Science Brain and Behavior contiuned 6

HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 55
Visit the central nervous system
module on the Foundations CD for a
three-dimensional view of the basal
ganglia. To see the model, go to the
overview and look in the section on
subcortical structures.
We can observe the functions of the basal ganglia by analyzing the behavior that
results from the many diseases that interfere with the normal functioning of these
nuclei. People afflicted with Parkinson’s disease, one of the most common disorders
of movement in the elderly, take short, shuffling steps, display bent posture, and
often require a walker to get around. Many have an almost continual tremor of the
hands and sometimes of the head as well. (We return to this disorder in Chapters
5 and 10.) Another disorder of the basal ganglia is Tourette’s syndrome, characterized
by various motor tics, involuntary vocalizations (including curse words and animal
sounds), and odd, involuntary movements of the body, especially of the face and
head.
Neither Parkinson’s disease nor Tourette’s syndrome is a disorder of producing
movements, as in paralysis. Rather they are disorders of controlling movements. The
basal ganglia, therefore,must play a role in the control and coordination of movement
patterns, not in activating the muscles.
The Limbic System In the 1930s, psychiatry was dominated by the theories of Sigmund
Freud, who emphasized the roles of sexuality and emotion in understanding
human behavior. At the time, regions controlling these behaviors had not been identified
in the brain, but a group of brain structures, collectively called the limbic system,
as yet had no known function. It was a simple step to thinking that perhaps the limbic
system played a central role in sexuality and emotion.
One sign that this hypothesis might be right came from James Papez, who discovered
that people with rabies have infections of limbic structures, and one of the symptoms
of rabies is emotionality. We now know that such a simple view of the limbic
system is inaccurate. In fact, the limbic system is not a unitary system at all, and, although
some limbic structures have roles in emotion and sexual behaviors, limbic
structures serve other functions, too, including memory and motivation.
The principal structures of the limbic system are shown in Figure 2-24. They include
the amygdala, the hippocampus, and the limbic, or cingulate, cortex, which lies in
the cingulate gyrus between the cerebral hemispheres. Removal of the amygdala produces
truly startling changes in emotional behavior. For example, a cat with the amygdala
removed will wander through a colony of monkeys, completely undisturbed by
their hooting and threats. No self-respecting cat would normally be caught anywhere
near such bedlam.
The hippocampus, the cingulate cortex, and associated structures have roles in certain
memory functions, as well as in the control of navigation in space. Many limbic
structures also are believed to be at least partly responsible for the rewarding properties
of psychoactive drugs.As we shall see in Chapter 7, repeated exposure to drugs such
as amphetamine or nicotine produces both chemical and structural changes in the cingulate
cortex and hippocampus, among other structures.
Caudate
nucleus
Thalamus
Lateral ventricle
Corpus callosum
Putamen
Basal
ganglia
Basal
ganglia
Globus
pallidus
Subthalamic
nucleus
Substantia
nigra
Figure 2-23
Basal Ganglia This frontal section
of the cerebral hemispheres shows the
basal ganglia relative to surrounding
structures. Two associated structures,
the substantia nigra and subthalamic
nucleus, instrumental in controlling and
coordinating movement, also are
illustrated.
Figure 2-24
Limbic System This medial view of the
right hemisphere illustrates the principal
structures of the limbic system that play
roles in emotional and sexual behaviors
and memory.
Amygdala
Cingulate cortex
(limbic cortex)
Temporal
lobe
Hippocampus
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The Olfactory System At the very front of the brain lie the olfactory
bulbs, the organs responsible for our sense of smell. The olfactory system
is unique among the senses, as Figure 2-25 shows, because it is almost
entirely a forebrain structure (see also the ventral view in Figure
2-7). Recall that the other sensory systems project most of their inputs
from the sensory receptors to the midbrain and thalamus. Olfactory
input takes a less direct route: the olfactory bulb sends most of its inputs
to a specialized region, the pyriform cortex, at the bottom of the brain
before progressing to the dorsal medial thalamus, which then provides
a route to the frontal cortex.
Compared with the olfactory bulbs of animals such as rats and dogs,
which depend more heavily on the sense of smell than we do, the human
olfactory bulb is relatively small. Nonetheless, it is still sensitive and
plays an important role in various aspects of our feeding and sexual behavior.We return
to the olfactory system in Chapter 11.
