Wayne Weiten

48 Pages·2008·3.78 MB·English

Weiten-Psych-7-SE 1/6/06 11:25 AM Page 1
There’s more to psychology than meets the eye. Weiten
p
s
y
c
h
o
What does this figure portray? Are you sure? If you counted your neurons—the cells in your
l
See page 140 to find out what you may have brain whose activity underlies everything that you
missed. do—at the rate of one per second, you’d be o
counting for 3000 years! See page 81.
s e v e n t h e d i t i o n
psychology
g
Psychology is an amazingly diverse field, with a legacy ThomsonNOW™ for Weiten’s Psychology:
of unity (its themes) as well as diversity (its variations). Themes and Variations, Seventh Edition
The process of research is applicable to so much in real This personalized online learning companion helps you y
life. In this text, you’ll study acrophobia (fear of heights)
focus your study time where you need it most. Get
and zoophobia (fear of animals), sleeplessness and the
started today at http://www.thomsonedu.com!
symbolic language of dreams. You’ll find out what
If an access card came with this text, you can start using
psychologists learnwhen they study behaviors such as
eating and fighting, and dig deeper to investigate how ThomsonNOW™right away. If not, just visit the site to t
hormones affect emotions and how the brain registers purchase electronic access. h
pain. You’ll explore broad social questions and personal,
Study Guide e
practical issues like the mysteries of love and the symp-
0-495-17034-8
toms of stress. Psychology is about youand yourlife— m
and it’safascinating, eye-opening experience. Organized around learning objectives, this guide reviews v
key ideas, people, and terms, complete with fill-in-the- a e
Also available to help you succeed in your blank, short-answer,and matching questions, as well as a
r s
psychology course self-quiz.
i
Art and Lecture Outlines a
PsykTrek 3.0: A Multimedia Introduction tt hh ee mm ee ss
0-495-17047-X
t
to Psychology
Focus moreeasily on lectures by taking notes directly on
i
Be part of the action as you boost your understanding these outlines of key ideas and prints of text artthat are
o
and have more fun on your journey through introduc- likely to be shown in class. vv aa rr ii aa tt ii oo nn ss
torypsychology.Icons in the text guide you to this n
interactive media experience, where you’ll explore Featured Studies Reader
s
psychological concepts and phenomena with animated 0-495-17039-9
graphics, video, and simulations that let you take an Investigate the full research articles cited in the “Featured
active role in experiments. You’ll even train a (virtual) Studies” sections of the main text, including explanatory seventh
rat! Available on CD-ROM (0-495-09035-2) or online. introductions and up to ten questions per article.
e d i t i o n
Wayne Weiten
Visit Wadsworth online at www.wadsworth.com
For your learning solutions:www.thomson.com/learning
SE/Weiten-Psychology-7 ISBN:0-495-09303-3 ©2007 Designer:Harasymczuk
Text and cover printer:Quebecor/Dubuque Binding:Case Trim:9" x 10.875" CMYK
3306_W_Weiten_FM 1/6/06 11:56 AM Page i
7
th E D I T I O N
Psychology
Themes and Variations
Wayne Weiten
University of Nevada, Las Vegas
Australia • Brazil • Canada • Mexico • Singapore • Spain
United Kingdom • United States
3306_W_Weiten_Ch03 1/5/06 7:18 AM Page 72
3
C H A P T E R
Communication in the Nervous System
Nervous Tissue: The Basic Hardware
The Neural Impulse: Using Energy to
Send Information
The Synapse: Where Neurons Meet
Neurotransmitters and Behavior
The Biological Bases
Organization of the Nervous System
The Peripheral Nervous System
The Central Nervous System
of Behavior Looking Inside the Brain:Research Methods
Electrical Recordings
Lesioning
Electrical Stimulation of the Brain
Transcranial Magnetic Stimulation
Brain-Imaging Procedures
FEATURED STUDY • Probing the Anatomy
of Sexual Arousal
The Brain and Behavior
The Hindbrain
The Midbrain
The Forebrain
The Plasticity of the Brain
Right Brain/Left Brain:Cerebral Laterality
Bisecting the Brain: Split-Brain Research
Hemispheric Specialization in the Intact Brain
The Endocrine System:Another Way
to Communicate
Heredity and Behavior:Is It All in the Genes?
