Science Brain and Behavior contiuned 4

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28 ! CHAPTER 1
Given these questions, it is not surprising that brain size and intelligence within
the human species, and between the sexes for that matter, do not seem particularly related.
The brains of people who virtually everyone agrees are very intelligent have been
found to vary in size from the low end to the high end of the range for our species. The
brilliant physicist Albert Einstein had a brain of average size. Women’s brains weigh
about 10 percent less than men’s brains on average, roughly equivalent to the average
difference in female and male body size, but the two sexes do not differ in measures of
average intelligence.
The lack of correlation between brain size and intelligence within a single species
is not limited to humans. Figure 1-18 plots the average brain size in different breeds of
dogs against each breed’s level of intelligence as ranked by dog experts. The brain sizes,
which range from less than 50 grams to nearly 130 grams, were not adjusted for the
breeds’ body sizes, because such adjustments are not made in studies of humans. The
results are all over the map, indicating no relation at all between the overall size of a
breed’s brain and that breed’s intelligence ranking.
Differences of some kind must exist in the brains of individual persons because
people differ in behavior and talents. Neuroscientists are not yet sure what structural
or functional measures are related to behavioral traits.Gross brain size is not very likely
to be one of them, however. Researchers who study this question believe that measures
of the relative size and function of particular brain regions will prove more helpful.
Culture
The most remarkable thing that our brains have allowed us to develop is an extraordinarily
rich culture, the complex learned behaviors passed on from generation to generation.
Here is a list, in alphabetical order, of major categories of behavior that are part
of human culture:
Age-grading, athletic sports, bodily adornment, calendar [use], cleanliness
training, community organization, cooking, cooperative labor, cosmology,
courtship, dancing, decorative art, divination, division of labor, dream interpretation,
education, eschatology, ethics, ethnobotany, etiquette, faith healing,
family feasting, fire making, folklore, food taboos, funeral rites, games,
gestures, gift giving, government, greetings, hair styles, hospitality, housing,
hygiene, incest taboos, inheritance rules, joking, kin groups, kinship nomenclature,
language, law, luck, superstitions, magic, marriage, mealtimes, medicine,
obstetrics, penal sanctions, personal names, population policy, postnatal
Brain weight (in grams)
40 60 80 100 120 140
100
Pekinese
Bulldog
Old English
sheepdog St. Bernard
Miniature
schnauzer
Labrador
retriever
Standard
poodle
80
60
40
20
0
(Higher- Intelligence
ranked)
(Lowerranked)
Dachshund
Figure 1-18
Brain Weight and Intelligence in Dogs
A within-species comparison of
intelligence rankings and brain size
among breeds of dogs yields a
correlation of 0.009, which indicates no
relationship at all. A correlation of 1.0
indicates a perfectly positive or negative
relationship. Intelligence rankings are from
The Intelligence of Dogs, by S. Coren, 1994,
Toronto: The Free Press. Size measures are from
“Brain Weight–Body Weight Scaling in Dogs and
Cats,” by R. T. Bronson, 1979, Brain, Behavior and
Evolution, 16, 227–236.
Culture. Learned behaviors that are
passed on from one generation to the
next through teaching and learning.
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WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 29
care, pregnancy usages, property rights, propitiation of supernatural beings,
puberty customs, religious ritual, residence rules, sexual restrictions, soul concepts,
status differentiation, surgery, tool making, trade, visiting, weaving, and
weather control. (Murdock, 1965)
Not all the items in this list are unique to humans.Many other animal species display
elements of some of these behaviors. For example,many other animals display age
grading (any age-related behavior or status), courtship behavior, rudimentary tool use,
and elements of language. Despite such behavioral similarities across species, humans
clearly have progressed much farther in the development of culture than other animals
have. For humans, every category of activity on Murdock’s list requires extensive learning
from other members of the species, and exactly how each behavior is performed
can differ widely from one group of people to another.
A human brain must function adequately to acquire these complex cultural skills.
When its functioning is inadequate, a person may be unable to learn even basic elements
of culture. “Learning Disabilities” describes how incapacitating it can be to have
a brain that has difficulty in learning to read.
Because of steady growth in cultural achievements, the behavior of Homo sapiens
today is completely unlike that of Homo sapiens living 100,000 years ago. The earliest
surviving art, such as carvings and paintings, dates back only some 30,000 years; agriculture
appears still more recently, about 10,000 to 15,000 years ago; and reading and
writing, the foundations of our modern literate and technical societies, were invented
only about 7000 years ago.
St.Ambrose, who lived in the fourth century, is reported to be the first person who
could read silently. Most forms of mathematics, another basis of modern technology,
were invented even more recently than reading and writing were. And many of our
skills in using mechanical devices are still more recent in origin.
Learning Disabilities
Focus on Disorders
Children absorb their society’s culture, and acquiring language
skills seems virtually automatic for most. Yet some people
have lifelong difficulties in mastering language-related
tasks, difficulties classified by educators under the umbrella
of learning disabilities.
Perhaps the most common learning disability is impairment
in learning to read, or dyslexia (from the Latin dys,
meaning “poor,” and lexia , meaning “reading”). Not surprisingly,
children with dyslexia have difficulty learning to write
as well as to read. In 1895, James Hinshelwood, an eye surgeon,
examined some schoolchildren who were having
reading problems, but he could find nothing wrong with
their vision. Hinshelwood was the first person to suggest that
these children were impaired in brain areas associated with
the use of language.
In more recent times, Norman Geshwind and Albert
Galaburda (1985) proposed how such impairment might
come about. These researchers were struck by the finding
that dyslexia is far more common in boys than in girls. Perhaps,
they reasoned, excessive amounts of the hormone
testosterone, which produces male physical characteristics
early in development, might also produce abnormal development
in language areas of the brain. Pursuing this hypothesis,
they examined postmortem the brains of a small
sample of people who had experienced dyslexia. They found
abnormal collections of neurons, or “warts,” in the language
areas of the left hemisphere. This relation between structural
abnormalities in the brain and learning difficulties is further
evidence that an intact brain is necessary for normal human
functioning.
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30 ! CHAPTER 1
These examples highlight a remarkable feature of the modern human brain: it performs
so many tasks in our modern world that were not directly selected for in our early
hominid evolution. The brains of early Homo sapiens certainly did not evolve to help
program computers or travel to distant planets. And yet the same brains are capable of
both these complex tasks and more. Apparently, the things that the human brain did
evolve to do contained all the elements necessary for adapting to far more sophisticated
skills. Thus, the human brain evolved a capacity allowing it to be highly flexible in accommodating
the variety of knowledge and achievements of modern culture.