The Somatic Nervous System
The somatic nervous system (SNS) is monitored and controlled by the CNS—the cranial
nerves by the brain and the spinal nerves by the spinal cord.
THE CRANIAL NERVES
The linkages provided by the cranial nerves between the brain and various parts of the
head and neck as well as various internal organs are illustrated and tabulated in Figure
2-26. Cranial nerves can have afferent functions, such as sensory inputs to the brain
from the eyes, ears,mouth, and nose, or they can have efferent functions, such as motor
56 ! CHAPTER 2
1 2
3
4
5
7
8
9
10
11
12
6
Cranial nerve Name Function
1 Olfactory Smell
2 Optic Vision
3 Oculomotor Eye movement
4 Trochlear Eye movement
Trigeminal Masticatory movements
and facial sensation
5
6 Abducens Eye movement
7 Facial Facial movement and sensation
8 Auditory vestibular Hearing and balance
Glossopharyngeal Tongue and pharynx movement
and sensation
9
Vagus Heart, blood vessels, viscera,
movement of larynx and pharynx
10
11 Spinal accessory Neck muscles
12 Hypoglossal Tongue muscles
Pyriform
cortex
To pyriform
cortex
Sensory input from nose
Olfactory bulb
Figure 2-25
Sense of Smell The relatively small
olfactory bulb of humans lies at the base
of the human brain and is connected to
receptor cells that lie in the nasal cavity.
Figure 2-26
Cranial Nerves Each of the 12 pairs of
cranial nerves has a different function. A
common mnemonic device for learning
the order of the cranial nerves is, On old
Olympus’s towering top, a Finn and
German vainly skip and hop. The first
letter of each word (except the last and)
is, in order, the first letter of the name
of each nerve.
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control of the facial muscles, tongue, and eyes. Some cranial nerves have both sensory
and motor functions, such as the modulation of both sensation and movement in the
face.
The 12 pairs of cranial nerves are known both by their numbers and by their names
(see Figure 2-26). One set of 12 controls the left side of the head, whereas the other set
controls the head’s right side. This arrangement makes sense for innervating duplicated
parts of the head (such as the eyes), but it is not so clear why separate nerves should control
the right and left sides of a singular structure (such as the tongue). Yet that is how
the cranial nerves work. If you have ever received novocaine for dental work, you know
that usually just one side of your tongue becomes anesthetized because the dentist injects
the drug into only one side of your mouth. The rest of the skin and muscles on
each side of the head are similarly controlled by cranial nerves located on that same side.
We consider many of the cranial nerves in some detail in later chapters in discussions
on topics such as vision, hearing, and responses to stress. For now, you simply
need to know that cranial nerves form part of the somatic nervous system, providing
inputs to the brain from the head’s sensory organs and muscles and controlling head
and facial movements. The cranial nerves also contribute to maintaining autonomic
functions by connecting the brain and internal organs and by influencing other autonomic
responses, such as salivation.
THE SPINAL NERVES
The spinal cord lies inside the bony spinal column,which is made up of a series of small
bones called vertebrae, categorized into five regions from top to bottom: cervical, thoracic,
lumbar, sacral, and coccygeal, as diagrammed in Figure 2-27A. You can think of
HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 57
(A) (B)
C8
C3
C4
C5
C6
C7
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L3
L4
L5
S1
S1
S2
S2
S3
S4
S5
C2
C7
L5 L5
Thoracic
nerves
Lumbar
nerves
Coccygeal
segment
Sacral
nerves
Cervical
nerves
Spinal cord
Dermatomes
Vertebrae
(spinal column)
C1
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L3
L4
L5
S1
S2
S3
S4
S5
Cranial nerve. One of a set of 12 nerve
pairs that control sensory and motor
functions of the head, neck, and internal
organs.
Vertebrae. The bones, or segments, that
form the spinal column.
Figure 2-27
Spinal Segments and Dermatomes
(A) The spinal column, illustrated in
sagittal view showing the five spinalcord
segments: cervical (C), thoracic (T),
lumbar (L), sacral (S), and coccygeal.
(B) Each spinal segment corresponds to
a dermatome supplied with afferent
peripheral nerve fibers by a single spinalcord
dorsal root and identified by the
segment number (examples are C5
and L2).
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each vertebra within these five groups as a very short segment of the spinal column.
