Science Brain and Behavior contiuned 8

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addition, a female cricket must often choose between competing
males, preferring, for example, the male that makes
the longest chirps. All these behaviors must be “wired into”
a successful cricket robot, making sure that one behavior
does not interfere with another. In simulating cricket behavior
in a robot, Webb is duplicating the rules of a cricket’s
nervous system, which are “programmed” by its genes.
Is the idea of comparing a living cricket’s nervous system
to a manufactured robot’s computer-driven parts disturbing?
In their attempts to explain behavior, remember,
scientists, like philosophers, frequently search for analogies
among the things they know. Descartes compared the nervous
system to simpler mechanical devices, such as a water
pump or a clock. Today’s comparison to computerized robots
is just a contemporary analogy.
Robots help neuroscientists to learn more about the
brain and behavior. Researchers such as Webb switch back
and forth between studying the nervous system and the behaviors
that it enables and writing computer programs and
building robots designed to simulate those behaviors. When
the animal under study and the computerized robot respond
in exactly the same way, researchers can be fairly sure
that they understand how some part of the nervous system
works.
At the present time, the construction of robots that display
principles of nervous system function is important to
the area of science called artificial intelligence (AI), which
attempts to produce machines that can think. At some time
in the future, robots may even help neuroscientists evaluate
the correctness of a complete theory of how the brain
works. Scientists who believe they have a complete understanding
of brain function might validate their theory by
building a robot whose behavior is indistinguishable from
a human being’s.
Programming Behavior
Focus on New Research
I n the search to discover how a nervous system produces
behavior, robots may help provide answers. Robots, after
all, engage in goal-oriented actions, just as animals do. A
robot’s computer must guide and coordinate those actions,
doing much the same work that an animal’s nervous system
does.
Barbara Webb’s little cricket robot, constructed from
Lego blocks, wires, and a motor and shown in the accompanying
photograph, illustrates this interesting use of electronic
technology. Although far more cumbersome than
nature’s model, Webb’s robot is designed to mimic a female
cricket that listens for and travels to the source of a male’s
chirping song. These behaviors are not as simple as they
may seem.
In approaching a male, a female cricket must avoid
open, well-lit places where a predator could detect her. In
Rules obtained from the study of crickets’ behavior can be
programmed into robots to be tested. From “A Cricket Robot,” by
B. Webb, 1996, Scientific American, 214(12), p. 99.
Robert P. Carr/Burce Coleman Inc. (animal); Barbara Webb (model)
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Computer software runs on programmed instructions written in lines of code.
In the living cell, genes are coded to make proteins, the building bocks of cells
and of interaction among cells. Genes thus intimately participate in the production
of behavior, but only indirectly.Genes located in the chromosomes of each cell
determine the proteins that a particular cell will make. Thus, genes are blueprints for
proteins, with each gene containing the code for one protein.
A gene’s workings, however, can go awry, sometimes with devastating consequences
for behavior. Many of the neurological disorders described in this book are
caused by errors in protein manufacture due to errors in genes passed from parent to
child, which is why we explore the process of genetic transmission in this chapter.
So one function of living cells is to act as organic factories that produce proteins.
Nerve cells allow us to respond to stimuli in the environment, process that information,
and act. These cells are of different types, each distinctive in its structure and
function. Just as we can explore the function of a robot by examining its overall structure,
so we can investigate the overall structure of a cell as a source of insight into its
work.
This chapter also investigates the internal structures of cells, the organelles that
perform various tasks. If you think of a cell as nature’s microscopic robot, the organelles
become the miniaturized components that allow the cell to do its job. Ultimately,
the genes of female and male crickets determine their behavior. Think of the
challenge of programming into a robot all the instructions needed to carry out its every
task. Yet a cell, nature’s tiny robot, contains all the instructions that it requires packed
away in its chromosomes.