The acquisition of complex culture was a gradual, step-by-step process, with one
achievement leading to another. Among our closest relatives, chimpanzees also have
culture in the sense that some groups display tool-using skills that others have not acquired.
In her book titled The Chimpanzees of Gombe, primatologist Jane Goodall describes
the process by which symbolic concepts, a precursor of language, might have
developed in chimpanzees. She uses the concept of “fig” as an example, explaining
how a chimp might progress from knowing a fig only as a tangible here-and-now entity
to having a special vocal call that represents this concept symbolically. Goodall
writes:
We can trace a pathway along which representations of . . . a fig become progressively
more distant from the fig itself. The value of a fig to a chimpanzee
lies in eating it. It is important that he quickly learn to recognize as fig the fruit
above his head in a tree (which he has already learned to know through taste).
He also needs to learn that a certain characteristic odor is representative of fig,
even though the fig is out of sight. Food calls made by other chimpanzees in
the place where he remembers the fig tree to be located may also conjure up a
concept of fig. Given the chimpanzees’ proven learning ability, there does not
seem to be any great cognitive leap from these achievements to understanding
that some quite new and different stimulus (a symbol) can also be representative
of fig. Although chimpanzee calls are, for the most part, dictated by emotions,
cognitive abilities are sometimes required to interpret them. And the
interpretations themselves may be precursors of symbolic thought. (Goodall,
1986, pp. 588–589)
Presumably, in our own distant ancestors, the repeated acquisition of concepts, as
well as the education of children in those concepts, gradually led to the acquisition of
language and other aspects of a complex culture. The study of the human brain, then,
is not just the study of the structure of a body organ. It is also the study of how that
organ acquires sophisticated cultural skills—that is, of how the human brain fosters
behavior in today’s world.
In Review .
Care must be taken in extending evolutionary principles of the brain and behavior. What
is true for comparisons across different species may not be true for comparisons within a
species. For instance, although a larger brain correlates with more complex behavior in
comparisons of different species, brain size and intelligence are not particularly related in
comparisons of individual members within a species such as modern humans. We humans
are distinguished in the animal kingdom by the amount of our behavior that is culturally
learned. We have progressed much farther in the development of culture than other
species have.
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WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 31
SUMMARY
What is the use of studying brain and behavior? The study of brain and behavior leads
us to an understanding of our origins, to an understanding of human nature, and to
an understanding of the causes of many behavioral disorders and their treatment.
What is the brain and what is behavior? Behavior can be defined as any kind of movement
in a living organism. As such, a behavior has both a cause and a function. The
flexibility and complexity of behavior vary greatly among different species. Humans
are capable of highly flexible and complex behaviors. Located inside the skull, the brain
is the organ that exerts control over behavior. The brain seems to need ongoing sensory
and motor stimulation to maintain its intelligent activity.
How is the nervous system structured? The nervous system is composed of the central
nervous system, which includes the brain and the spinal cord, and the peripheral
nervous system, through which the brain and spinal cord communicate with sensory
receptors, with muscles and other tissues, and with the internal organs.
How has Western tradition viewed the relation between the brain and behavior? There
have been three major explanations: mentalism, dualism, and materialism. In antiquity,
Aristotle believed that the brain has no role in behavior, but rather that behavior
is the product of an intangible entity called the psyche, or mind. Descartes modified
mentalism in the European Renaissance, proposing the dualist explanation that only
rational behavior is produced by the mind, whereas other behaviors are produced mechanically
by the brain. Finally, 150 years ago, Darwin’s proposal that all living organisms
are descended from a common ancestor led materialists to conclude that the
source of all behavior is the brain.
How did brain cells and the nervous system evolve? Brain cells and the nervous system
evolved in some groups of animals over millions of years. The evolutionary stages
through which the human brain evolved can be traced through the human lineage to
their common ancestors. The nervous system evolved only in the animal kingdom, and
a true brain and spinal cord evolved only in the chordate phylum.Mammals are a class
of chordates characterized by especially large brains relative to body size.
What species were the early ancestors of modern humans? One of our early hominid ancestors
was probably Australopithecus, or a primate very much like it, who lived in Africa
several million years ago. From an australopith species, more humanlike species likely
evolved. Among them are Homo habilis and Homo erectus.Modern humans,Homo sapiens,
did not appear in Asia and North Africa until about 200,000 to 100,000 years ago.
How did the human brain evolve? The human brain evolved through the lineage of
hominid species that are the ancestors of modern humans. Since Australopithecus, the
hominid brain has increased in size almost threefold. Environmental challenges and
opportunities that favored the natural selection of more complex behavior patterns,
changes in physiology, and neoteny stimulated brain evolution in humans.
What are some important considerations in studying the brain and behavior of modern
humans? Principles learned in studying the evolution of the brain and behavior
across species may not apply to the brain and behavior within a single species, such as
Homo sapiens.As animals evolved, a larger brain was associated with more complex behavior;
yet, within our species, the most able and intelligent people do not necessarily
have the largest brains. In the study of modern humans, the great extent to which our
behavior is not inherent in our nervous systems but rather is culturally learned must
be recognized.
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REVIEW QUESTIONS
1. Summarize the ideas of Aristotle, Descartes, and Darwin regarding the relation
between the brain and behavior.
2. How would you go about tracing your own lineage by using the taxonomic
system described in this chapter?
3. Recall the number of species of living organisms in each taxonomic
subgrouping.What do you think accounts for the apparent relation between
numbers of species and brain size?
4. Brain size is one way of accounting for behavioral complexity in interspecies
comparisons but not for intraspecies comparisons.What is the reasoning behind
this distinction?
5. How does the existence of culture increase the difficulty of understanding
human brain function?
FOR FURTHER THOUGHT
Darwin’s principle of natural selection is based on the existence of a broad range of
individual differences within a species. There are large individual differences in brain
size among modern humans. Under what conditions could a new human species
with a still-larger brain evolve?
RECOMMENDED READING
Campbell, N. A. (2003). Biology, 4th ed. Menlo Park, CA: Benjamin Cummings. This
introductory biology textbook provides a comprehensive overview of the structure and
function of living organisms.