Basic Principles of Genetics
Investigating Hereditary Influence: Research Methods
The Interplay of Heredity and Environment
The Evolutionary Bases of Behavior
Darwin’s Insights
Subsequent Refinements to Evolutionary Theory
Behaviors as Adaptive Traits
Reflecting on the Chapter’s Themes
PERSONAL APPLICATION • Evaluating the
Concept of “Two Minds in One”
Cerebral Specialization and Cognitive Processes
Complexities and Qualifications
CRITICAL THINKING APPLICATION • Building
Better Brains:The Perils of Extrapolation
The Key Findings on Neural Development
The Tendency to Overextrapolate
Recap
Practice Test
Corbis
© Royalty-Free/
3306_W_Weiten_Ch03 1/5/06 7:18 AM Page 73
If you have ever visited an aquarium, you may Organisms as diverse as humans and squid share
have encountered one of nature’s more captivat- many biological processes. However, their unique be-
ing animals: the octopus. Although this jellylike havioral capacities depend on the differences in their
mass of arms and head appears to be a relatively sim- physiological makeup. You and I have a larger reper-
ple creature, it is capable of a number of interesting toire of behaviors than the octopus in large part be-
behaviors. The octopus has highly developed eyes cause we come equipped with a more complex brain
that enable it to respond to stimuli in the darkness of and nervous system. The activity of the human brain
the ocean. When threatened, it can release an inky is so complex that no computer has ever come close
cloud to befuddle enemies while it makes good its es- to duplicating it. Your nervous system contains as
cape by a kind of rocket propulsion. If that doesn’t many cells busily integrating and relaying informa-
work, it can camouflage itself by changing color and tion as there are stars in our galaxy. Whether you are
texture to blend into its surroundings. Furthermore, scratching your nose or composing an essay, the ac-
the animal is surprisingly intelligent. In captivity, an tivity of those cells underlies what you do. It is little
octopus can learn, for example, to twist the lid off a wonder, then, that many psychologists have dedi-
jar with one of its tentacles to get at a treat inside. cated themselves to exploring the biological bases of
Despite its talents, there are many things an octo- behavior.
pus cannot do. An octopus cannot study psychology, How do mood-altering drugs work? Are the two
plan a weekend, dream about its future, or discover halves of the brain specialized to perform different
the Pythagorean theorem. Yet the biological processes functions? What happens inside the body when you
that underlie these uniquely human behaviors are feel a strong emotion? Are some mental illnesses the
much the same as the biological processes that en- result of chemical imbalances in the brain? To what
able an octopus to escape from a predator or forage extent is intelligence determined by biological in-
for food. Indeed, some of science’s most important heritance? These questions only begin to suggest the
insights about how the nervous system works came countless ways in which biology is fundamental to
from studies of a relative of the octopus, the squid. the study of behavior.
CCoommmmuunniiccaattiioonn iinn tthhee NNeerrvvoouuss SSyysstteemm
Imagine that you are watching a scary movie. As the your brain. In essence, your nervous system is a com- PREVIEW QUESTIONS
tension mounts, your palms sweat and your heart plex communication network in which signals are
What are the key parts of
beats faster. You begin shoveling popcorn into your constantly being transmitted, received, and inte- the neuron, and what are
mouth, carelessly spilling some in your lap. If some- grated. The nervous system handles information, their functions?
one were to ask you what you are doing at this mo- just as the circulatory system handles blood. In this How do glial cells con-
tribute to the functioning
ment, you would probably say, “Nothing—just watch- section, we take a close look at how communication
of the nervous system?
ing the movie.” Yet some highly complex processes occurs in the nervous system.
What is an action
are occurring without your thinking about them. A potential?
setyiems.u Alulms (otshte i nlisgthant tfarnoemo uthsley ,s ycoreuern b)r iasi nst irsi kinintegr pyoreutr- Nervous Tissue: Psyk Trek Htraonws mdoisessio sny ntaakpet icplace?
The Basic Hardware 22aa Which neurotransmitters
ing the light stimulus, and signals are flashing to other
regulate which aspects
parts of your body, leading to a flurry of activity. Your of behavior?
