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Science Brain and Behavior contiuned 5

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46 ! CHAPTER 2
We can see the internal structures of the brain in much more detail by dyeing their
cells with special stains (Figure 2-13). For example, if we use a dye that selectively stains
cell bodies, we can see that the distribution of cells within the gray matter of the cerebral
cortex is not homogeneous but rather forms layers, as shown in Figure 2-13A
and C. Each layer contains similarly staining cells. Stained subcortical regions are seen
to be composed of clusters, or nuclei, of similar cells.
Although layers and nuclei are very different in appearance, both form functional
units within the brain.Whether a particular brain region has layers or nuclei is largely
an accident of evolution. By using a stain that selectively dyes the fibers of neurons, as
shown in Figure 2-13B and D, we can see the borders of the subcortical nuclei more
clearly. In addition, we can see that the cell bodies stained in the right-hand panels of
Figure 2-13 lie in regions adjacent to the regions with most of the fibers.
A key feature of neurons is that they are connected to one another by fibers known
as axons. When axons run along together, much like the wires that run from a car engine
to the dashboard, they form a nerve or a tract. By convention, the term tract is
usually used to refer to collections of nerve fibers found within the brain and spinal
cord, whereas bundles of fibers located outside these CNS structures are typically
referred to simply as nerves. Thus, the pathway from the eye to the brain is known as the
optic nerve, whereas the pathway from the cerebral cortex to the spinal cord is known
as the corticospinal tract.
Figure 2-12
Brain Cells A prototypical neuron (left)
and glial cell (right) show that both
have branches emanating from the cell
body. This branching organization
increases the surface area of the cell
membrane. The neuron is called a
pyramidal cell because the cell body is
shaped somewhat like a pyramid; the
glial cell is called an astrocyte because
of its star-shaped appearance.
(A) (B)
(C) (D)
Figure 2-13
Cortical Layers and Glia Brain sections
from the left hemisphere of
a monkey (midline is to the left in each
image). Cells are stained with (A) a
selective cell-body stain (Nissl stain) for
neurons and (B) a selective fiber stain,
staining for insulating glial cells, or
myelin. The images reveal very different
pictures of the brain at a microscopic
level. Closer up (C and D), notice the
difference in appearance between these
higher-power micrographs through the
gray- and white-matter sections of
different cortical regions.
Several axons running
together are a nerve (when
outside the brain) or a tract
(when inside the brain).
Neuron 1
Neuron 2
Axon
Cell body Terminal
Neuron
(pyramidal cell)
Glial cell
(astrocyte)
CNRI/Science Photo Library
N. Kedesha/Science Photo Library
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NEUROANATOMY AND FUNCTIONAL ORGANIZATION
OF THE NERVOUS SYSTEM
When we look under the hood,we can make some pretty good guesses about what each
part of a car engine does. The battery must provide electrical power to run the radio
and lights, for example, and, because batteries need to be charged, the engine must contain
some mechanism for charging them.We can take the same approach to deduce the
functions of the parts of the brain. For example, the part of the brain connected to the
optic nerve coming from each eye must have something to do with vision. Similarly,
brain structures connected to the auditory nerve coming from each ear must have
something to do with hearing.
From these simple observations we can begin to understand how the brain is organized.
The real test of inferences about the brain comes in analyzing actual brain
function: how this seeming jumble of parts produces behaviors as complex as human
thought. The place to start is the brain’s anatomy.
Evolutionary Development of
the Nervous System
The developing brain is less complex than the mature adult brain and provides a clearer
picture of its basic structural plan. As detailed in Chapter 7, the biological similarity of
embryos of vertebrate species as diverse as amphibians and mammals is striking in the
earliest stages of development. The brain of a young vertebrate embryo begins as a
sheet of cells that folds into a hollow tube and develops into three regions: forebrain,
midbrain, and hindbrain.
These three regions of the primitive developing brain are recognizable as a series
of three enlargements at the end of the embryonic spinal cord (Figure 2-14A). The
adult brain of a fish, amphibian, or reptile is roughly equivalent to this three-part brain.
The prosencephalon (front brain) is responsible for olfaction, the mesencephalon (middle
brain) is the seat of vision and hearing, and the rhombencephalon (hindbrain) controls
movement and balance. The spinal cord is considered part of the hindbrain.