The corresponding spinal-cord segment within each vertebral region functions as that
segment’s “minibrain.”
This arrangement may seem a bit odd, but it has a long evolutionary history. Think
of a simpler animal, such as a snake, which evolved long before humans did. A snake’s
body is a tube divided into segments.Within that tube is another tube, this one of neurons
of the spinal cord, which also is segmented. Each of the snake’s nervous system
segments receives fibers from sensory receptors in the part of the body adjacent to it,
and that nervous system segment sends fibers back to the muscles in that body part.
Each segment, therefore, works independently.
A complication arises in animals lsuch as humans, who have limbs that may originate
at one spinal-segment level but extend past other segments of the spinal column.
Your shoulders, for example,may begin at C3 (cervical segment 3), but your arms hang
down well past the sacral segments. So, unlike the snake, which has spinal-cord segments
that connect to body segments fairly directly adjacent to them, human body
segments are schematically in more of a patchwork pattern, as shown in Figure 2-27B,
but make sense if the arms are extended as they are if we walk on “all fours.”
Regardless of their complex pattern, however, the segments of our bodies still
correspond to segments of the spinal cord. Each of these body segments is called a
dermatome (meaning “skin cut”).A dermatome has both a sensory nerve,which sends
information from the skin, joints, and muscles to the spinal cord, and a motor nerve,
which controls the movements of the muscles in that particular segment of the body.
These sensory and motor nerves, known as spinal nerves, are functionally equivalent
to the cranial nerves of the head.Whereas the cranial nerves receive information
from sensory receptors in the eyes, ears, facial skin, and so forth, the spinal nerves receive
information from sensory receptors in the rest of the body. Similarly, whereas the
cranial nerves move the muscles of the eyes, tongue, and face, the peripheral nerves
move the muscles of the limbs and trunk.
SNS Connections Like the central nervous system, the somatic nervous system is bilateral
(two sided). Just as the cranial nerves control functions on the same side of the
head on which they are found, the spinal nerves on the left side of the spinal cord control
the left side of the body, and those on the right side of the spinal cord control the
body’s right side.
Figure 2-28 shows the spinal column in cross section. Look first at the nerve fibers
entering the spinal cord’s dorsal side (in the body of a normally upright animal such as
a human, the dorsal side means the back, as illustrated in Figure 2-4). These dorsal fibers
are afferent: they carry information from the body’s sensory receptors. The fibers collect
together as they enter a spinal-cord segment, and this collection of fibers is called a
dorsal root.
Fibers leaving the spinal cord’s ventral side (ventral here means the front) are efferent,
carrying information from the spinal cord to the muscles. They, too, bundle together
as they exit the spinal cord and so form a ventral root. As you can see in the cross
section at the top of the drawing in Figure 2-28, the outer part of the spinal cord consists
of white matter, or CNS nerve tracts. These tracts are arranged so that, with some
exceptions, dorsal tracts are sensory and ventral tracts are motor. The inner part of the
cord, which has a butterfly shape, is gray matter composed largely of cell bodies.
The observation that the dorsal spinal cord is sensory and the ventral side is motor
is one of the nervous system’s very few established laws, the law of Bell and Magendie.
Combined with an understanding of the spinal cord’s segmental organization, this law
enables neurologists to make quite accurate inferences about the location of spinal-cord
58 ! CHAPTER 2
Dermatome. Area of the skin supplied
with afferent nerve fibers by a single
spinal-cord dorsal root.
Law of Bell and Magendie. The
general principle that sensory fibers are
located dorsally and ventral fibers are
located ventrally.
Go to the Foundations CD and find
the spinal-cord area of the central
nervous system module. There you can
see a detailed illustration of the spinal
cord.
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HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 59
damage or disease on the basis of changes in sensation or movement that patients experience.
For instance, if a person experiences numbness in the fingers of the left hand
but can still move the hand fairly normally, one or more of the dorsal nerves in spinalcord
segments C7 and C8 must be damaged. In contrast, if sensation in the hand is normal
but the person cannot move the fingers, the ventral roots of the same segments must
be damaged. The topic of diagnosing spinal-cord injury or disease is further discussed
in “Magendie, Bell, and Bell’s Palsy” on page 60.
So far we have emphasized the segmental organization of the spinal cord, but the
spinal cord must also somehow coordinate inputs and outputs across different segments.