Your nervous system is living tissue composed of cells.
sweat glands are releasing perspiration, your heart-
The cells in the nervous system fall into two major
beat is quickening, and muscular movements are en-
categories: neurons andglia.
abling your hand to find the popcorn and, more or less
successfully, lift it to your mouth. Psyk Trek
Neurons 22aa
Even in this simple example, you can see that be-
havior depends on rapid information processing. In- Neuronsare individual cells in the nervous system
formation travels almost instantaneously from your that receive, integrate, and transmit information.
eyes to your brain, from your brain to the muscles of They are the basic links that permit communication
your arm and hand, and from your palms back to within the nervous system. The vast majority of them
The Biological Bases of Behavior 73
3306_W_Weiten_Ch03 1/5/06 7:18 AM Page 74
Dendrites
Soma
Myelin
Axon
sheath
Terminal buttons and synapses
Figure 3.1 communicate only with other neurons. However, a terial that encases some axons.The myelin sheath
Structure of the neuron. small minority receive signals from outside the ner- speeds up the transmission of signals that move along
Neurons are the communica- vous system (from sensory organs) or carry messages axons. If an axon’s myelin sheath deteriorates, its sig-
tion links of the nervous sys-
from the nervous system to the muscles that move nals may not be transmitted effectively. The loss of
tem. This diagram highlights
the key parts of a neuron, the body. muscle control seen with the disease multiple sclerosis
including specialized recep- A highly simplified drawing of two “typical” neu- is due to a degeneration of myelin sheaths (Schwartz
tor areas (dendrites), the cell
rons is shown in Figure 3.1. Actually, neurons come & Westbrook, 2000).
body (soma), the axon fiber
along which impulses are in such a tremendous variety of types and shapes that The axon ends in a cluster of terminal buttons,
transmitted, and the terminal no single drawing can adequately represent them. which are small knobs that secrete chemicals called
buttons, which release chem-
Trying to draw the “typical” neuron is like trying to neurotransmitters. These chemicals serve as mes-
ical messengers that carry
signals to other neurons. Neu- draw the “typical” tree. In spite of this diversity, the sengers that may activate neighboring neurons. The
rons vary considerably in size drawing in Figure 3.1highlights some common fea- points at which neurons interconnect are called syn-
and shape and are usually
tures of neurons. apses.A synapse is a junction where information
densely interconnected.
The soma, or cell body, contains the cell nucleus is transmitted from one neuron to another (synapse
and much of the chemical machinery common to is from the Greek for “junction”).
most cells (somais Greek for “body”). The rest of the To summarize, information is received at the den-
neuron is devoted exclusively to handling informa- drites, is passed through the soma and along the axon,
tion. The neurons in Figure 3.1 have a number of and is transmitted to the dendrites of other cells at
branched, feelerlike structures called dendritic trees meeting points called synapses. Unfortunately, this
(dendriteis a Greek word for “tree”). Each individual nice, simple picture has more exceptions than the
branch is a dendrite. Dendrites are the parts of a U.S. Tax Code. For example, some neurons do not
neuron that are specialized to receive information. have an axon, while others have multiple axons. Also,
Most neurons receive information from many other although neurons typically synapse on the dendrites
cells—sometimes thousands of others—and so have of other cells, they may also synapse on a soma or
extensive dendritic trees. an axon.
Web Link 3.1
From the many dendrites, information flows into
Neuropsychology Central the cell body and then travels away from the soma Glia
This content-rich site,
along the axon(from the Greek for “axle”). The axon Gliaare cells found throughout the nervous sys-
maintained by Professor
Jeffrey Browndyke of is a long, thin fiber that transmits signals away tem that provide various types of support for neu-
Louisiana State University, from the soma to other neurons or to muscles or rons.Glia (literally “glue”) tend to be much smaller
is dedicated to all aspects
glands. Axons may be quite long (sometimes several than neurons, but they outnumber neurons by about
of human neuropsychol-
ogy, from the perspectives feet), and they may branch off to communicate with 10 to 1, so glial cells appear to account for over 50%
of the experimental re- a number of other cells. of the brain’s volume. Among other things, glial cells
search laboratory as well
In humans, many axons are wrapped in cells with supply nourishment to neurons, help remove neu-
as the applied clinical set-
a high concentration of a white, fatty substance rons’ waste products, and provide insulation around
ting of the hospital and
professional office. called myelin.The myelin sheath is insulating ma- many axons. The myelin sheaths that encase some
74 C H A P T E R 3
3306_W_Weiten_Ch03 1/24/06 12:55 PM Page 75
axons are derived from special types of glial cells. ions inside the cell. The resulting voltage means that
Glia also play a complicated role in orchestrating the the neuron at rest is a tiny battery, a store of poten-
development of the nervous system in the human tial energy. The resting potential of a neuron is its
embryo. stable, negative charge when the cell is inactive. As
These functions, which have been known for shown in Figure 3.2(a), this charge is about (cid:1)70 milli-
many years, made glial cells the unsung heroes of the volts, roughly one-twentieth of the voltage of a flash-
nervous system. Until recently, it was thought that light battery.