In mammals, the prosencephalon develops further to form the cerebral hemispheres,
the cortex and subcortical structures known collectively as the telencephalon
(endbrain), and the diencephalon (between brain) containing the thalamus, among other
structures (Figure 2-14B). The hindbrain also develops further into the metencephalon
(across brain), which includes the enlarged cerebellum, and the myelencephalon (spinal
brain), including the medulla and the spinal cord.
The human brain is a more complex mammalian brain, possessing especially
large cerebral hemispheres while retaining most of the features of other mammalian
In Review .
Inside the skull and under the meninges, we find two main brain structures: the cerebrum
and the cerebellum. Both are separated into roughly symmetrical hemispheres that have
many gyri and sulci covering their surfaces. At the base of the brain, we see the brainstem,
of which the cerebellum is a part. Cutting open the brain, we observe the fluid-filled ventricles,
the corpus callosum that connects the two cerebral hemispheres, and the cortex
and subcortical regions below it. We also see that brain tissue is of two main types: white
matter and gray matter.
HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 47
Nucleus (pl. nuclei). A group of cells
forming a cluster that can be identified
with special stains to form a functional
grouping.
Nerve. Large collection of axons
coursing together within the central
nervous system.
Tract. Large collection of axons coursing
together outside the central nervous
system.
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48 ! CHAPTER 2
brains (Figure 2-14C). Most behaviors are not the product of a single locus in the
brain but rather of many brain areas and levels. These several nervous system layers
do not simply replicate function; rather, each region adds a different dimension to the
behavior. This hierarchical organization affects virtually every behavior in which humans
engage.
The Central Nervous System
With its literally thousands of parts, learning the name of a particular CNS structure
is pointless without also learning something about its function. In this section, therefore,
we focus on the names and functions of the major components of the CNS outlined
in Table 2-2: the spinal cord, the brainstem,
and the forebrain.
These three subdivisions reinforce the concept
of levels of function, with newer levels partly
replicating the work of older ones. A simple analogy
to this evolutionary progress is learning to
read.When you began to read, you learned simple
words and sentences. As you progressed, you mastered
new, more challenging words and longer,
more complicated sentences, but you still retained
the simpler skills that you had learned first. Much
later, you encountered Shakespeare, with a complexity
and subtlety of language unimagined in
grade school, taking you to a new level of reading
comprehension.
Each new level of training adds new abilities
that overlap and build on previously acquired skills.
Yet all the functional levels deal with reading. Likewise,
in the course of natural selection, the brain
has evolved functional levels that overlap one another
in purpose but allow for a growing complex-
(A) Vertebrate (B) Mammalian embryo (C) Fully developed human brain
Prosencephalon
Prosencephalon (forebrain) Forebrain
Brainstem
Telencephalon (end brain) Neocortex, basal ganglia, limbic system
olfactory bulb, lateral ventricles
Thalamus, hypothalamus, pineal body,
third ventricle
Tectum, tegmentum, cerebral aqueduct
Diencephalon (between brain)
Rhombencephalon (hindbrain)
Metencephalon (across-brain) Cerebellum, pons, fourth ventricle
Medulla oblongata, fourth ventricle
Spinal Spinal cord Spinal cord cord Spinal cord
Myelencephalon (spinal brain)
Mesencephalon (midbrain) Mesencephalon
Telencephalon Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Spinal cord
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Spinal cord
Mesencephalon
Rhombencephalon
Spinal cord
Figure 2-14
Stages in Brain Evolution and
Development The forebrain grows
dramatically in the evolution of the
mammalian brain.
Anatomical Divisions of the Central Nervous System
Anatomical division Functional division Principal structures
Forebrain Forebrain Cerebral cortex
Basal ganglia
Limbic system
Brainstem Diencephalon Thalamus
Hypothalamus
Midbrain Tectum
Tegmentum
Hindbrain Cerebellum
Pons
Medulla oblongata
Reticular formation
Spinal cord Spinal nerves Cervical nerves
Thoracic nerves
Lumbar nerves
Sacral nerves
Table 2-2
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ity of behavior. For instance, the brain has functional levels that control movements.
With the evolution of each new level, the complexity of movement becomes increasingly
refined.We expand on the principle of evolutionary levels of function later in this
chapter.
THE SPINAL CORD
Although producing movement is one function of the brain, it is ultimately the spinal
cord that controls most body movements. To understand how important the spinal cord
is, think of the old saying “running around like a chicken with its head cut off.”When a
chicken’s head is lopped off to provide dinner for the farmer’s family, the chicken is still
capable of running around the barnyard until it collapses from loss of blood.The chicken
accomplishes this feat because the spinal cord can act independently of the brain.