For example, many body movements require the coordination of muscles that
are controlled by different segments, just as many sensory experiences require the
coordination of sensory inputs to different parts of the spinal cord. How is this coordination
of spinal-cord activities accomplished? The answer is that the spinal-cord segments
are interconnected in such a way that adjacent segments can operate together to
direct rather complex coordinated movements.
The integration of spinal-cord activities does not require the brain’s participation,
which is why the headless chicken can run around in a reasonably coordinated way.
Still, a close working relation must exist between the brain and the spinal cord. Otherwise,
how could we consciously plan and execute our voluntary actions? Somehow
information must be relayed back and forth, and examples of this information sharing
are numerous. For instance, tactile information from sensory nerves in the skin
travels not just to the spinal cord but also to the cerebral cortex through the thalamus.
Similarly, the cerebral cortex and other brain structures can control movements because
of their connections to the ventral roots of the spinal cord. So, even though the
brain and spinal cord can function independently, the two are intimately connected in
their functions.
Sympathetic
nerve chain
Spinal
nerve
White
matter
Ventral root Gray matter
(motor nerves)
Dorsal root
(sensory nerves)
Layers of
dura mater
Vertebra
Dorsal
Dorsal fibers carry
information from
body to spinal cord.
Ventral fibers carry
information from
spinal cord to muscles.
Gray matter consists
mostly of cell bodies.
White matter is
arranged in dorsal
tracts and ventral
tracts.
Ventral
Bassett/Visuals Unlimited Figure 2-28
Spinal-Cord Connections The spinal
cord runs inside the vertebral column.
The sympathetic nerve chain, which is
part of the autonomic nervous system,
lies outside the spinal column. As in
the brain, spinal gray matter is made
up largely of cell bodies, whereas the
white matter is made up of fiber tracts
that ascend dorsally (superiorly) and
descend ventrally (inferiorly) to and
from the brain, respectively. Note that
you are viewing the spine from the back
in this diagram. A photograph shows
the exposed spinal column from this
dorsal view.
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60 ! CHAPTER 2
The Autonomic Nervous System
The internal autonomic nervous system (ANS) is a hidden partner in controlling behavior.
Even without our conscious awareness, it stays on the job to keep the heart beating,
the liver releasing glucose, the pupils of the eyes adjusting to light, and so forth.
Without the ANS, which regulates the internal organs and glands by connections
through the SNS to the CNS, life would quickly cease. Although it is possible to learn
to exert some conscious control over some of these vegetative activities, such conscious
interference is unnecessary. One important reason is that the ANS must keep working
during sleep when conscious awareness is off-duty.
The two divisions of the ANS, sympathetic and parasympathetic, work in opposition.
The sympathetic system arouses the body for action, for example, by stimulating
the heart to beat faster and inhibiting digestion when we exert ourselves during exercise
or times of stress; that is, the familiar “fight or flight” response. The parasympathetic
Magendie, Bell, and Bell’s Palsy
Focus on Disorders
François Magendie, a volatile and committed French experimental
physiologist, reported in a three-page paper in 1822
that he had succeeded in cutting the dorsal and ventral roots
of puppies, animals in which the roots are sufficiently segregated
to allow such surgery. Magendie found that cutting the
dorsal roots caused loss of sensation, whereas cutting the
ventral roots caused loss of movement.
Eleven years earlier, a Scotsman named Charles Bell had
proposed functions for these nerve roots on the basis of
anatomical information and the results of somewhat inconclusive
experiments on rabbits. Although Bell’s findings were
not identical with Magendie’s, they were similar enough to
ignite a controversy. Bell hotly disputed Magendie’s claim to
the discovery of dorsal and ventral root functions. As a result,
the principle of sensory and motor segregation in the
nervous system has been given both researchers’ names: the
law of Bell and Magendie.
Magendie’s conclusive experiment on puppies was considered
extremely important because it enabled neurologists
for the first time to localize nervous system damage from the
symptoms that a patient displays. Bell went on to describe
an example of such localized, cranial motor-nerve dysfunction
that still bears his name—Bell’s palsy, which is a facial
paralysis that occurs when the motor part of the facial nerve
on one side of the head becomes inflamed (see the accompanying
photograph).
