in t he I nt ersect ion.
A computerized reconstruction of
Phineas T. Gage’s accident, based on
measurements taken of his skull.
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66 // CHAPTER 2 // The Brain and Behavior
The map in Figure 2.16 shows which parts of the somatosensory and motor
cortexes are associated with different parts of the body. It is based on research
done by Wilder Pen eld (1947), a neurosurgeon at the Montreal Neurological
Institute. Pen eld worked with patients who had severe epilepsy, and he often
performed surgery to remove portions of the epileptic patients’ brains. How-
ever, he was concerned that removing a portion of the brain might impair
some of the individuals’ functions. Pen eld’s solution was to map the cortex
during surgery by stimulating different cortical areas and observing the
responses of the patients, who were given a local anesthetic so they would
remain awake during the operation. He found that when he stimulated certain
somatosensory and motor areas of the brain, patients reported feeling differ-
ent sensations, or different parts of a patients body moved. For both somato-
sensory and motor areas, there is a point-to-point relation between a part of
the body and a location on the cerebral cortex. In Figure 2.16, the face and
hands are given proportionately more space than other body parts because the
face and hands are capable of ner perceptions and movements than are other
body areas and therefore need more cerebral cortex representation.
Motor Cortex Somatosensory Cortex
Motor cortex
Somatosensory cortex
fingers, and
fingers, and
Upper face
Teeth and
and pharynx
Upper leg
Lower leg
Lower leg
and toes
and toes
Frontal lobes
Parietal lobes
Top view of the brain
F IGURE 2.16 Disproportionate Representation of Body Parts in the Motor and Somatosensory Areas of the
Cortex The amount of cortex allotted to a body part is not proportionate to the body part’s size. Instead, the brain has more space for body parts that
require precision and control. Thus, the thumb, ngers, and hand require more brain tissue than does the arm.
Penf i el d’ s t echni que
has i nf l uence t o t hi s day.
Speci fical ly, during br ai n surger y,
pat i ent s ar e of t en awake. The
br ai n cannot f eel pai n, so k eepi ng
pat i ent s awake al lows surgeons
to ask about what they are feeling,
hear ing, and seei ng, t o be sure that
the surger y does not damage
br ai n ar eas t hat ar e cr uci al
for consciousness, speech,
and ot her i mp o r t a n t
funct i ons.
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Broca’s area
Wernicke’s area
FIGURE 2.17 Broca’s Area and
Wernicke’s Area Broca’s area is located in the
brain’s left hemisphere, and it is involved in the control
of speech. Individuals with damage to Broca’s area
have problems saying words correctly. Also shown is
Wernicke’s area, the portion of the left hemisphere that
is involved in understanding language. Individuals with
damage to this area cannot comprehend words; they
hear the words but do not know what they mean.
Structures of the Brain and Their Functions // 67
T h e p o i n t - t o - p o i n t m a p p i n g o f s o m a t o s e n s o r y elds
onto the cortexs surface is the basis of our orderly and
accurate perception of the world (Chen & others,
2011). When something touches your lip, for example,
your brain knows what body part has been touched
because the nerve pathways from your lip are the only
pathways that project to the lip region of the somato-
sensory cortex.
A S S O C I A T I O N C O R T E X Embedded in the
brains lobes, the association cortex makes up 75 per-
cent of the cerebral cortex (see Figure 2.15). Process-
ing information about sensory input and motor output
is not all that is taking place in the cerebral cortex. The
association cortex ( s o m e t i m e s c a l l e d association
areas) is the region of the cerebral cortex that inte-
grates this information. The highest intellectual functions, such as thinking and prob-
lem solving, occur in the association cortex.
Interestingly, damage to a speci c part of the association cortex often does not result
in a speci c loss of function. With the exception of language areas (which are localized),
loss of function seems to depend more on the extent of damage to the association cortex
than on the speci c location of the damage. By observing brain-damaged individuals and
using a mapping technique, scientists have found that the association cortex is involved
in linguistic and perceptual functioning.
The largest portion of the association cortex is located in the frontal lobes, directly
under the forehead. Damage to this area does not lead to somatosensory
or motor loss but rather to problems in planning and problem solving.
Personality also may be linked to the frontal lobes. Recall the mis-
fortune of Phineas Gage, whose personality radically changed
after he experienced frontal lobe damage.