Coren, S. (1994). The intelligence of dogs. Toronto: The Free Press. This very popular book
includes a number of tests that are supposed to tell you how smart your dog is. The
book also provides comparisons of intelligence for different dog breeds as rated by dog
trainers. Check your dog out against other breeds by using easy-to-perform tests.
Remember, if your dog is not well trained, it might not do well on the tests.
Darwin, C. (1965). The expression of the emotions in man and animals. Chicago: University
of Chicago Press. (Original work published 1872.) If a dog growls at you, is the dog
32 ! CHAPTER 1
neuroscience interact ive KEY TERMS
There are many resources available for
expanding your learning online:
www.worthpublishers.com/kolb
Try the Chapter 1 quizzes and
flashcards to test your mastery of the
chapter material. You’ll also be able to
link to other sites that will reinforce
what you’ve learned.
http://neurolab.jsc.nasa.gov/
timeline.htm
Review a timeline of the pioneers in
brain study from René Descartes to
Roger Sperry in the Spotlight on
Neuroscience, National Aeronautics and
Space Administration (NASA).
http://serendip.brynmawr.edu/
Mind/Table.html
Visit this site by R. H.Wozniak of Bryn
Mawr College to learn more about the
philosophical underpinnings of dualism
and materialism and to read a detailed
history of the origins of the mind–body
question and the rise of experimental
psychology.
On your Foundations CD-ROM, you’ll
be able to begin learning about the
anatomy of the brain in the module on
the Central Nervous System. This
module is composed of a rotatable,
three-dimensional brain as well as a
number of sections of the brain that
you can move through with the click of
a mouse. In addition, the Research
Methods module contains various
computer tomographic and magnetic
resonance images of the brain,
including a video clip of a coronal MRI
scan.
bilateral symmetry, p. 16
central nervous system
(CNS), p. 6
cerebellum, p. 18
cerebral cortex, p. 5
chordates, p. 16
cladogram, p. 16
common ancestor, p. 14
culture, p. 28
dualism, p. 9
encephalization quotient
(EQ), p. 23
frontal lobe, p. 6
ganglia, p. 16
hemisphere, p. 5
homeobox gene cluster,
p. 16
hominid, p. 20
materialism, p. 11
mentalism, p. 9
mind, p. 9
mind–body problem, p. 9
motor neuron, p. 6
natural selection, p. 12
neoteny, p. 26
nerve net, p. 16
neuron, p. 6
occipital lobe, p. 6
parietal lobe, p. 6
peripheral nervous system
(PNS), p. 6
psyche, p. 9
radiator hypothesis, p. 25
segmentation, p. 16
sensory neuron, p. 6
species, p. 12
species-typical behavior,
p. 27
spinal cord, p. 6
temporal lobe, p. 6
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angry? Darwin thought so. Darwin’s only book on psychology is one in which he argues
that the expression of emotions is similar in animals, including humans, which suggests
the inheritance of emotions from a common ancestor. This view is becoming popular
today, but Darwin proposed it more than 130 years ago.
Darwin, C. (1963).On the origin of species by means of natural selection, or the preservation
of favored races in the struggle for life. New York: New American Library. (Original work
published 1859.) This book is the most important one ever written in biology. Darwin
extensively documents the evidence for his theory of natural selection. The book is an
enjoyable account of natural life, and one chapter, titled “Instincts,” describes behavior
in both wild and domesticated animals.
Goodall, J. (1986). The chimpanzees of Gombe. Cambridge, MA: Harvard University Press.
Goodall’s three-decade-long study of wild chimpanzees, begun in 1960, rates as one of
the most scientifically important studies of animal behavior ever undertaken. Learn
about chimpanzee family structure and chimpanzee behavior, and look at the beautiful
photographs of chimpanzees engaged in various behaviors.
Gould, S. J. (1981). The mismeasure of man. New York: Norton. Gould criticizes and
repudiates extensive literature of the nineteenth and twentieth centuries that claims
that differences in human intelligence and differences in the intelligence of the sexes
are due to differences in brain size. The appealing feature of this book is that Gould is
highly critical of the methodology of the proponents of the brain-size hypothesis
while also giving reasons derived from modern genetics for criticizing their position.
Lorenz, K. Z. (1981). The foundations of ethology. New York: Springer Verlag. Learn how to
study animals and learn how they behave from one of the founders of ethology, the
study of animal behavior.
Martin, R. D. (1990). Primate origins and evolution: A phylogenetic reconstruction. Princeton,
NJ: Princeton University Press. Martin provides a detailed description of the origins
and the evolution of primates. This book is an excellent primate reference.
Weiner, J. (1995). The beak of the finch. New York: Vintage. This book is a marvelous study
of evolution in action.Weiner documents how the populations of Galápagos finches are
affected by changes in the availability of certain kinds of food. Careful measurements of
the finches’ beaks demonstrate that certain beak sizes and shapes are favored when
certain kinds of food are available; however, when the appropriate food becomes
unavailable, populations of birds with differently shaped beaks become favored.
WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 33
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Focus on New Research: Optimizing Connections in
the Brain
An Overview of Brain Function
and Structure
The Brain’s Primary Functions
Basic Brain Terminology
The Brain’s Surface Features
Focus on Disorders: Meningitis and Encephalitis
Focus on Disorders: Stroke
The Brain’s Internal Features
Microscopic Inspection: Cells and Fibers
Neuroanatomy and Functional
Organization of the Nervous System
Evolutionary Development of the Nervous System
The Central Nervous System
The Somatic Nervous System
Focus on Disorders: Magendie, Bell, and Bell’s Palsy
The Autonomic Nervous System
Eight Principles of Nervous
System Function
Principle 1: The Sequence of Brain Processing Is
“In Integrate Out”
Principle 2: Sensory and Motor Divisions Exist
Throughout the Nervous System
Principle 3: Many of the Brain’s Circuits Are Crossed
Principle 4: The Brain Is Both Symmetrical
and Asymmetrical
Principle 5: The Nervous System Works Through
Excitation and Inhibition
Principle 6: The Central Nervous System Functions on
Multiple Levels
Principle 7: Brain Systems Are Organized Both
Hierarchically and in Parallel
Principle 8: Functions in the Brain Are Both Localized
and Distributed
34 !
C H A P T E R2
How Does the Nervous
System Function?
Left: Carolina Biological Supply/Phototake. Middle: Corbis.
Right: CNRI/Phototake.
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we learned in Chapter 1, increasing brain size has complications
because, for example, it is necessary to provide
more blood and to keep the brain cooled (recall the radiator
hypothesis of brain evolution).