The onset of Bell’s palsy is typically sudden. Often the
stricken person wakes up in the morning and is shocked to
discover that the face is paralyzed on one side. He or she
cannot open the mouth on that side of the head or completely
close the eye on that side. Most people fully recover from
Bell’s palsy, although it may take several months. But, in rare
instances, such as that of Jean Chretien, the former prime
minister of Canada, paralysis of the mouth is permanent.
A young man suffering from Bell’s palsy, a paralysis of the facial
nerve that causes weakness over one side of the face. He was
photographed during an involuntary tic (a nervous reaction) that
affects the right side of the face, causing his right eye to close
tightly.
Dr. P. Marazzi/Science Photo Library/
Photo Researchers
Sympathetic system. Part of the
autonomic nervous system; arouses the
body for action, such as mediating the
involuntary fight or flight response to
alarm by increasing heart rate and blood
pressure.
Parasympathetic system. Part of
the autonomic nervous system; acts in
opposition to the sympathetic system—for
example, preparing the body to rest and
digest by reversing the alarm response or
stimulating digestion.
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HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 61
system calms the body down, for example, by slowing the heartbeat and stimulating
digestion to allow us to “rest and digest” after exertion and during quiet times.
Like the SNS, the ANS interacts with the rest of the nervous system. Activation of
the sympathetic system starts in the thoracic and lumbar spinal-cord regions. But the
spinal nerves do not directly control the target organs. Rather, the spinal cord is connected
to autonomic control centers, which are collections of neural cells called ganglia.
The ganglia control the internal organs.
The sympathetic ganglia are located near the spinal cord, forming a chain that runs
parallel to the cord, as illustrated on the left in Figure 2-29. The parasympathetic system
also is connected to the spinal cord—specifically, to the sacral region—but the
greater part of it derives from three cranial nerves: the vagus nerve, which calms most
of the internal organs, and the facial and oculomotor nerves, which control salivation
and pupil dilation, respectively. In contrast with the sympathetic system, the parasympathetic
system connects with ganglia that are near the target organs, as shown on the
right in Figure 2-29.
Cervical
Thoracic
Lumbar
Sacral
Cervical
Oculomotor
nerve
Facial nerve
Vagus
nerve
Thoracic
Lumbar
Sacral
Sympathetic system
Stimulation: “fight or flight”
Parasympathetic system
Inhibitory: “rest and digest”
Liver
Pancreas
Sympathetic
chain
ganglion
Stomach
Intestines
Rectum
Inhibits salivatio
Inhibits salivation
Relaxes airways
Inhibits digestion
esoculg seta lumitS
esaeler
Contracts bladder
Constricts blood vessels
Dilates blood
vessels
Stimulates ejacula tion
Stimulates erect ion
Stimulates bladd
Accel era t es heartbeat Slows heartbeat
Constricts airway
Stimulates salivation
Dila t es pupil Contracts pupil
Kidney
Sympathetic
prevertebral ganglia
Genitals
Adrenal gland
Stimulates digestion
Gall bladder
Cranial Cranial
Liver
Lungs
Heart
Secretion of
adrenalin
Figure 2-29
Autonomic Nervous System The
pathways of the two divisions of the
autonomic nervous system exert
opposing effects on the organs that they
innervate. All autonomic fibers connect
at “stops” en route from the CNS to
their target organs. (Left) Arousing
sympathetic fibers connect to a chain of
ganglia near the spinal cord. (Right)
Calming parasympathetic fibers connect
to individual parasympathetic ganglia
near the target organs.
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62 ! CHAPTER 2
EIGHT PRINCIPLES OF NERVOUS SYSTEM FUNCTION
Knowing the parts of a car engine is the place to start if you want to understand how
an engine works. But even though you know which part the carburetor is, you will not
understand its function until you grasp the principle of air and fuel mixing, igniting
and powering the cylinders. Knowing the parts, unfortunately, is not enough. You also
need some guiding principles about how the parts work together.
Now that you know the basic parts of the nervous system, learning some general
principles will help you understand how its different parts work together. Table 2-3 lists
eight principles that form the basis for many discussions throughout this book. Spending
the time needed to understand these principles fully before moving on will place
you at an advantage in your study of brain and behavior.
Principle 1: The Sequence of Brain Processing
Is “In Integrate Out”
The parts of the brain make a great many connections with one another. Recall, for example,
that meddling cerebral cortex that appears to be connected to everything. This
connectivity of the brain is the key to its functioning.