The Cerebral Hemispheres
and Split-Brain Research
Recall that the cerebral cortex is divided into two halves—left and
right (see Figure 2.14). Do these hemispheres have different func-
tions? In 1861, French surgeon Paul Broca saw a patient who had
received an injury to the left side of his brain about 30 years ear-
lier. The patient became known as Tan because tan was the only word
he could speak. Tan suffered from aphasia, a language disorder asso-
ciated with brain damage. Tan died several days after Broca evaluated
him, and an autopsy revealed that the injury was to a precise
area of the left hemisphere. Today we refer to this area of the
brain as Broca’s area, and we know that it plays an important
role inthe production of speech. Another area of the brains
left hemisphere that significantly figures in language is
Wernickes area, which, if damaged, causes problems in com-
prehending language. Figure 2.17 locates Broca’s area and
Wernickes area. Today there continues to be considerable
interest in the degree to which the brain’s left hemisphere or
right hemisphere is involved in various aspects of thinking,
association cortex
Sometimes called associa-
tion areas, the region of the
cerebral cortex that is the
site of the highest intellec-
tual functions, such as think-
ing and problem solving.
Used by permission of The Funny Times.
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68 // CHAPTER 2 // The Brain and Behavior
Plane of cut
Corpus callosum
feeling, and behavior (Hauk & Pulvermuller, 2011;
Meng & others, 2012).
For many years, scientists speculated that the
corpus callosum, t h e l a r g e b u n d l e o f a x o n s
connecting the brain’s two hemispheres, has
something to do with relaying information
between the two sides (Figure 2.18). Roger
Sperry (1974) confirmed this in an experiment in
which he cut the corpus callosum in cats. He also
severed certain nerves leading from the eyes to the
brain. After the operation, Sperry
trained the cats to solve a series of
visual problems with one eye blind-
folded. After a cat learned the task—
say, with only its left eye uncoveredits
other eye was blindfolded, and the ani-
mal was tested again. Thesplit-brain
cat behaved as if it had never learned
the task. It seems that the memory was
stored only in the left hemisphere,
which could no longer directly com-
municate with the right hemisphere.
F u r t h e r e v i d e n c e o f t h e c o r p u s c a l l o -
sum’s function has come from studies of patients with severe, even life-threatening, forms
of epilepsy. Epilepsy is caused by electrical “brainstorms” that ash uncontrollably across
the corpus callosum. In one famous case, neurosurgeons severed the corpus callosum of
an epileptic patient, now known as W. J., in a nal attempt to reduce his unbearable
seizures. Sperry (1968) examined W. J. and found that the corpus callosum functions the
same in humans as in animals—cutting the corpus callosum seemed to leave the patient
with “two separate minds” that learned and operated independently.
As it turns out, the right hemisphere receives information only from the left side
of the body, and the left hemisphere receives information only from the right side of
the body. When you hold an object in your left hand, for example, only the right
hemisphere of your brain detects the object. When you hold an object in your right
hand, only the left hemisphere of the brain detects it (Figure 2.19). In individuals
with a normally functioning corpus callosum, both hemispheres receive this infor-
mation eventually, as it travels between the hemispheres through the corpus
callosum. In fact, although we might have two minds, we usually use them
in tandem.
You can appreciate how well and how rapidly the corpus callosum inte-
grates your experience by thinking about the challenge of doing two things
at once (Stirling, 2002). Recall, for example, when you were a kid and you
tried to tap your head and rub your stomach at the same time. Even with two
separate hands controlled by two separate hemispheres, such dual activity is
very dif cult.
H E M I S P H E R I C D I F F E R E N C E S I N F U N C T I O N I N G In people with
intact brains, specialization of function, or what is sometimes called lateraliza-
tion , occurs in some areas. Researchers have uncovered evidence for hemispheric
differences in function by sending different information to each ear. Remember, the
left hemisphere gets its information ( rst) from the right ear, and the right hemisphere
hears what is going on ( rst) in the left ear. This research has shown that the brain
corpus callosum
The large bundle
of axons that con-
nects the brain’s
two hemispheres,
responsible for
relaying informa-
tion between the
two sides.
FIGURE 2.18 Corpus
Callosum The corpus callosum is a
thick band of about 80 million axons
that connects the brain cells in one
hemisphere to those in the other. In
healthy brains, the two sides engage in
a continuous ow of information via
this neural bridge.
Fi gur e 2 . 19 i s t r i cky.
Not i ce t hat , un l i ke t he hand,
each eye i s act ual l y s pl i t i n hal f , s o
that half of the information that
each eye s ees goes t o a di f f er ent
hemi spher e. T he i n f or mat i on on t he
same si de of t he eye as t he nose
cr osses ov er , and t he i n f or mat i on
that is on the outside of each
eye s t ays put .
Split Brain
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Fixation point
Left visual field Right visual field
Main language
half field
half field
Optic nerve
Corpus callosum severed
tends to divide its functioning into one hemi-
sphere or the other as follows:
Left hemisphere: The most extensive
research on the brain’s two hemispheres
has focused on language. Speech and
grammar are localized to the left hemi-
sphere (Lazard, Collette, & Perrot, 2012).