You can demonstrate movement controlled by the spinal cord in your own body
by tapping your patellar tendon, just below your kneecap (the patella). Your lower leg
kicks out and, try as you might, it is very hard to prevent the movement from occurring.
Your brain, in other words, has trouble inhibiting the reaction. This type of automatic
movement is known as a spinal reflex, a topic that we return to in Chapter 10.
THE BRAINSTEM
The brainstem begins where the spinal cord enters the skull and extends upward to the
lower areas of the forebrain. The brainstem receives afferent nerves from all of the
body’s senses, and it sends efferent nerves to control all of the body’s movements except
the most complex movements of the fingers and toes. The brainstem, then, both
produces movements and creates a sensory world.
In some animals, such as frogs, the entire brain is largely equivalent to the brainstem
of mammals or birds. And frogs get along quite well, demonstrating that the
brainstem is a fairly sophisticated piece of machinery. If we had only a brainstem, we
would still be able to create a world, but it would be a far simpler, sensorimotor world,
more like what a frog experiences.
The brainstem can be divided into three regions: hindbrain, midbrain, and diencephalon,
sometimes called the “between brain” because it borders upper and lower
parts of the brain. In fact, the “between brain” status of the diencephalon can be seen in
a neuroanatomical inconsistency: some anatomists place it in the brainstem and others
place it in the forebrain. The left side of Figure 2-15 illustrates the location of these three
brainstem regions under the cerebral hemispheres, and the right side of the figure compares
the shape of the brainstem regions to the lower part of your arm held upright. The
hindbrain is long and thick like your forearm, the midbrain is short and compact like
your wrist, and the diencephalon at the end is bulbous like your hand forming a fist.
The hindbrain and midbrain are essentially extensions of the spinal cord; they developed
first as simple animals evolved a brain at the anterior end of the body. It makes
HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 49
Patellar
tendon
Hindbrain. Evolutionarily the oldest part
of the brain; contains the pons, medulla,
reticular formation, and cerebellum
structures that coordinate and control
most voluntary and involuntary
movements.
Midbrain. Central part of the brain that
contains neural circuits for hearing and
seeing as well as orienting movements.
Diencephalon. The “between brain”
that contains the hypothalamus, thalamus,
and epithalamus; thought to coordinate
many basic instinctual behaviors,
including temperature regulation, sexual
behavior, and eating.
On the Foundations CD, visit the
module on the central nervous system for
a detailed, three-dimensional view of the
brainstem.
Diencephalon
Hindbrain
Cerebellum
Midbrain
Fist
(analogous
to diencephalon)
Wrist
(analogous
to midbrain)
Forearm
(analogous
to hindbrain)
Figure 2-15
Brainstem Structures Medial
view of the brain at left shows the
relation of the brainstem to the
cerebral hemisphere. Brainstem
structures perform both sensory
and motor functions.
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50 ! CHAPTER 2
sense, therefore, that these lower brainstem regions should retain a division between
structures having sensory functions and those having motor functions, with sensory
structures located dorsally and motor ones ventrally.
Each brainstem region performs more than a single task. Each contains various
subparts, made up of groupings of nuclei that serve different purposes. All three
regions, in fact, have both sensory and motor functions. However, the hindbrain is
especially important in motor functions, the midbrain in sensory functions, and the
diencephalon in integrative tasks.Here we consider the central functions of these three
regions; later chapters contain more detailed information about them.
The Hindbrain The hindbrain controls various motor functions ranging from
breathing to balance to fine movements, such as those used in dancing. Its most distinctive
structure, and one of the largest structures of the human brain, is the cerebellum.
Recall from Chapter 1 that the size of the cerebellum increases with the physical
speed and dexterity of a species, as shown in Figure 2-16A. Animals that move relatively
slowly (such as a sloth) have relatively small cerebellums for their body size,
whereas animals that can perform rapid, acrobatic movements (such as a hawk or a
cat) have very large cerebellums. The cerebellum, which resembles a cauliflower when
viewed in sagittal section in Figure 2-16B, is important in controlling complex movements
and apparently has a role in a variety of cognitive functions as well.