In Review .
Traditional discussions of the nervous system that focus on anatomy distinguish between the
central nervous system, which consists of the brain and spinal cord, and the peripheral nervous
system, which encompasses everything else. We focus instead on a functional categorization
in which the CNS interacts with the divisions of the PNS: the somatic nervous system
that includes the spinal and cranial nerves of the head and body and the autonomic nervous
system that controls the body’s internal organs. Each nervous system segment can be further
subdivided into functionally distinct subsections such as the forebrain, hindbrain, and spinal
cord of the CNS. Similarly, within each CNS subsection, we find more functional subregions,
such as the limbic system and the basal ganglia of the forebrain. Finally, each functional
system can be further divided into areas that have their own unique functions, such as
the caudate nucleus, putamen, and globus pallidus of the basal ganglia. An effective process
for learning the anatomy of the nervous system is to work from the general to the more specific
in each category and, in each case, to remember to associate structure with function.
Principles of Nervous System Functioning
1. Information-processing sequence in the brain is “in integrate out.”
2. Sensory and motor functions throughout the nervous system are separated.
3. Inputs and outputs to the brain are crossed.
4. Brain anatomy and function display both symmetry and asymmetry.
5. The nervous system operates by a juxtaposition of excitation and inhibition.
6. The nervous system has multiple levels of function.
7. Brain components operate both in parallel and hierarchically.
8. Functions in the brain are both localized in specific regions and distributed.
Table 2-3
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HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 63
For an animation of how neurons
integrate information, go to the section
on neural integration in the neural
communication module on the
Foundations CD.
Sensory and motor systems interact constantly to control an organism’s interaction
with its environment. The fundamental points of connection between neurons,
known as synapses, allow cells in different brain regions to influence one another.
Chapter 5 explains the organization of the synapse. The key point here is that most
neurons have afferent (incoming) connections with tens or sometimes hundreds of
thousands of other neurons, as well as efferent (outgoing) connections to neurons and
many other cell types.
CREATING NEW INFORMATION
Figure 2-30 charts a basic example of creating new information in the brain through
the sense of vision, beginning with receptor cells, neurons located in the retina of the
eye. Different receptor cells are most receptive to light of a particular
wavelength: red, green, or blue. In the brain, neurons receive inputs
from one or more of these color-sensitive receptors.
A neuron might be able to receive inputs from only one receptor
type, from two receptor types, or from all three. A neuron receiving
input only from green-type receptor cells “knows” only about green
and forwards only green information. In contrast, a neuron receiving
input from both green- and red-type receptors “knows” about two
colors and forwards a very different message, as would a neuron receiving
input from all three receptor types.
The neurons that receive more than one kind of input sum the
information that they get. In a sense, they create new information that
did not previously exist. This summation is the “integration operation”
of the brain.
SUMMATION OF INPUTS
The top panel in Figure 2-31 shows how multiple connections enable neurons to integrate
information at the cellular level and thus create new information. The inputs to
a neuron at any given moment are summed up, and the signal sent by that neuron to
other neurons incorporates this summation.
Summation is more than just adding up equally weighted inputs. Some inputs
have a greater influence than do others on the receiving neuron, and the simultaneous
occurrence of certain inputs may have effects that far exceed their simple sum. The
summation of information, then, can transform information before it’s passed on to
other neurons. This transformation makes the summation process partly one of creating
new information.
The same principle holds for the functioning of nuclei within the brain or of a
layer of brain tissue, as illustrated in the middle panel of Figure 2-31. The inputs to
each neuron in a nucleus are not identical, and so there are internal (intrinsic) connections
between the neurons. Several areas of the nucleus might each send different
information to another area, which integrates that information and sends a combined
message along to several other areas.
This process is much like the summation of information in a single neuron, but,
in this case, summation takes place in a collection of neurons. As a result of summation,
the output of all the cells in the nucleus is changed. Once again, there is information
integration.
The logical extension of this discussion is to view the entire brain as an organ that
receives inputs, creates information, and produces behavior, as illustrated in the bottom
panel of Figure 2-31. To the animal whose brain is engaged in this process, the creation
of information from inputs represents reality. The more complex the brain
B
Color in
the world
Neuron
in brain
Receptor
cell
G
R
B
B G
B G R
Figure 2-30
Integrating Information Receptor cells
B (blue), G (green), and R (red), or
“input,” each code information about
one hue and pass it along to neurons B,
B/G, and B/G/R in the brain. The neurons
sum (integrate) this information to send
a new message (output) that we
perceive as color.