Recent studies have directly examined whether evolution
favors economy of connective distance as a factor in
locating different brain regions. Several principles can be
extracted from this research:
1. Brain structures do not enlarge in isolation. There is
a correlated evolution of different brain regions and,
especially, the cerebral hemispheres and cerebellum.
Thus, as the neocortex enlarges with evolution, so,
too, does the cerebellum. Furthermore, as sensory areas
have expanded in the course of evolution, so have associated
motor areas.
2. Evolution selects for developmental processes that minimize
the length of neural connections. The shorter the
connections between regions, the faster the transmission,
the less space taken up by fiber tracts, and the less
likelihood of errors in making these connections in the
course of development.
3. When all possible connections between different brain
regions are considered, adjacent areas have more connections
than do areas that are not adjacent. For
example, Klyachko and Stevens (2003) examined all
possible permutations of connections in 11 areas in
the frontal lobe of the rhesus monkey. They calculated
39.9 million possible arrangements of connections,
but the actual arrangement observed in rhesus monkeys
was optimal, and any deviation increased axonal
volume.
We can conclude that the best predictor of the function
of any given brain region is probably the function
of the adjacent areas. Areas that are adjacent are heavily
interconnected and so presumably have interdependent
functions.
Optimizing Connections in the Brain
Focus on New Research
A s stated in Chapter 1, compared with other mammals,
primates have evolved a larger brain than
would be predicted from their body size. Overall, scientists
believe that this increase in brain size is due to an expansion
of existing structures rather than to the production of
entirely new structures. As the brain has enlarged with evolution,
so, too, have the number of connections among different
brain regions.
Connections take up space. When the fraction of the
neocortex that is occupied by connections is factored out
from cell bodies, synapses, and other brain constituents, we
find that roughly 60 percent of the volume in the neocortex
is taken up by axons and dendrites, as shown in the accompanying
illustration. The amount of space required by
these connections poses an important problem of economy:
if brain regions have extensive interconnections with
one another, the increase in brain size could be larger if
the regions are distant than if they are adjacent. Connections
between distant regions would have be in fiber bundles
that would traverse some distance and take up space,
whereas adjacent regions could connect directly and save
space. Direct connections would reduce brain size, and, as
This estimate of the volume of neocortex made up of different
components shows that the wires (axons and dendrites) account
for nearly 60 percent of the volume.
Dendrites
22%
Axons
37%
Other
12%
Glia
12%
Spines
5%
Boutons
13%
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36 ! CHAPTER 2
This chapter builds on the foundation laid in Chapter 1 for studying brain and
behavior.Here we consider the biology of the nervous system and how its basic
components function. We focus first on the brain and then elaborate on how
the brain works in concert with the rest of the nervous system. This focus on function
suggests eight basic principles of nervous system organization that are given in detail
in the concluding section. These “big ideas” apply equally to the micro- and macroviews
of the nervous system presented in this chapter and the following ones on neurobiology
and to the broader picture of behavior that emerges in later chapters.
AN OVERVIEW OF BRAIN FUNCTION
AND STRUCTURE
When buying a new car, people like to open the hood and examine the engine, the part of
the car responsible for most of its behavior—and misbehavior. In doing so,we see a maze
of tubes,wires, boxes, and fluid reservoirs.All most of us can do is gaze, because what we
see simply makes no sense, except in the most general way.We
know that the engine burns gasoline to make the car move and
somehow generates electricity to run the radio and lights. But
this tells us nothing about what all the engine’s many parts do.
What we need is information about how such a system works.
When it comes to behavior, the brain is the engine. In
many ways, examining a brain for the first time is similar to
looking under the hood of a car.We have a vague sense of what
the brain does but no sense of how the parts that we see accomplish
these tasks.We may not even be able to identify the
parts. In fact, at first glance, the outside of a brain may look
more like amass of folded tubes divided down the middle than
like a structure with many interconnected pieces.
What can you make of the human brain shown in Figure 2-1? Can you say anything
about how it works? At least car engines have parts with regular shapes that are recognizably
similar, which is not true of mammals’ brains.When we compare the brain of a cat
with that of a human, as shown in Figure 2-2, for example,we see enormous differences
not just in overall size but also in the structure and relative sizes of the parts. In fact, some
parts present in one brain are totally absent in the other.What is it that these parts do that
makes one animal stalk mice and another read textbooks?
The Brain’s Primary Functions
Perhaps the simplest summary of brain function is that it produces behavior, or movement,
as you learned in Chapter 1. To produce behavior as we search, explore, and
manipulate our environments, the brain must have information about the world—
about the objects around us, their size, shape, and movement, for instance. Without
Rat Cat Monkey Human
Figure 2-2
Mammalian Brains On the outside, the
brains of a rat, cat, monkey, and human
differ dramatically in size and in general
appearance. The rat brain is smooth,
whereas the other brains have furrows of
varying patterns in the cerebral cortex.
The cerebellum, located above the
brainstem, is wrinkled in all these species.
The brainstem is the route by which most
information enters and exits the brain.
The olfactory bulb, which controls the
perception of smells, is relatively larger in
cats and rats but is not visible in monkeys
and humans, because it is small and lies
on the underside of the brain. Photographs
courtesy of Wally Welker/University of Wisconsin
Comparative Mammalian Brain Collection.
Figure 2-1
Human Brain in Situ When the skull
is opened the gyri (bumps) and sulci
(cracks) of the cerebral hemispheres are
visible, but their appearance offers little
information about their function.
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such information, the brain cannot orient and direct the body to produce an appropriate
response to stimulation.
The need for such information is especially true when the required response is
some complex behavior, such as catching a ball. The organs of the nervous system are
designed to admit information from the world and convert this information into biological
activity that produces subjective experiences of reality. The brain thus produces
what we believe is reality in order for us to move. These subjective experiences of reality
are essential to carrying out any complex task.
This view of the brain’s primary purpose may seem abstract, but it is central to understanding
how the brain functions. Consider the task of answering a telephone. The brain
directs the body to pick up the phone when the nervous system responds to vibrating
molecules of air by creating the subjective experience of a ring.We perceive this sound and
react to it as if it actually existed,when in fact the sound is merely a fabrication of the brain.
That fabrication is produced by a chain reaction that takes place when vibrating
air molecules hit the eardrum.Without the nervous system, especially the brain, there
is no such thing as sound. Rather, there is only the movement of air molecules.