As we look below the cerebellum at the rest of the hindbrain, shown in Figure 2-17,
we find three subparts: the reticular formation, the pons, and themedulla. Extending the
length of the entire brainstem at its core, the reticular formation is a netlike mixture of
neurons (gray matter) and nerve fibers (white matter) that gives this structure
the mottled appearance from which its name derives (from the Latin
rete, meaning “net”). The reticular formation has a variety of functions that
are localized along its length into small patches, each with a special
function in stimulating the forebrain, such as in awakening
from sleep.Not surprisingly, the reticular formation is sometimes
also called the reticular activating system.
The pons and medulla contain substructures that control
many vital movements of the body. Nuclei within the pons receive
inputs from the cerebellum and actually bridge it (the Latin
word pons means “bridge”) to the rest of the brain. At the rostral
Threetoed
sloth
Leopard
Hawk
(A) (B)
Gray matter
(cerebellar cortex)
White matter
(cerebellar cortex)
Subcortical
nuclei
Figure 2-16
The Cerebellum and Movement
(A) Their relatively large cerebellums
enable fine, coordinated movements such
as flight and landing in birds and preycatching
in cats. Like the sloth, slowmoving
animals have relatively smaller
cerebellums. (B) Like the cerebrum, the
cerebellum has a cortex with gray and
white matter and subcortical nuclei.
Medulla Cerebellum
Reticular
formation
Pons
Figure 2-17
Hindbrain The principal structures of
the hindbrain integrate both voluntary
and involuntary body movement.
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HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 51
tip of the spinal cord, the medulla’s nuclei control such vital functions
as the regulation of breathing and the cardiovascular system.
For this reason, a blow to the back of the head can kill you—your
breathing stops if the control centers in the hindbrain are injured.
The Midbrain In the midbrain, shown in Figure 2-18, the sensory
component, the tectum, is located dorsally, whereas a motor structure,
the tegmentum, is ventral (tectum meaning roof of the ventrical
and tegmentum meaning floor of the ventrical). The tectum
receives a massive amount of sensory information from the eyes and
ears. The optic nerve sends a large bundle of nerve fibers to the superior
colliculus, whereas the inferior colliculus receives much of its
input from auditory pathways. (Collis in Latin means “hill”; thus the
colliculi appear to be four little hills on the upper surface of the midbrain.)
The colliculi function not only to process sensory information
but also produce orienting movements related to sensory
inputs, such as turning your head to see the source of a sound.
This orienting behavior is not as simple as it may seem. To produce
it, the auditory and visual systems must share some sort of
common “map” of the external world so that the ears can tell the eyes where to look. If
the auditory and visual systems had different maps, it would be impossible to use the
two systems together. In fact, the colliculi also have a tactile map. After all, if you want
to look at the source of an itch on your leg, your visual and tactile systems need a common
representation of where that place is.
Lying ventral to the tectum, the tegmentum (shown in cross section in Figure
2-18) is not a single structure but rather is composed of many nuclei, largely with
movement-related functions. Several of its nuclei control eye movements. The socalled
red nucleus controls limb movements, and the substantia nigra is connected to
the forebrain, a connection especially important in initiating movements. The periacqueductal
gray matter, made up of cell bodies that surround the acqueduct joining the
third and fourth ventricles, contains circuits controlling species-typical behaviors (e.g.,
female sexual behavior). These nuclei also play an important role in the modulation of
pain by opiates.
The Diencephalon The diencephalon, shown in sagittal section at the top left in Figure
2-19, has more anatomical structures than the hindbrain and midbrain have, owing
to its roles in integrating both motor and sensory functions. The two principal structures
of the diencephalon are the hypothalamus and the thalamus. Both are visible on
the ventral view in Figure 2-7, where the thalamus is just to the left of the tip of the
brainstem, and the hypothalamus is to the left of the thalamus.
The hypothalamus is composed of about 22 small nuclei, as well as nerve-fiber
systems that pass through it. Attached to the base of the hypothalamus is the pituitary
gland, shown at the bottom left in Figure 2-19. Although comprising only about 0.3
percent of the brain’s weight, the hypothalamus takes part in nearly all aspects of behavior,
including feeding, sexual behavior, sleeping, temperature regulation, emotional
behavior, hormone function, and movement.
The hypothalamus is organized more or less similarly in different mammals, largely
because the control of feeding, temperature, and so on, is carried out similarly. But there
are sex differences in the structures of some parts of the hypothalamus, which are probably
due to differences between males and females in activities such as sexual behavior
and parenting. A critical function of the hypothalamus is to control the body’s production
of hormones, which is accomplished by interactions with the pituitary gland.