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circuitry, the more complex the reality that can be created and, subsequently, the more
complex the thoughts that can be expressed. The emergence of thought that enables
consciousness may be the brain’s ultimate act of integration.
Principle 2: Sensory and Motor Divisions Exist
Throughout the Nervous System
The segregation of sensory and motor functions described by the Bell and Magendie
law exists throughout the nervous system. However, distinctions between motor and
sensory functions become subtler in the forebrain.
SENSORY AND MOTOR DIVISIONS AT THE PERIPHERY
The spinal nerves, as diagrammed in Figure 2-28, are either sensory or motor in function.
Some cranial nerves are exclusively sensory; some are exclusively motor; and some
have two parts, one sensory and one motor, much like spinal nerves serving the skin
and muscles.
SENSORY AND MOTOR DIVISIONS IN THE BRAIN
Essentially extensions of the spinal cord, the lower brainstem regions—hindbrain and
midbrain—retain the spinal-cord division, with sensory structures located dorsally
and motor ones ventrally. Recall that an important function of the midbrain is to orient
the body to stimuli. This orientation requires both sensory input and motor output.
The midbrain’s colliculi, which are located dorsally in the tectum, are the sensory
component, whereas the tegmentum, which is ventral (below the colliculi), is a motor
structure that plays a role in controlling various movements, including orienting ones.
Brainstem structures are illustrated in Figures 2-15 through 2-18.
64 ! CHAPTER 2
Source Inputs Integration
Cellular
level
Outputs
N1 = neuron 1
N4
N5
N1
N2
N3 N6
N7
N8
Nuclei
level
Area
A
Area
B
Area
X
Area
Y
Area
Area Z
C
Brain
level World Brain Behaviors
Figure 2-31
Levels of Neural Processing (Top) At
the cellular level, each of neurons 1
through 5 sends some message to
neuron 6, which essentially now
“knows” what neurons 1 through 5
signaled. This “knowledge” is a form of
integration. The output from neuron 6 is
sent to neurons 7 and 8, whose activity is
affected by the “knowledge” of neuron
6. (Center) At the level of brain nuclei,
areas A through C signal area X, which
can integrate (combine) the information
from areas A through C. The integrated
information is then sent to areas Y and
Z. (Bottom) At the macro level, the
world provides information to the brain,
which produces behavior.
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Distinct sensory nuclei are present in the thalamus, too, although they are no
longer located dorsally. Because all sensory information reaches the forebrain through
the thalamus, it is not surprising to find separate nuclei associated with vision, hearing,
and touch. Separate thalamic nuclei also control movements. Other nuclei have
neither sensory nor motor functions but rather connect to cortical areas, such as the
frontal lobe, that perform more integrative tasks.
Finally, sensory and motor functions are divided in the cortex as well. This division
exists in two ways. First, there are separate sensory and motor cortical regions.
Some primarily process a particular sensory input, such as vision, hearing, or touch.
Others control detailed movements of discrete body parts, such as the fingers. Second,
the entire cortex can be viewed as being organized around the sensory and motor distinction.
For instance, layer IV of the cortex always receives sensory inputs,whereas layers
V and VI always send motor outputs, as shown in Figure 2-21. Layers I, II, and III
integrate sensory and motor operations.
Principle 3: Many of the Brain’s Circuits
Are Crossed
A most peculiar organizational feature of the brain is that most of its inputs and outputs
are “crossed,” as shown in Figure 2-32. Each hemisphere receives sensory stimulation
from the opposite (contralateral) side of the body and controls muscles on the
opposite side as well.Crossed organization explains why people who experience strokes
(blood clots or bleeding) in the left cerebral hemisphere may have difficulty in sensing
stimulation to the right side of the body or in moving body parts on the right side. The
opposite is true of people with strokes in the right cerebral hemisphere.
The human visual system is crossed in a more complicated way than are systems
for other parts of the body or for animals with eyes on the sides of their head, such as
the rat shown on the left in Figure 2-32. Two eyes facing forward instead of sideward
inevitably see much the same thing, except on the far sides of the field of vision. The
problem with this arrangement is that, to see an object with both eyes, information
about it must go to the same place in the brain. Duplicate information cannot be sent
to two different places.