Although it is a common misconception
that all language processing occurs in the
left hemisphere, much language processing
and production does come from this hemi-
sphere (Ibrahim & Eviatar, 2012). For exam-
ple, in reading, the left hemisphere comprehends
syntax (rules for combining words into phrases
and sentences) and grammar, but the right hemi-
sphere does not. The left hemisphere is also keenly
involved in singing the words of a song.
Right hemisphere: The right hemisphere dominates in
processing nonverbal information such as spatial percep-
tion, visual recognition, and emotion (Gainotti, 2012). For example, as we saw in the
Intersection, the fusiform face area in the right hemisphere is mainly at work when
we process information about people’s faces (Kanwisher, 2006).
Structures of the Brain and Their Functions // 69
FIGURE 2.19 Information Pathways
from the Eyes to the Brain Each of our eyes
receives sensory input from both our left and our right
eld of vision. Information from the left half of our visual
eld goes to the brain’s right hemisphere (which is
responsible for simple comprehension), and information
from the right half of our visual eld goes to the brain’s
left hemisphere (the brain’s main language center, which
controls speech and writing). The input received in either
hemisphere passes quickly to the other hemisphere
across the corpus callosum. When the corpus callosum
is severed, however, this transmission of information
cannot occur.
The brain’s left hemisphere is intricately involved in speech and language, and so it plays a role when we recall song lyrics. The fusiform face area of
the brain’s right hemisphere is dominant when we process information about people’s faces.
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70 // CHAPTER 2 // The Brain and Behavior
The right hemisphere also may be more involved than the left hemisphere in process-
ing information about emotions, both when we express emotions ourselves and when
we recognize others’ emotions (Carmona, Holland, & Harrison, 2009). People are
more likely to remember emotion words if they hear them in the left ear. Much
of our sense of humor resides in the right hemisphere (Marinkovic & others,
2011). In fact, if you want to be sure that someone laughs at your joke, tell it
to the person’s left ear.
The right hemisphere is also adept at interpreting story meanings and voice
intonations. Further, the right hemisphere excels at picking up a song melody.
Importantly, though, it is dif cult to learn exactly what the right hemisphere
can do, because it cannot just tell us. We have to come up with a way for the
right hemisphere to communicate what it knows. The right hemisphere certainly
has some verbal abilities, for instance, because people with split brains can draw
(with their left hand) pictures of words that have been spoken to them (in the
Because differences in the functioning of the brain’s two hemispheres are known to
exist, people commonly use the phrases left-brained (meaning logical and rational) and
right-brained (meaning creative or artistic) as a way of categorizing themselves and
others. Such generalizations have little scienti c basis, and that is a good thing.
We have both hemispheres because we use them both. Regardless of how much
fun it might be to label ourselves “right-brained” or “left-brained, we are
fortunate to be whole-brained, period. The reality is that most day-to-day
activities involve a complex interplay between the brain’s two hemispheres
(Abbassi & others, 2012; Ibrahim & Eviatar, 2012).
Integration of Function in the Brain
H o w d o a l l o f t h e r e g i o n s o f t h e b r a i n c o o p e r a t e t o p r o d u c e t h e w o n d r o u s c o m p l e x i t y
of thought and behavior that characterizes humans? Neuroscience still does not have
answers to questions such as how the brain solves a murder mystery or composes a
poem or an essay. Even so, we can get a sense of integrative brain function by using a
real-world scenario, such as the act of escaping from a burning building.
Imagine that you are sitting at your computer, writing an e-mail, when a re breaks
out behind you. The sound of crackling ames is relayed from your ear through the
thalamus, to the auditory cortex, and on to the auditory association cortex. At each stage,
the stimulus is processed to extract information, and at some stage, probably at the
association cortex level, the sounds are nally matched with something like a neural
memory representing sounds of res you have heard previously. The association re”
sets new machinery in motion. Your attention (guided in part by the reticular formation)
shifts to the auditory signal being held in your association cortex and on to your auditory
association cortex, and simultaneously (again guided by reticular systems) your head
turns toward the noise. Now your visual association cortex reports in: “Objects matching
ames are present. In other regions of the association cortex, the visual and auditory
reports are synthesized (“We have things that look and sound like re’), and neural
associations representing potential actions (“ ee’’) are activated. However, ring the neu-
rons that code the plan to ee will not get you out of the chair. The basal ganglia must
become engaged, and from there the commands will arise to set the brain stem, motor
cortex, and cerebellum to the task of transporting you out of the room. All of this hap-
pens in mere seconds.
W h i c h p a r t o f y o u r b r a i n d i d y o u u s e t o e s c a p e ? V i r t u a l l y a l l s y s t e m s h a d a r o l e . B y
the way, you would probably remember this event because your limbic circuitry would
The r i ght hemi spher e
is expert at recognizing faces.