the “glamorous” work in the nervous system—the Psyk Trek
transmission and integration of informational sig- The Action Potential 22aa
nals—was the exclusive province of the neurons. New
As long as the voltage of a neuron remains constant,
research, however, suggests that glia may also send
the cell is quiet, and no messages are being sent. When
and receive chemical signals (Fields, 2004; Fields &
the neuron is stimulated, channels in its cell mem-
Stevens-Graham, 2002). Some types of glia can de-
brane open, briefly allowing positively charged so-
tect neural impulses and send signals to other glial
dium ions to rush in. For an instant, the neuron’s
cells. Surprised by this discovery, neuroscientists are
charge is less negative, or even positive, creating an
now trying to figure out how this signaling system
action potential (Koester & Siegelbaum, 2000). An
interfaces with the neural communication system.
action potential is a very brief shift in a neuron’s
Although glia may contribute to information pro-
electrical charge that travels along an axon. The fir-
cessing in the nervous system, the bulk of this crucial
ing of an action potential is reflected in the voltage
work is handled by the neurons. Thus, we need to
spike shown in Figure 3.2 (b). Like a spark traveling
examine the process of neural activity in more detail.
along a trail of gunpowder, the voltage change races
down the axon.
TEhnee rNgeyu troa lS Iemnpdu Ilnsfeo:rUmsaintgio n Psyk Trek22aa nelAs fitner tthhee ficerilnl gm oefm abnr aacntei otnh apto toepnetniaeld, thtoe clehta nin-
sodium close up. Some time is needed before they
What happens when a neuron is stimulated? What
is the nature of the signal—the neural impulse—that Figure 3.2.
moves through the neuron? These were the questions The neural impulse. The electrochemical properties of the neuron allow it to transmit signals.
The electric charge of a neuron can be measured with a pair of electrodes connected to an oscillo-
that Alan Hodgkin and Andrew Huxley set out to an-
scope, as Hodgkin and Huxley showed with a squid axon. Because of its exceptionally thick axons,
swer in their groundbreaking experiments with axons the squid has frequently been used by scientists studying the neural impulse. (a)At rest, the neu-
removed from squid. Why did they choose to work ron’s voltage hovers around (cid:1)70 millivolts. (b)When a neuron is stimulated, a brief jump occurs in
the neuron’s voltage, resulting in a spike on the oscilloscope recording of the neuron’s electrical
with squid axons? Because the squid has a pair of
activity. This change in voltage, called an action potential, travels along the axon like a spark travel-
“giant” axons that are about a hundred times larger
ing along a trail of gunpowder.
than those in humans (which still makes them only
(a)
about as thick as a human hair). Their large size per-
mitted Hodgkin and Huxley to insert fine wires called
50
microelectrodes into them. By using the microelectrodes
to record the electrical activity in individual neurons, Inside
0
Hodgkin and Huxley unraveled the mystery of the electrode olts
v
neural impulse. Axon Milli –50
Outside
The Neuron at Rest: Psyk Trek electrode –100 Milliseconds
A Tiny Battery 22aa (b)
Hodgkin and Huxley (1952) learned that the neural
impulse is a complex electrochemical reaction. Both 50
inside and outside the neuron are fluids containing Inside
electrode
ePloescittriivceallyly c chhaarrggeedd saotdoimusm a nand dm pooletacsuslieusm ca ilolends iaonnsd. Axon + + – Millivolts –500
negatively charged chloride ions flow back and forth
Outside
across the cell membrane, but they do not cross at
electrode –100
the same rate. The difference in flow rates leads to a Milliseconds
slightly higher concentration of negatively charged
The Biological Bases of Behavior 75
3306_W_Weiten_Ch03 1/5/06 7:18 AM Page 76
are ready to open again, and until that time the neu- to read this description of the neural impulse, bil-
Web Link 3.2 ron cannot fire. The absolute refractory period is lions of such impulses have been transmitted in your
the minimum length of time after an action poten- nervous system!