Dorsal
Tegmentum
Cerebellum
Superior colliculus
(receives visual
input)
Superior
colliculus
Red
nucleus
Cerebral
aqueduct
Periaqueductal
gray matter
Reticular
formation
Substantia
nigra
Tectum
Inferior colliculus
(receives auditory
input)
Ventral
Reticular formation. Midbrain area in
which nuclei and fiber pathways are
mixed, producing a netlike appearance;
associated with sleep–wake behavior and
behavioral arousal.
Tectum. Roof (area above the ventricle)
of the midbrain; its functions are sensory
processing, particularly visual and
auditory, and producing orienting
movements.
Tegmentum. Floor (area below the
ventricle) of the midbrain; a collection of
nuclei with movement-related, speciesspecific,
and pain-perception functions.
Orienting movement. Movement
related to sensory inputs, such as turning
the head to see the source of a sound.
Hypothalamus. Diencephalon structure
that contains many nuclei associated with
temperature regulation, eating, drinking,
and sexual behavior.
Figure 2-18
Midbrain Structures in the midbrain
are critical in producing orienting
movements, species-specific behaviors,
and the perception of pain.
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52 ! CHAPTER 2
Medial geniculate nucleus
to auditory cortex
Optic tract from
left eye
Diencephalon Thalamus
Hypothalamus and pituitary gland
Auditory input
Lateral geniculate nucleus
to visual cortex
Dorsomedial nucleus
(connects to frontal lobe)
Hypothalamus
Pituitary stalk
Pituitary gland
Figure 2-19
Diencephalon The connections of only 3 of
the 20-odd thalamic nuclei are shown for the
right thalamus, but each nucleus connects to a
discrete region of cortex. Lying below (hypo)
the thalamus, at the base of the brain, the
hypothalamus and pituitary lie above the roof
of the mouth. The pituitary gland lies adjacent
to the optic chiasm, where the left and right
optic tracts (originating from the eyes) cross
over en route to the occipital lobe.
For a three-dimensional view of the
hypothalamus and thalamus, visit the
central nervous system module on the
Foundations CD. To examine the
structures, go to the overview and look in
the section on subcortical structures.
Figure 2-20
Forebrain Structures The major
structures of the forebrain integrate
sensation, emotion, and memory to
enable advanced cognitive functions
such as thinking, planning, and
language.
Basal ganglia
(caudate nucleus,
putamen, globus
pallidus)
Cerebral
cortex
Amygdala
Hippocampus
The other principal structure of the diencephalon, the thalamus, is much larger
than the hypothalamus. Like the hypothalamus, the thalamus contains about 20 nuclei,
although the thalamic nuclei are much larger than those in the hypothalamus. Perhaps
most distinctive among the functions of the thalamus is its role as a kind of gateway
for channeling sensory information traveling to the cerebral cortex.
All sensory systems send inputs to the thalamus for information integration and
relay to the appropriate area in the cortex. The optic nerve, for example, sends information
through a large bundle of fibers to a region of the thalamus, the lateral geniculate
nucleus, shown on the right in Figure 2-19. In turn, the lateral geniculate nucleus
processes some of this information and then sends it to the visual region of the cortex.
The routes to the thalamus may be somewhat indirect; for example, the route for
olfaction traverses several synapses before entering the dorsomedial nucleus of the thalamus.
Analogous sensory regions of the thalamus receive auditory and tactile information,
which is subsequently relayed to the respective auditory and tactile cortical regions.
Some thalamic regions have motor functions or perform an integrative task. One
region with an integrative function is the dorsomedial thalamic nucleus (see Figure
2-19). It has connections to most of the frontal lobe of the cortex.We return to the thalamic
sensory nuclei in Chapters 8 through 10, where we examine how sensory information
is processed. Other thalamic regions are considered in Chapters 11 and 13,
where we explore motivation and memory.
THE FOREBRAIN
The forebrain, whose major internal and external structures are shown in Figure 2-20,
is the largest region of the mammalian brain. Each of its three principal structures has
multiple functions. To summarize briefly, the neocortex, another name for the cerebral
cortex, regulates a host of mental activities ranging from perception to planning;
the basal ganglia control voluntary movement; and the limbic system regulates emotions
and behaviors that create and require memory. You will encounter each of these
forebrain structures in detail later in this book.