There is more to a telephone ring than just the movement of air molecules however.
Our creation of reality is based not only on the sensory information received but
also on the cognitive processes each of us might use to interact with the incoming information.
A telephone ringing when we are expecting a call has a different meaning
from its ringing when we are not expecting a call, such as at 3:00 AM.
The subjective reality created by the brain can be better understood by comparing
the realities of two different kinds of animals. You are probably aware that dogs perceive
sounds that humans do not. This difference in perception does not mean that a
dog’s nervous system is better than ours or that our hearing is poorer. Rather, the perceptual
world created by a dog brain simply differs from that created by a human brain.
Neither subjective experience is “right.” The difference in experience is merely due to
two different systems for processing physical stimuli.
The same differences exist in visual perceptions. Dogs see very little color, whereas
our world is rich with color because our brains create a different reality. Such subjective
differences exist for good reason: they allow different animals to exploit different
features of their environments.Dogs use their hearing to detect the movements of mice
in the grass; early humans probably used color vision for identifying ripe fruit in trees.
Evolution, then, equips each species with a view of the world that helps it survive.
These sensory examples show how a brain helps guide an organism’s behavior. To
make this link between sensory processing and behavior, the brain must also have a system
for accumulating, integrating, and using knowledge.Whenever the brain collects
sensory information, it is essentially creating knowledge about the world, knowledge
that it can use to produce more-effective behaviors. The knowledge currently being
created in one sensory domain can be compared both with past knowledge and with
knowledge gathered in other domains.
We can now identify the brain’s three primary functions:
1. Creating a sensory reality
2. Integrating information
3. Producing behavior
Each function requires specific neural systems to create the sensory world, to produce
movement (behavior), and to integrate the two.
As introduced in Chapter 1, the central nervous system (CNS) consists of the brain
and the spinal cord, and the peripheral nervous system (PNS) encompasses everything
else. The CNS–PNS distinction, diagrammed in Figure 2-3A, is based on anatomy.
In a functional organization, little changes, but the focus shifts to how the parts of the
HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 37
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38 ! CHAPTER 2
system work together (Figure 2-3B). That is, the major divisions of the PNS, the somatic
and autonomic nervous systems, step up to constitute, along with the CNS, a
three-part functional scheme:
The CNS includes the brain and the spinal cord.
The somatic nervous system (SNS), all the spinal and cranial nerves to and from
the muscles, joints, and skin, produces movement and transmits incoming sensory information,
including the position and movement of body parts, to the CNS.
The autonomic nervous system (ANS) balances the body’s internal organs to “rest
and digest” through the parasympathetic (calming) nerves or to “fight or flee” or engage
in vigorous activity through the sympathetic (arousing) nerves.
Basic Brain Terminology
The place to start our structural overview is to “open the hood” by observing the brain
snug in its home within the skull. Figure 2-1 shows a brain viewed from this perspective.
The features that you see are part of what is called the brain’s “gross anatomy,” not
because they are ugly, but because they constitute a broad overview. The hundreds,
even thousands, of discrete brain regions make the task of mastering brain terminology
seem daunting. Many structures have several names, and terms are often used interchangeably.
This peculiar nomenclature arose because research on brain and behavior
spans several centuries and includes scientists of many nationalities and languages.
When the first anatomists began to examine the brain with the primitive tools of
their time, they made many erroneous assumptions about how the brain works, and
the names that they chose for brain regions are often manifestations of those errors.
For instance, they named one region of the brain the gyrus fornicatus because they
thought that it had a role in sexual function. In fact,most of this region has nothing to
do with sexual function. Another area was named the red nucleus because it appears
reddish in fresh tissue. This name denotes nothing of the area’s potential functions,
which turn out to be the control of limb movements.
As time went on, the assumptions and tools of brain research changed, but the
naming continued to be haphazard and inconsistent. Early investigators named structures
after themselves or objects or ideas. They used different languages, especially
Somatic nervous system (SNS). Part
of the PNS that includes the cranial and
spinal nerves to and from the muscles,
joints, and skin that produce movement,
transmit incoming sensory input, and
inform the CNS about the position and
movement of body parts.
Autonomic nervous system (ANS).
Part of the PNS that regulates the
functioning of internal organs and glands.
Somatic
nervous system
(B)
Nervous
system
Brain Spinal
cord
Spinal
nerves
Sympathetic
division
Parasympathetic
division
Parasympathetic
division
Sympathetic
division
Cranial
nerves
Central
nervous system
Autonomic
nervous system
(A)
Nervous
system
Brain Spinal
cord
Somatic
nervous
system
Central
nervous system
Peripheral
nervous system
Autonomic
nervous
system
Figure 2-3
Parsing the Nervous System The
nervous system can be conceptualized
anatomically (A) and functionally (B).
The functional approach employed in
this book focuses on how the parts of
the nervous system interact.
Visit the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
and go to the Chapter 2 Web links to view
an index listing the roots of neuroanatomical
terms.
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HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 39
Latin, Greek, and English.More recently, investigators
have often used numbers or letters, but
even this system lacks coherence, because the
numbers may be Arabic or Roman and are often
used in combination with letters, which may be
either Greek or Latin. When we look at current
brain terminology, then, we see a mixture of all
these naming systems.
Many names for nervous system structures
include information about anatomical location.
Table 2-1 summarizes these location-related
terms.Note in Figure 2-4A that structures found
on the top of the human brain or on the top of
some other structure within the brain are dorsal.
Structures located toward the bottom of the
human brain or one of its parts are ventral.
Structures found toward the brain’s midline are
medial, whereas those located toward the sides
are lateral. (Your, heart, for example, is medial;
your hips are lateral.)
Figure 2-4B contrasts anatomical directions
relative to the body orientations of a human
and a dog. Now compare how these directionals
are applied to the human brain in Figure 2-4A.
Structures located toward the front of the brain
are anterior, whereas those located toward the
back of the brain are posterior. Sometimes the
terms rostral and caudal are used instead of anterior
and posterior, respectively. And, occasionally,
the terms superior and inferior are used to
refer to structures that are located dorsally or
ventrally. These terms do not label structures
according to their importance, just their location.
It is also common to combine terms. For
example, a structure may be described as dorsolateral,
which means that it is located “up and to
the side.”