Molecular Neurobiology:
A Gallery of Animations tial during which another action potential cannot
SNieteil eBduistiosr b arnindg ps htoysgiecitahner b2 emgiilnli.s eTchoins d“sd. oIwt ins ftoimlloew” eidsn b’ty vae bryri elof nregla, toivnel yre f1r aocr- TWhhee Srey nNaepusreo:ns Meet Psyk Trek22bb
a set of QuickTime anima-
tions demonstrating activi- tory period,during which the neuron can fire, but its
ties at the molecular level threshold for firing is elevated, so more intense stim-
In the nervous system, the neural impulse functions
of the synapse, such as
ulation is required to initiate an action potential.
the fusion of synaptic vesi- as a signal. For that signal to have any meaning for
cmleesm wbirtahn teh.e presynaptic The All-or-None Law Psyk Trek22aa tthhee nsyesutreomn atos oa twhehro cleel,l sit. Amsu nsot tbeed teraarnlisemr, itthteisd t frraonms-
The neural impulse is an all-or-none proposition, like mission takes place at special junctions called synapses,
firing a gun. You can’t half-fire a gun. The same is true which depend on chemicalmessengers.
of the neuron’s firing of action potentials. Either the
neuron fires or it doesn’t, and its action potentials Sending Signals: Psyk Trek
are all the same size (Kandel, 2000). That is, weaker Chemicals as Couriers 22bb
stimuli do not produce smaller action potentials. A “typical” synapse is shown in Figure 3.3. The first
Even though the action potential is an all-or- thing that you should notice is that the two neurons
nothing event, neurons can convey information about don’t actually touch. They are separated by the syn-
the strength of a stimulus. They do so by varying the aptic cleft, a microscopic gap between the termi-
rateat which they fire action potentials. In general, a nal button of one neuron and the cell membrane
stronger stimulus will cause a cell to fire a more rapid of another neuron. Signals have to jump this gap to
volley of neural impulses than a weaker stimulus will. permit neurons to communicate. In this situation,
Various neurons transmit neural impulses at dif- the neuron that sends a signal across the gap is called
ferent speeds. For example, thicker axons transmit the presynaptic neuron,and the neuron that receives the
neural impulses more rapidly than thinner ones do. signal is called the postsynaptic neuron.
Although neural impulses do not travel as fast as How do messages travel across the gaps between
electricity along a wire, they arevery fast, moving at neurons? The arrival of an action potential at an
up to 100 meters per second, which is equivalent to axon’s terminal buttons triggers the release of neuro-
more than 200 miles per hour. The entire, compli- transmitters—chemicals that transmit informa-
cated process of neural transmission takes only a few tion from one neuron to another. Within the but-
thousandths of a second. In the time it has taken you tons, most of these chemicals are stored in small sacs,
concept check 3.1
Understanding Nervous System Hardware Using Metaphors
A useful way to learn about the structures and functions of parts of the nervous system is through metaphors.
Check your understanding of the basic components of the nervous system by matching the metaphorical
descriptions below with the correct terms in the following list: (a) glia, (b) neuron, (c) soma, (d) dendrite,
(e) axon, (f) myelin, (g) terminal button, (h) synapse. You’ll find the answers in Appendix A.
_______ 1. Like a tree. Also, each branch is a telephone wire that carries incoming messages to you.
_______ 2. Like the insulation that covers electrical wires.
_______ 3. Like a silicon chip in a computer that receives and transmits information between input and output
devices as well as between other chips.
_______ 4. Like an electrical cable that carries information.
_______ 5. Like the maintenance personnel who keep things clean and in working order so the operations of
the enterprise can proceed.