Extending our analogy between the brainstem and your forearm, imagine that the
“fist” of the brainstem (the diencephalon) is thrust inside a watermelon. The watermelon
represents the forebrain,with the rind being the cortex and the fruit inside being
the limbic system and the basal ganglia. By varying the size of the watermelon, we can
vary the size of the brain, which in a sense is what evolution has done. The forebrain
varies considerably in size across species (see Figure 2-2).
The Cortex There are actually two types of cortex, the old and the new. The neocortex
(new bark) has six layers of gray matter on top of a layer of white matter. The neocortex
is the tissue that is visible when we view the brain from the outside, as in Figure
2-7. The neocortex is unique to mammals, and its primary function is to create and respond
to a perceptual world.
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The older cortex, sometimes called limbic cortex, has three or four layers of gray
matter on top of a layer of white matter. This tissue is not easily observed on the outside
surface of the human brain, except where it forms the cingulate cortex, a region visible
in medial views lying just above the corpus callosum (see the medial view in Figure
2-7). The limbic cortex is more primitive than the neocortex. It is found in the brains
of other chordates in addition to mammals, especially in birds and reptiles.
The limbic cortex is thought to play a role in controlling motivational states. Although
anatomical and functional differences exist between the neocortex and the limbic
cortex, the distinctions are not critical for most discussions in this book. Therefore,
we will usually refer to both types of tissue simply as cortex.
Measured by volume, the cortex makes up most of the forebrain, comprising 80
percent of the human brain overall. It is the brain region that has expanded the most
during mammalian evolution. The human neocortex has a surface area as large as 2500
square centimeters but a thickness of only 1.5 to 3.0 millimeters. This area is equivalent
to about four pages of this book. (In contrast, a chimpanzee has a cortical area
equivalent to about one page.)
The pattern of sulci and gyri formed by the folding of the cortex varies across
species. Some species, such as rats, have no sulci or gyri, whereas carnivores have gyri
that form a longitudinal pattern (look back at the cat brain in Figure 2-2). In primates,
the sulci and gyri form a more diffuse pattern.
As you know, the human cortex consists of
two nearly symmetrical hemispheres, the left
and the right, which are separated by the longitudinal
fissure. Each hemisphere is subdivided
into the four lobes, corresponding to the skull
bones overlying each hemisphere, introduced in
Chapter 1: frontal, temporal, parietal, and occipital.
Unfortunately, bone location and brain
function are unrelated. As a result, the lobes of
the cortex are rather arbitrarily defined regions
that include many different functional zones.
Nonetheless,we can attach some gross functions
to each lobe.The three posterior lobes have
sensory functions: the occipital lobe is visual; the
parietal lobe is tactile; and the temporal lobe is
visual, auditory, and gustatory. In contrast, the
frontal lobe is motor and is sometimes referred to as the brain’s “executive” because it
integrates sensory and motor functions and formulates plans of action.
Fissures and sulci often establish the boundaries of cortical lobes. For instance, in
humans, the central sulcus and lateral fissure form the boundaries of each frontal lobe.
They also form the boundaries of each parietal lobe, but in this case the lobes lie posterior
to the central sulcus. The lateral fissure demarcates each temporal lobe as well,
forming its dorsal boundary. The occipital lobes are not so clearly separated from the
parietal and temporal lobes, because no large fissure marks their boundaries. Traditionally,
the occipital lobes are defined on the basis of other anatomical features, which
are presented in Chapter 8.
The layers of the cortex have several distinct characteristics:
Different layers have different cell types.
The density of cells in each layer varies, ranging from virtually no cells in layer I (the
top layer) to very dense cell packing in layer IV (Figure 2-21).
Other differences in appearance relate to the functions of cortical layers in different
regions. These visible differences led neuroanatomists of the early twentieth century to
HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 53
Thalamus. Diencephalon structure
through which information from all
sensory systems is integrated and
projected into the appropriate region of
the neocortex.
Forebrain. Evolutionarily the newest
part of the brain; coordinates advanced
cognitive functions such as thinking,
planning, and language; contains the
limbic system, basal ganglia, and the
neocortex.
Neocortex (cerebral cortex). Newest,
outer layer (new bark) of the forebrain
and composed of about six layers of gray
matter that creates our reality.
Basal ganglia. Group of nuclei in the
forebrain that coordinates voluntary
movements of the limbs and the body;
located just beneath the neocortex and
connected to the thalamus and to the
midbrain.