Resear cher s have asked peopl e
to watch images on a computer
scr een and t o pr ess a but t on
when they see a face. Even right-
han ded peopl e ar e much f ast er
at t hi s t ask whe n t hey u se t hei r
left hand because t he informat ion
goes di rectly from the right
hemi spher e t o t he han d t hat
hemi spher e con t r ol s.
Coul d t hi s be why women,
even r i ght - handed women ( but not
me n ) , a u t o ma t i c a l l y c a r r y a b a b y i n
the left hand?
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The Endocrine System // 71
likely have started memory formation when the association rewas triggered. The
next time the sounds of crackling ames reach your auditory association cortex, the
associations triggered would include this most recent escape. In sum, considerable inte-
gration of function takes place in the brain (Rissman & Wagner, 2012; Squire & Wixted,
2011). All of the parts of the nervous system work together as a team to keep you safe
and sound.
1. Four ways that researchers study the
brain and the nervous system are elec-
trical recording, imaging, staining, and
A. biopsy.
B. lesioning.
C. lobotomy.
D. neurosurgery.
2. The brain’s three major regions are the
hindbrain, the midbrain, and the
A. brain stem.
B. reticular formation.
C. forebrain.
D. temporal lobes.
3. The most recently developed level of the
human brain is the
A. midbrain.
B. forebrain.
C. reticular formation.
D. brain stem.
A P P L Y I T ! 4. Because Miles suffers
from extreme seizures, a surgeon severs his
corpus callosum. Using a special technique,
researchers present a picture of a flower to
Miles’s right brain and a picture of a bum-
blebee to Miles’s left brain. When Miles is
asked to say out loud what he sees, he is
likely to answer
A. A fl ower.”
B. “I don’t know.”
C. A bee.”
D. There is no way to know.
The endocrine system consists of a set of glands that regulate the activities of certain
organs by releasing their chemical products into the bloodstream. Glands a r e o r g a n s o r
tissues in the body that create chemicals that control many bodily functions. Neurosci-
entists have discovered that the nervous system and endocrine system are intricately
interconnected. They know that the brain’s hypothalamus connects the nervous system
and the endocrine system and that the two systems work together to control the body’s
activities. Yet the endocrine system differs signi cantly from the nervous system in a
variety of ways. For one thing, the parts of the endocrine system are not all connected
in the way that the parts of the nervous system are. For another thing, the endocrine
system works more slowly than the nervous system, because the chemicals released by
the endocrine glands are transported through the circulatory system, in the blood. The
heart does a mind-boggling job of pumping blood through the body, but blood moves
far more slowly than neural impulses do.
The chemical messengers produce d by the endocrine glands are called hormones. T h e
bloodstream carries hormones to all parts of the body, and the membrane of every cell
has receptors for one or more hormones.
The endocrine glands consist of the pituitary gland, the thyroid and parathyroid
glands, the adrenal glands, the pancreas, the ovaries in women, and the testes in men
(Figure 2.20). In much the same way that the brain’s control of muscular activity is
constantly monitored and altered to suit the information received by the nervous system,
the action of the endocrine glands is continuously monitored and changed by nervous,
hormonal, and chemical signals (Enger, Ross, & Bailey, 2012). Recall from earlier in
the chapter that the autonomic nervous system regulates processes such as respiration,
heart rate, and digestion. The autonomic nervous system acts on the endocrine glands
endocrine system
The body system consisting
of a set of glands that regu-
late the activities of certain
organs by releasing their
chemical products into the
Organs or tissues
in the body that
create chemicals
that control many
of our bodily
Chemical mes-
sengers that are
produced by the
endocrine glands
and carried by
the bloodstream
to all parts of the
The Endocrine System
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72 // CHAPTER 2 // The Brain and Behavior
Thyroid gland
Parathyroid gland
Adrenal gland
(in females)
(in males)
Pituitary gland
to produce a number of important
physiological reactions to strong emo-
tions, such as rage and fear.
T h e pituitary gland, a pea-sized
gland just beneath the hypothalamus,
controls growth and regulates other
glands (Figure 2.21). The anterior
(front) part of the pituitary is known as
the mas ter gland because almost all of
its hormones direct the activity of tar-
get glands elsewhere. In turn, the
anterior pituitary gland is controlled
by the hypothalamus.
T h e adrenal glands, located at the
top of each kidney, regulate mood,
energy level, and the ability to cope
with stress. Each adrenal gland secretes
epinephrine (also called adrenaline) and
norepinephrine (also called noradrena-
line). Unlike most hormones, epinephrine
and norepinephrine act quickly. Epinephrine helps a person get ready for an emer-
gency by acting on smooth muscles, the heart, stomach, intestines, and sweat glands.
In addition, epinephrine stimulates the reticular formation, which in turn arouses
the sympathetic nervous system, and this system subse