_______ 6. Like the nozzle at the end of a hose, from which water is squirted.
_______ 7. Like a railroad junction, where two trains may meet.
76 C H A P T E R 3
3306_W_Weiten_Ch03 1/5/06 7:18 AM Page 77
called synaptic vesicles. The neurotransmitters are re-
leased when a vesicle fuses with the membrane of
the presynaptic cell and its contents spill into the
synaptic cleft. After their release, neurotransmitters
diffuse across the synaptic cleft to the membrane of
the receiving cell. There they may bind with special
molecules in the postsynaptic cell membrane at vari-
ous receptor sites. These sites are specifically “tuned”
to recognize and respond to some neurotransmitters
but not to others.
Receiving Signals: Psyk Trek A(pxroens yonf aspetnicd)i nngeuron
Postsynaptic Potentials 22bb
When a neurotransmitter and a receptor molecule
Neural impulse
combine, reactions in the cell membrane cause a post-
synaptic potential (PSP), a voltage change at a re- Synaptic vesicles
ceptor site on a postsynaptic cell membrane. Post- containing
neurotransmitter
synaptic potentials do not follow the all-or-none law
as action potentials do. Instead, postsynaptic poten- Terminal
button
tials are graded. That is, they vary in size and they in-
Neurotransmitter
crease or decrease the probability of a neural impulse molecules
in the receiving cell in proportion to the amount of
voltage change.
Two types of messages can be sent from cell to cell:
excitatory and inhibitory. An excitatory PSP is a pos-
itive voltage shift that increases the likelihood that
the postsynaptic neuron will fire action potentials.
Synaptic
An inhibitory PSP is a negative voltage shift that cleft
decreases the likelihood that the postsynaptic neu-
ron will fire action potentials. The direction of the
voltage shift, and thus the nature of the PSP (excita- Cell membrane
tory or inhibitory), depends on which receptor sites of receiving
(postsynaptic)
are activated in the postsynaptic neuron (Kandel,
Receptor sites neuron
2000).
The excitatory or inhibitory effects produced at a Transmitter fits receptor Transmitter does not fit at
synapse last only a fraction of a second. Then neuro- site, binds to cell membrane, receptor site, cannot bind
transmitters drift away from receptor sites or are in- producing postsynaptic to cell membrane
potential
activated by enzymes that metabolize (convert) them
into inactive forms. Most are reabsorbed into the
presynaptic neuron through reuptake,a process in
which neurotransmitters are sponged up from the
simply relay messages it receives. It must integrate Figure 3.3
synaptic cleft by the presynaptic membrane. Re-
signals arriving at many synapses before it “decides” The synapse. When a
uptake allows synapses to recycle their materials.
whether to fire a neural impulse. If enough excitatory neural impulse reaches an
Reuptake and the other key processes in synaptic axon’s terminal buttons, it
PSPs occur in a neuron, the electrical currents can
transmission are summarized in Figure 3.4on the triggers the release of chemi-
add up, causing the cell’s voltage to reach the thresh- cal messengers called neuro-
next page.
old at which an action potential will be fired. How- transmitters. The neurotrans-
mitter molecules diffuse across
ever, if many inhibitory PSPs also occur, they will
Integrating Signals: Psyk Trek tend to cancel the effects of excitatory PSPs. Thus, ttho er escyenpatpotric s ictleesf to ann tdh eb ipnods t-
Neural Networks 22bb the state of the neuron is a weighted balance between synaptic neuron. A specific
neurotransmitter can bind
A neuron may receive a symphony of signals from excitatory and inhibitory influences (Kandel & Siegel-
only to receptor sites that its
thousands of other neurons. The same neuron may baum, 2000). molecular structure will fit
pass its messages along to thousands of neurons as As Rita Carter (1998) has pointed out in Mapping into, much like a key must
fit a lock.
well. Thus, a neuron must do a great deal more than the Mind, “The firing of a single neuron is not enough
The Biological Bases of Behavior 77
3306_W_Weiten_Ch03 1/5/06 7:18 AM Page 78
Figure 3.4
Overview of synaptic
transmission. The main
elements in synaptic trans-
mission are summarized here, 1Synthesis and storage of
superimposed on a blowup 5Reuptake of neurotransmitters
neurotransmitter molecules in
of the synapse seen in Figure sponged up by the presynaptic
synaptic vesicles
3.3. The five key processes neuron
involved in communication
at synapses are (1) synthe-
sis and storage, (2) release,
(3) binding, (4) inactivation
or removal, and (5) reuptake
of neurotransmitters. As you’ll
see in this chapter and the 2Release of neurotransmitter 4Inactivation (by enzymes) or
remainder of the book, the molecules into synaptic cleft removal (drifting away) of
effects of many phenom- neurotransmitters
ena—such as pain, drug use,
and some diseases—can be
explained in terms of how
they alter one or more of
these processes (usually at
synapses releasing a specific
neurotransmitter).