Anatomical Locations
Term Meaning with respect to the nervous system
Anterior Located near or toward the front or the head
Caudal Located near or toward the tail
Dorsal On or toward the back or, in reference to brain nuclei, located above
Frontal “Of the front“ or, in reference to brain sections, a viewing
orientation from the front
Inferior Located below
Lateral Toward the side of the body
Medial Toward the middle, specifically the body’s midline; sometimes written
as mesial
Posterior Located near or toward the tail
Rostral ”Toward the beak”; located toward the front
Sagittal Parallel to the length (from front to back) of the skull; used in
reference to a plane
Superior Located above
Ventral On or toward the belly or the side of the animal in which the belly is
located or, in reference to brain nuclei, located below
Table 2-1
(A)
Meaning “above,”
sometimes referred
to as superior
Meaning “tail,”
sometimes referred
to as caudal
Meaning “below” or
“belly,” sometimes
referred to as inferior
Meaning “middle”
Meaning “front,”
sometimes referred
to as frontal or rostral
Posterior Anterior
Lateral Media l
Ventral Dorsal
Meaning “side”
(B)
Anterior Posterior Dorsal Ventral Anterior Posterior
Ventral Dorsal
Figure 2-4
Anatomical Orientation Because humans walk
upright, anatomical directions relative to the head
and brain, shown in part A, take a shift compared
with the head orientation of a four-legged animal,
shown in part B. (A) The concepts “dorsal” and
“ventral” take a 90° turn counterclockwise when
they reach the level of the human brain. Similarly,
posterior and caudal (both mean “tail”) refer to a
slightly different orientation for the vertically
situated human head. (B) Anatomical directions
relative to body orientation.
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40 ! CHAPTER 2
Afferent. Conducting toward a central
nervous system structure.
Efferent. Conducting away from a
central nervous system structure.
Meninges. Three layers of protective
tissue—dura mater, arachnoid, and pia
mater—that encase the brain and spinal
cord.
Cerebrospinal fluid (CSF). Clear
solution of sodium chloride and other
salts that fills the ventricles inside the
brain and circulates around the brain and
spinal cord beneath the arachnoid layer
in the subarachnoid space.
Cerebrum. Major structure of the
forebrain, consisting of two virtually
identical hemispheres (left and right).
This afferent nerve
carries information
from sensory
receptors in skin
to the brain.
Sensory
endings
This efferent nerve
carries information
from the brain to the
neurons controlling
leg muscle, causing
Figure 2-5 a response.
Information Flow Afferent sensory nerves
carry information into the CNS; efferent motor
nerves carry information out of the CNS.
The direction of neural information flow also is important. Afferent (incoming)
refers to information coming into the CNS or one of its parts, whereas efferent (outgoing)
refers to information leaving the CNS or one of its parts. Thus, the sensory
signals transmitted from the body into the brain are afferent, and efferent signals
from the brain trigger some response (Figure 2-5). The words are similar but easy to
keep straight. The letter “a” in afferent comes alphabetically before the “e” in efferent,
and sensory information must come into the brain before an outward-flowing signal
can trigger a response. Therefore, afferent means “incoming” and efferent means
“outgoing.”
The Brain’s Surface Features
Return to the brain in the open skull. The first thing that you encounter is not the brain
but rather a tough, triple-layered covering, the meninges, illustrated in Figure 2-6. The
outer dura mater (from Latin, meaning “hard mother”) is a tough double layer of fibrous
tissue that encloses the brain and spinal cord in a kind of loose sack. In the
middle is the arachnoid layer (from Greek, meaning “like a spider’s web”), a very thin
sheet of delicate connective tissue that follows the brain’s contours. The inner layer, or
pia mater (from Latin, meaning “soft mother”), is a moderately tough membrane of
connective-tissue fibers that cling to the brain’s surface.
Between the arachnoid layer and pia mater flows cerebrospinal fluid (CSF), a
colorless solution of sodium chloride and other salts. The cerebrospinal fluid cushions
the brain so that it can move or expand slightly without pressing on the skull. The
symptoms of meningitis, an infection of the meninges and CSF, are described in
“Meningitis and Encephalitis” on page 42.
After removing the meninges,we can lift the brain from the
skull and examine its parts. As we look at the brain in the dorsal
view at the top of Figure 2-7, it appears to have two major
parts, each wrinkly in appearance, resembling a walnut meat
taken whole from its shell. They are the left and right hemispheres
of the cerebrum, the most recently evolved structure of
the central nervous system.
From the opposite, ventral view shown in the second panel
in Figure 2-7, the hemispheres of the smaller “little brain,”
or cerebellum, are visible. Both the cerebrum and the cerebellum
are visible in the lateral and medial views shown in the
Skull
Dura
mater
Arachnoid Meninges
layer
Subarachnoid space
Brain (filled with CSF)
Pia mater
Figure 2-6
Cerebral Security A triple-layered
covering, the meninges, encases the
brain and spinal cord, and the
cerebrospinal fluid (CSF) cushions them.
On the Foundations of Behavioral
Neuroscience CD, visit the module on
the central nervous system to better
visualize the brain’s surface features. To
look at the various structures go to the
detailed anatomy view in the section on
the cortex. (See the Preface for more
information about this CD.)
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HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 41
bottom panels of Figure 2-7. These structures appear wrinkled in large-brained animals
because their outer surface, or cortex, is a relatively thin sheet of tissue that is crinkled
up to fit into the skull, as described in Chapter 1 (see also Figure 2-2).
Thus, much of the cortex is invisible from the brain’s surface. All we can see are
bumps, or gyri (singular: gyrus), and cracks, or sulci (singular: sulcus). Some sulci are
so very deep that they are called fissures. The longitudinal fissure between the cerebral
Lateral view
Medial view
Frontal
lobe
Central sulcus
Temporal
lobe
Lateral
fissure
Parietal
lobe
Occipital
lobe
Central sulcus
Cerebellum
Frontal
lobe
Temporal
lobe Brainstem
Parietal
lobe
Occipital
lobe
Dorsal view
Frontal
lobe
Longitudinal
fissure
Parietal
lobe
Central sulcus
Occipital
lobe
Ventral view
Frontal
lobe
Temporal lobe Cerebellum
Brainstem
Cranial nerves
Olfactory
bulbs
Figure 2-7
Views of the Human Brain Locations
of the lobes of the cerebral hemispheres
are shown in these top, bottom, side,
and midline views, as are the cerebellum
and the three major sulci. Photographs
courtesy of Yakolev Collection/AFIP.