Figure 2-32 (right) shows how the human brain solves this problem by dividing
each eye’s visual field into a left half and a right half. The information that either eye
HOW DOES THE NERVOUS SYSTEM FUNCTION? ! 65
Contralateral
side of cortex
Contralateral
side of cortex
Right visual field
Left visual field Right visual field
Left visua l field
Motor
Sensor
Motor
Sensory
Contralateral
side of body
Contralateral
side of body
Fixation point
Figure 2-32
Crossed Neural Circuits These
schematic representations of a rat and a
human brain in dorsal view show the
projection of visual and somatosensory
input to contralateral (opposite-side)
areas of the cortex and the crossed
projection of the motor cortex to the
contralateral side of the body. The brain
splits the visual input from each eye in
two: input from the right side of the
world as seen by both eyes goes to the
left hemisphere, and input from the left
side of the world as seen by both eyes
goes to the right hemisphere. For
simplicity, these diagrams show neural
activity as going directly to and from the
brain; in reality, intricate neural
connections en route to the sensory areas
and between the brain’s left and right
sides knit our perceptual world together.
(Left) Because the eyes of rats are laterally
placed, most of the input from each
eye travels to the opposite hemisphere.
(Right) The frontally placed eyes of
humans create a far narrower visual
field than that which the rat perceives.
For an animation and illustration of
how the visual system is crossed, go to the
section on the optic chiasm in the visual
system module on the Foundations CD.
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receives from the left half of its visual field is sent to the right side of the brain, and the
information that either eye receives from the right half of its visual field is sent to the
left side of the brain.
A crossed nervous system must join the two sides of the perceptual world together
somehow. To do so, innumerable neural connections link the left and right sides of the
brain. The most prominent connecting cable is the corpus callosum, which joins the left
and right cerebral hemispheres with about 200 million nerve fibers (see Figure 2-9B).
Two important exceptions to the crossed-circuit principle are in the olfactory and
somatic systems. Olfactory information does not cross but rather projects directly into
the same (ipsilateral) side of the brain. Further, the cranial and peripheral nerves do
not cross but are connected ipsiliaterally.
Principle 4: The Brain Is Both Symmetrical
and Asymmetrical
Although the left and the right hemispheres look like mirror images, they also have
some asymmetrical features. Asymmetry is essential for integrative tasks, language and
body control among them.
Consider speaking. If a language zone existed in both hemispheres, each connected
to one side of the mouth, we would have the strange ability to talk out of both sides of
our mouths at once. This would not make talking easy, to say the least. One solution is
to locate language control of the mouth on one side of the brain only. Organizing the
brain in this way allows us to speak with a single voice.
A similar problem arises in controlling the body’s movement in space.We would
not want the left and the right hemispheres each trying to take us to a different place.
Again, the problem can be solved if a single brain area controls this sort of spatial
processing.
In fact, processes such as language and spatial navigation are localized on only one
side of the brain. Language is usually on the left side, and spatial functions are usually
on the right. The brains of many species have both symmetrical and asymmetrical features.
The control of singing is located in one hemisphere in the bird brain. Like human
language, birdsong is usually located on the left side. It seems likely that the control of
song by two sides of the brain would suffer the same problems as the control of language
and that birds and humans independently evolved the same solution—namely,
to place the control only on one side of the brain.
Principle 5: The Nervous System Works Through
Excitation and Inhibition
Imagine that the telephone rings while you are reading this page. You stop reading, get
up, walk to the telephone, pick it up, and talk to a friend who convinces you that going
to a movie would be more fun than reading. To carry out this series of actions, not only
must your brain initiate certain behaviors, it must also stop other behaviors.When you
were talking, for example, you were not engaged in reading. To walk and talk, you had
to first stop reading. Producing behavior, then, requires both initiating some actions
and stopping others.
In talking about the nervous system, we refer to the initiation of an activity as
excitation and the cessation of an activity as inhibition. Recall, for example, that the
sympathetic and parasympathetic systems produce opposite actions. The sympathetic
system excites the heart, whereas the parasympathetic inhibits it.
66 ! CHAPTER 2
Excitation. A process by which the
activity of a neuron or brain area is
increased.
Inhibition. A process by which the
activity of a neuron or brain area is
decreased or stopped.