Limbic system. Disparate forebrain
structures lying between the neocortex
and the brainstem that form a functional
system controlling affective and motivated
behaviors and certain forms of memory;
includes cingulate cortex, amygdala,
hippocampus, among other structures.
Longitudinal
Left fissure
hemisphere
Right
hemisphere
Frontal
lobe
Central
sulcus
Temporal
lobe
Lateral
fissure
Parietal
lobe
Occipital
lobe
Visit the module on the central
nervous system on the Foundations CD to
view a three-dimensional model of the
cortex, along with photographs of cortical
sections.
Visit the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
and go to the Chapter 2 Web links to see
how the cortex looks in other animals.
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make maps of the cortex, like the one in Figure 2-22A that was developed by Albert
Brodmann in about 1905. Because these maps are based on cell characteristics,
the subject of cytology, they are called cytoarchitectonic maps. For example, sensory
cortex in the parietal lobe, shown in red in Figure 2-21, has a distinct layer IV.
Motor cortex in the frontal lobe, shown in blue in the same illustration, has a more
distinctive layer V. Layer IV is an afferent layer, whereas layer V is efferent. It makes
sense that a sensory region would have a large input layer, whereas a motor
region would have a large output layer.
Chemical differences in the cells in different cortical layers can be revealed
by staining the tissue. Some regions are rich in one chemical,
whereas others are rich in another. These differences are presumably related
to functional specialization of different areas of the cortex.
The one significant difference between the organization of the cortex
and the organization of other parts of the brain is its range of connections.
Unlike most brain structures that connect to only selective brain regions,
the cortex is connected to virtually all other parts of the brain. The cortex,
in other words, is the ultimate meddler. It takes part in everything. This fact
not only makes it difficult to identify specific functions of the cortex but
also complicates our study of the rest of the brain because the cortex’s role in other
brain regions must always be considered.
To illustrate, consider your perception of clouds. You have no doubt gazed up at
clouds on a summer’s day and imagined sailing ships, elephants, faces, and countless
other objects. Although a cloud does not really look exactly like an elephant, you can
concoct an image of one if you impose your frontal cortex—that is, your imagination—
on the sensory inputs. This kind of cortical activity is known as top-down processing
because the top level of the nervous system, the cortex, is influencing how information
is processed in lower regions—in this case, the midbrain and hindbrain.
The cortex influences many behaviors besides the perception of objects. It influences
our cravings for foods, our lust for things (or people), and how we interpret the
meaning of abstract concepts, words, and images. The cortex is the ultimate creator of
our reality, and one reason that it serves this function is that it is so well connected.
The Basal Ganglia A collection of nuclei that lie within the forebrain just below the
white matter of the cortex, the basal ganglia consist of three principal structures: the
caudate nucleus, the putamen, and the globus pallidus, all shown in Figure 2-23. Together
with the thalamus and two closely associated structures, the substantia nigra and
subthalamic nucleus, the basal ganglia form a system that functions primarily to control
certain aspects of voluntary movement.
54 ! CHAPTER 2
Cytoarchitectonic map. Map of the
neocortex based on the organization,
structure, and distribution of the cells.
Motor
cortex
Sensory
cortex
I
Input of
sensory
information
Output
to other
parts of
brain
Integrative
functions
II
III
IV
V
VI
I
II
III
IV
V
VI
Motor
cortex
Sensory
cortex
Figure 2-21
Layering in the Neocortex As this
comparison of cortical layers in the
sensory and motor cortices shows, layer
IV is relatively thick in the sensory cortex
and relatively thin in the motor cortex.
Afferents go to layer IV (from the
thalamus) as well as to layers II and III.
Efferents go to other parts of the cortex
and to the motor structures of the brain.
(A) (B) Touch (3–1–2)
Vision (17)
Hearing (41)
12
4 3
5
6
7
8
9
10
11
19
17
19 18
20
21
22
37
42
40
46
45 44
47
43
39
18
38
Figure 2-22
Brain Maps (A) In his cytoarchitectonic
map of the cortex, Brodmann defined
areas by the organization and characteristics
of the cells. (B) This schematic
map shows the regions associated with
the simplest sensory perceptions of
touch, vision, and audition. As we shall
see, the areas of the cortex that process
sensory information are far greater than
these basic areas.
CH02.qxd 1/28/05 9:28 AM Page 54

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