Visit the central nervous system
module on the Foundations CD to
examine, locate, and rotate the parts of
the brain. To view the three-dimensional
models, go to the overview and look in
the section on the subdivisions of the
CNS.
Gyrus (pl. gyri). A groove in brain
matter, usually a groove found in the
neocortex or cerebellum.
Sulcus (pl. sulci). A small cleft formed
by the folding of the cerebral cortex.
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hemispheres and the lateral fissure at the side of the brain are both shown in various
views in Figure 2-7, along with the central sulcus at the top of the cerebrum.
If we now look at the bottom of the brain, the ventral view in Figure 2-7, we see
something completely different. In the midst of the wrinkled cerebrum and cerebellum
emerges a smooth whitish structure with little tubes attached. This central set of structures
is the brainstem, and the little tubes signify the cranial nerves that run to and
from the brain.
One final gross feature is obvious: the brain appears to be covered in blood vessels.
Like the rest of the body, the brain receives blood through arteries and sends it
back through veins to the kidneys and lungs for cleaning and oxygenation. The cerebral
arteries emerge from the neck to wrap around the outside of the brainstem,
cerebrum, and cerebellum, finally piercing the brain’s surface to nourish its inner
regions.
Three major arteries feed blood to the cerebrum—namely, the anterior, middle,
and posterior cerebral arteries shown in Figure 2-8. Because the brain is very sensitive
42 ! CHAPTER 2
Meningitis and Encephalitis
Focus on Disorders
Various harmful microorganisms can invade the layers of the
meninges, particularly the pia mater and the arachnoid layer,
as well as the CSF flowing between them, and cause infections
called meningitis. One symptom, inflammation, places
pressure on the brain. Because the space between meninges
and skull is slight, unrelieved pressure can lead to delirium
and, if the infection progresses, to drowsiness, stupor, and
even coma.
Usually, the earliest symptom of meningitis is severe
headache and a stiff neck (cervical rigidity). Head retraction
(titling the head backward) is an extreme form of cervical
rigidity. Convulsions, a common symptom in children, indicate
that the brain also is affected by the inflammation.
Infection of the brain itself is called encephalitis. Some
of the many forms of encephalitis have great historical significance.
Early in the past century, in World War I, a form
of encephalitis called sleeping sickness (encephalitis lethargica)
reached epidemic proportions.
Its first symptom is sleep disturbance. People sleep
all day and become wakeful, even excited, at night. Subsequently,
they show symptoms of Parkinson’s disease—severe
tremors and difficulty in controlling body movements. Many
are completely unable to make any voluntary movements,
such as walking or even combing their hair. Survivors of
sleeping sickness were immortalized by the neurologist
Oliver Sacks in the book and movie Awakenings.
The cause of these encephalitis symptoms is death of
a brain area known as the substantia nigra (black substance),
which you will learn about later in this chapter. Other forms
of encephalitis may have different effects on the brain. For example,
Rasmussen’s encephalitis attacks one cerebral hemisphere
in children. In most cases, the only effective treatment
is radical: the removal of the entire affected hemisphere.
Surprisingly, some young children who lose a hemisphere
adapt rather well. They may even complete college,
literally with half a brain. But retardation is a more common
outcome of hemispherectomy after encephalitis.
Pus is visible over the anterior surface of this brain infected with
meningitis.
Biophoto Associates/Science Source/
Photo Researchers
Brainstem. Central structures of the
brain including the hindbrain, midbrain,
thalamus, and hypothalamus.
Stroke. Sudden appearance of
neurological symptoms as a result of
severe interruption of blood flow.
White matter. Areas of the nervous
system rich in fat-sheathed neural axons.
Gray matter. Areas of the nervous
system composed predominantly of cell
bodies and blood vessels.
Ventricle. A cavity in the brain that
contains cerebral spinal fluid.
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HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 43
to loss of blood, a blockage or break in a cerebral artery is likely to lead to the death of
the affected region, a condition known as stroke. Because the three cerebral arteries
service different parts of the brain, strokes disrupt different brain functions, depending
on the artery affected (see “Stroke” on page 44).
The Brain’s Internal Features
The simplest way to examine the inside of something is to
cut it in half. The orientation in which we cut makes a difference
in what we see, however. Consider what happens
when we slice through a pear. If we cut from side to side,
we cut across the core; if we cut it from top to bottom,
we cut parallel to the core. Our impression of what the
inside of a pear looks like is clearly influenced by the way in which we slice it. The
same is true of the brain.
We can reveal the brain’s inner features by slicing it downward through the middle,
parallel to the front of the body, as shown in Figure 2-9A. The result, shown in Figure
2-9B, is known as a frontal section because we can now see the inside of the brain
from the front. It is immediately apparent that the interior is not homogeneous.
Both light and dark regions of tissue are visible, and though these regions may not
be as distinctive as the parts of a car’s engine, they nevertheless represent different brain
components. The light regions, called white matter, are mostly nerve fibers with fatty
coverings that produce the white appearance, much as fat droplets in milk make it appear
white. In the darker regions, called gray matter, capillary blood vessels and cell
bodies predominate.
A second feature apparent at the middle of our frontal section are two wing-shaped
cavities. The brain contains four such fluid-filled ventricles, which are shown in place
Use the Foundations CD to examine
a three-dimensional model of the
ventricular system in the section on
subcortical structures in the central
nervous system module.
Lateral ventricles
Gray matter
Temporal lobe
Lateral sulcus
White matter
Corpus callosum
(A) (B)
Glauberman/Photo Researchers
Figure 2-9
Frontal Section Through the Brain
The brain is (A) cut through the middle
parallel to the front of the body and
then (B) viewed at a slight angle. This
frontal section displays white matter,
gray matter, and the lateral ventricles.
A large bundle of fibers, the corpus
callosum, visible above the ventricles
joins the two hemispheres.
Middle cerebral
artery
Posterior cerebral
artery
Anterior cerebral
artery
Figure 2-8
Major Cerebral Arteries Each major
artery feeds a different region of the
cerebrum.
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44 ! CHAPTER 2
Stroke
Focus on Disorders
A severe interruption of blood flow to the brain kills brain
cells and causes the sudden appearance of the neurological
symptoms of stroke. In the United States, someone suffers a
stroke approximately every minute, producing more than
a half million new stroke victims every year. Worldwide,
stroke is the second leading cause of death.