3Binding of neurotransmitters
at receptor sites on
postsynaptic membrane
to create the twitch of an eyelid in sleep, let alone a be made while some old connections whither away
conscious impression. . . . Millions of neurons must (Hua & Smith, 2004).
fire in unison to produce the most trifling thought” Ironically, the elimination of old synapsesappears
(p. 19). Most neurons are interlinked in complex to play a larger role in the sculpting of neural net-
chains, pathways, circuits, and networks. Our percep- works than the creation of new synapses.The nervous
tions, thoughts, and actions depend on patterns of system normally forms more synapses than needed
neural activity in elaborate neural networks. These and then gradually eliminates the less-active syn-
networks consist of interconnected neurons that fre- apses. For example, the number of synapses in the
quently fire together or sequentially to perform cer- human visual cortex peaks at around age one and
tain functions (Song et al., 2005). The links in these then declines, as diagrammed in Figure 3.5(Hutten-
networks are fluid, as new synaptic connections may locher, 1994). Thus, synaptic pruning is a key process
Figure 3.5
in the formation of the neural networks that are cru-
Synaptic pruning. This
cial to communication in the nervous system.
graph summarizes data on 24
the estimated number of syn-
apses in the human visual
c(Horuttetxe nalso cah feurn, c1t9io9n4 )o.f Aasg e 20 Neurotransmitters Psyk Trek
and Behavior 22bb,,44dd
you can see, the number of
synapses in this area of the 16
brain peaks around age 1 and 110)
tchoeunr sme oosf ttlyh ed elicfeli nsepsa onv. eTrh tihse mber of(cid:2)es (112 Aicsa lw ceo huarvieer ss eteon c, othmem nuernviocautse s yinstfeomrm raetliieosn o bne ctwheemen-
decline reflects the process Nuaps neurons. These neurotransmitters are fundamental to
n
ofsynaptic pruning,which sy 8 behavior, playing a key role in everything from mus-
involves the gradual elimina-
tion of less active synapses. cle movements to moods and mental health.
4 You might guess that the nervous system would
Source:Data based on Huttenlocher,
P. R. (1994). Synaptogenesis in human require only two neurotransmitters—one for excita-
cerebral cortex. In G. Dawson & K. W.
Fischer (Eds.), Human behavior and the 0 tory potentials and one for inhibitory potentials. In
developing brain. New York: Guilford 7 2 4 6 81012 2 5 10 20 30 50 70
reality, there are nine well-established, classic (small-
Press. Graphic adapted from Kolb, B. & Months
Months Years
Whishaw, I. Q. (2001). An introduction gestation molecule) transmitters, about 40 additional neuro-
to brain and behavior. New York: Worth
Publishers. Birth peptide chemicals that function, at least part-time,
78 C H A P T E R 3
3306_W_Weiten_Ch03 1/5/06 7:18 AM Page 79
as neurotransmitters, and a variety of recently recog- Table 3.1 Common Neurotransmitters and Some of Their Functions
nized “novel” neurotransmitters (Schwartz, 2000;
Neurotransmitter Functions and Characteristics
Snyder, 2002). As scientists continue to discover new
Acetylcholine Released by motor neurons controlling skeletal muscles
and increasingly diverse transmitter substances, they
(ACh) Contributes to the regulation of attention, arousal, and memory
are being forced to reevaluate their criteria regarding Some ACh receptors stimulated by nicotine
what qualifies as a neurotransmitter (Snyder & Fer-
Dopamine Contributes to control of voluntary movement, pleasurable emotions
ris, 2000). (DA) Decreased levels associated with Parkinson’s disease
Specific neurotransmitters work at specific kinds Overactivity at DA synapses associated with schizophrenia
Cocaine and amphetamines elevate activity at DA synapses
of synapses. You may recall that transmitters deliver
their messages by binding to receptor sites on the post- Norepinephrine Contributes to modulation of mood and arousal
(NE) Cocaine and amphetamines elevate activity at NE synapses
synaptic membrane. However, a transmitter cannot
bind to just any site. The binding process operates Serotonin Involved in regulation of sleep and wakefulness, eating, aggression
Abnormal levels may contribute to depression and obsessive-compulsive
much like a lock and key, as was shown in Figure 3.3.