Even with the best and fastest medical attention, most
who endure stroke suffer some residual motor, sensory, or
cognitive deficit. For every ten people who have a stroke, two
die, six are disabled to varying degrees,
and two recover to a degree but still endure
a diminished quality of life. One in
ten who survive risks further stroke.
Consequences of stroke are significant
for victims, their families, and their
life styles. Consider Mr. Anderson, a 45-
year-old electrical engineer who took his
three children to the movies one Saturday
afternoon in 1998 and collapsed.
Rushed to the hospital, he was diagnosed
as having a massive stroke of the middle
cerebral artery of his left hemisphere. The
stroke has impaired Mr. Anderson’s language
and his motor control on the right
side ever since.
Seven years after his stroke, Mr. Anderson remained unable
to speak, but he could understand simple conversations.
Severe difficulties in moving his right leg required him to use
a walker. He could not move the fingers of his right hand and
so had difficulty feeding himself, among other tasks. Mr. Anderson
will probably never return to his engineering career
or be able to drive or to get around on his own.
Like Mr. Anderson, most stroke survivors require help to
perform everyday tasks. Their caregivers are often female relatives
who give up their own careers and other pursuits. Half
of these caregivers develop emotional illness, primarily depression
or anxiety or both, after a year’s time. Lost income
and stroke-related medical bills have a significant effect on
the family’s standard of living.
Although we tend to speak of stroke as a single disorder,
two major types of strokes have been identified. In the more
common and often less severe ischemic stroke, a blood vessel
is blocked (such as by a clot). The more severe hemorrhagic
stroke results from a burst vessel bleeding into the brain.
The hopeful news is that ischemic stroke can be treated
acutely with a drug called tissue plasminogen activator (t-PA)
that breaks up clots and allows a return of
normal blood flow to an affected region.
(Unfortunately, no treatment exists for hemorrhagic
stroke, where the use of clotpreventing
t-PA would be disastrous.) The
results of clinical trials showed that, when
patients are given t-PA within 3 hours
of suffering an ischemic stroke, the number
who make a nearly complete recovery
increases by 32 percent compared with
those who are given a placebo (Chiu et al.,
1998). In addition, impairments are reduced
in the remaining patients who survive
the stroke.
One difficulty is that many people
are unable to get to a hospital soon
enough for treatment with t-PA. Most stroke victims do not
make it to an emergency room until about 24 hours after
symptoms appear, too late for treatment with t-PA. Apparently,
most people fail to realize that stroke is an emergency.
Other drugs producing an even better outcome than
does t-PA will likely become available in the future. It is
hoped that these drugs will extend the 3-hour window for
administering treatment after a stroke. There is also intense
interest in developing treatments in the postacute period that
will stimulate the brain to initiate repairative processes. Such
treatment will facilitate the patient’s functional improvement
(see a review by Teasell et al., 2002).
In this CT scan of a brain with a stroke,
the dark area of the right is the area
that has been damaged by the loss of
blood flow.
Canadian Stroke Network
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in Figure 2-10. Cells that line the ventricles make the cerebrospinal fluid that fills them.
The ventricles are connected; so the CSF flows from the two lateral ventricles (also visible
in Figure 2-9) to the third and fourth ventricles that lie on the brain’s midline and
into the canal that runs the length of the spinal cord. The CSF is also found in the space
between the lower layers of the meninges wrapping around the brain and spinal cord.
Although the functions of the ventricles are not well understood, researchers think
that the ventricles play an important role in maintaining brain metabolism. The cerebral
spinal fluid may allow certain compounds access to the brain, and it probably helps
the brain excrete metabolic wastes. In the event of head or spinal trauma, CSF cushions
the blow.
Another way to cut through the brain is perpendicular to front to back. The result
is a side view, or sagittal section (Figure 2-11A). If we make our cut down the brain’s
midline, we divide the cerebrum into its two hemispheres, revealing several distinctive
brain components (Figure 2-11B). One is a long band of white matter that runs much
of the length of the cerebral hemispheres. This band, the corpus callosum, contains
about 200 million nerve fibers that join the two hemispheres and allow communication
between them.
Figure 2-11B clearly shows that the cortex covers the cerebral hemispheres above
the corpus callosum, whereas below it are various internal structures. Owing to their
location below the cortex, these structures are known as subcortical regions. These
older brain regions generally control basic physiological functions, whereas the newer
cortical structures process motor, sensory, perceptual, and cognitive functions.
Recall from Chapter 1 that bilateral symmetry and segmentation are two important
structural features of the human nervous system. If you were to compare the left
and right hemispheres in sagittal section, you would be struck by their symmetry. The
brain, in fact, has two of nearly every structure, one on each side. The few one-of-akind
structures, such as the third and fourth ventricles, are found along the brain’s midline.
Another one-of-a-kind example is the pineal gland,mentioned in Chapter 1 as the
seat of the mind in Descartes’s theory about how the brain works.
Microscopic Inspection: Cells and Fibers
Although the parts of a car engine are all large enough to be seen with the naked eye,
the fundamental units of the brain—its cells—are so small that they can be viewed only
with the aid of a microscope. By using a microscope, we quickly discover that the brain
has two main types of cells: neurons and glia, illustrated in Figure 2-12. The human
brain contains about 80 billion neurons and 100 billion glia. Neurons carry out the
brain’s major functions, whereas glia aid and modulate the neurons’ activities—for
example, forming the fatty covering, or insulation, over neurons. Both neurons and glia
come in many forms, each determined by the work done by particular cells.We examine
the structures and functions of neurons and glia in Chapter 3.
HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 45
(A) (B)
Cortex
Brainstem
Corpus
callosum
Fourth
ventricle
Cerebellum
Plane
of cut
Third
ventricle
Corpus callosum. Fiber system
connecting the two cerebral hemispheres.
Figure 2-11
Sagittal Section Through the Brain The brain is (A) cut
from front to back and then (B) viewed from the side. This
medial sagittal section separates the hemispheres, allowing
a view of the midline structures of the brain, including the
subcortical structures that lie below the corpus callosum.
Figure 2-10
Cerebral Ventricles The four ventricles
are interconnected. There are two
symmetrical lateral ventricles, one in
each hemisphere, and the third and
fourth cerebral ventricles, each of which
lies in the midline of the brain.
(A)
(B)
Right lateral
ventricle
Lateral
ventricle
Cerebral
aqueduct
Third
ventricle
Fourth
ventricle
Left lateral
ventricle
Third ventricle
Fourth ventricle