disorder
Just as a key has to fit a lock to work, a transmitter has Prozac and similar antidepressant drugs affect serotonin circuits
to fit into a receptor site for binding to occur. As a re-
GABA Serves as widely distributed inhibitory transmitter
sult, specific transmitters can deliver signals only at
Valium and similar antianxiety drugs work at GABA synapses
certain locations on cell membranes.
Endorphins Resemble opiate drugs in structure and effects
Why are there many neurotransmitters, each of
Contribute to pain relief and perhaps to some pleasurable emotions
which works only at certain synapses? This variety
and specificity reduces crosstalk between densely
packed neurons, making the nervous system’s com-
munication more precise. Let’s briefly review some as antagonists. An antagonist is a chemical that op-
of the most interesting findings about how neuro- poses the action of a neurotransmitter. For exam-
transmitters regulate behavior, which are summa- ple, the drug curare is an ACh antagonist. It blocks
rized in Table 3.1. action at the same ACh synapses that are fooled by
Psyk Trek nicotine. As a result, muscles are unable to move.
Acetylcholine 22bb
Some South American natives put a form of curare
The discovery that cells communicate by releasing on arrow tips. If they wound an animal, the curare
chemicals was first made in connection with the blocks the synapses from nerve to muscle, paralyzing
transmitter acetylcholine (ACh). ACh has been found the animal.
throughout the nervous system. It is the only trans- Psyk Trek
Monoamines 22bb,,44dd
mitter between motor neurons and voluntary mus-
cles. Every move you make—typing, walking, talking, The monoamines include three neurotransmitters:
breathing—depends on ACh released to your mus- dopamine, norepinephrine, and serotonin. Neurons
cles by motor neurons (Kandel & Siegelbaum, 2000). using these transmitters regulate many aspects of
ACh also appears to contribute to attention, arousal, everyday behavior. Dopamine (DA), for example, is
and perhaps memory. used by neurons that control voluntary movements.
The activity of ACh (and other neurotransmitters) The degeneration of such neurons apparently causes
may be influenced by other chemicals in the brain. Parkinsonism, a disease marked by tremors, muscular
Although synaptic receptor sites are sensitive to rigidity, and reduced control over voluntary move-
specific neurotransmitters, sometimes they can be ments (DeLong, 2000).
“fooled” by other chemical substances. For example,
if you smoke tobacco, some of your ACh synapses
will be stimulated by the nicotine that arrives in your
Muhammed Ali and Michael
brain. At these synapses, the nicotine acts like ACh
J. Fox are two well-known
itself. It binds to receptor sites for ACh, causing post- victims of Parkinson’s dis-
synaptic potentials (PSPs). In technical language, ease. Roughly one million
Americans suffer from Park-
nicotine is an ACh agonist. An agonist is a chemi- inson’s disease, which is
cal that mimics the action of a neurotransmitter. caused by a decline in the
synthesis of the neurotrans-
atog opNnrooistdt aus.lc lSe co ahm ePmeS Pcic h(atelhms eti hkcaaetlys f sobloiidnl edssy tinona trpoet citceh prete olcoercsp ktb,o ubrtsu fata riietl World Photos mtoicoicntut erirsn dbdoeocppaaaummseiin noeef. tsThyhene td hreeetdseiursico--
dacecopteitosonnr’ tso ifwt etosh,re kr e)n.na Idtnue rreianflf get rctathn, estmmhe iuyttn etuer smbaybp looecr. acTruhipluyys i,bn tlgoh ceiktys t arhceet- © 2002 AP/Wide rina ttihone mofi dab srtariunc.ture located
The Biological Bases of Behavior 79