CELLS OF THE NERVOUS SYSTEM
If Barbara Webb’s little robot mysteriously arrived in a box on your doorstep, could you
guess what it is designed to do? The robot’s wheels imply that it is meant to move, and
the gears next to the wheels suggest that it can vary its speed or perhaps change directions
by varying the speed of one wheel relative to the other. The robot’s many exposed
wires show that it is not intended to go into water.And, because this robot has no lights
or cameras, you can infer that it is not meant to see. The structure of the robot suggests
its function. So it is with cells.
There are problems in examining the cells of the nervous system for insights into
their function. These cells are very small, are packed tightly together, and have the consistency
of jelly. To see a brain cell, you must first distinguish it from surrounding cells,
stain it to make it visible, and then magnify it by using a microscope. Anatomists have
developed ways of removing most of the water from the brain by soaking it in formaldehyde,
after which the brain can be sliced thin and stained with various dyes that
either color its cells completely or color some of the cells’ components. Now the cells
can be placed under a microscope for viewing.Anatomists can also “culture” brain cells
in a dish where living cells can be viewed and studied more simply than they can when
they are in a living brain.
There remains, however, the problem of making sense of what you see. Different
brain samples can yield different images, and different people can interpret those images
in different ways. So began a controversy between two great scientists over what
neurons really are. One was the Italian Camillo Golgi and the other the Spaniard Santiago
Ramón y Cajal. Both men were awarded the Nobel Prize for medicine in 1906.
Imagine that you are Camillo Golgi hard at work in your laboratory staining
and examining cells of the nervous system. You immerse a thin slice of brain tissue in
a solution containing silver nitrate and other chemicals, a technique used at the time
76 ! CHAPTER 3
Use the Foundations of Behavioral
Neuroscience CD to learn about different
ways to look at the brain. In the module
on research methods, you’ll find a section
on histology that includes samples of six
common stains for brain sections. (See
the Preface for more information about
this CD.)
Visit the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
and go to the Chapter 3 Web links to
view an article on robotics and artificial
intelligence.
Cell body (soma). Core region of the
cell containing the nucleus and other
organelles for making proteins.
Dendrite. Branch of a neuron that
consists of an extension of the cell
membrane, thus greatly increasing the
area of the cell.
Axon. “Root,” or single fiber, of a neuron
that carries messages to other neurons.
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to produce black-and-white photographic prints. A contemporary method,
shown in Figure 3-1, produces a color-enhanced microscopic image that resembles
the images Golgi saw.
The image is beautiful and intriguing, but what do you make of it? To Golgi,
this structure suggested that the nervous system is composed of a network of interconnected
fibers. He thought that information, like water running through
pipes, somehow flowed around this “nerve net” and produced behavior.His theory
was not implausible, given what he saw.
But Santiago Ramón y Cajal came to a different conclusion. He studied the
brain tissue of chick embryos because he assumed that their nervous systems
would be simpler and easier to understand than would an adult nervous system. Figure
3-2 shows one of the images that he rendered from the neural cells of a chick embryo.
Cajal concluded that the nervous system is made up of discrete cells that begin life with
a rather simple structure that becomes more complex with age.When mature, these cells
consist of a main body with extensions projecting from it.
The structure looks something like a radish, with branches coming out of the top
and roots coming out of the bottom. Cajal’s belief that these complexly shaped cells
are the functional units of the nervous system is now universally
accepted. The idea proposed by Cajal, that neurons
are the units of brain function, is called the neuron
hypothesis.
Figure 3-2 shows the three basic subdivisions of a
neuron. The core region is called the cell body or soma
(Greek, meaning “body”). Most of a neuron’s branching
extensions are called dendrites (Latin for “branch”), but
the main “root” is called the axon (Greek for “axle”). A
neuron has only one axon, but most neurons have many
dendrites. Some small neurons have so many dendrites
that they look like a garden hedge.
The nervous system is composed not only of neurons but also of cells called glia
(the name comes from the Greek word for “glue”; see Figure 2-12). Neurons are the
functional units that enable us to receive information, process it, and act.Glial cells help
the neurons out, binding them together (some do act as glue) and providing support,
nutrients, and protection, among other functions detailed later in the chapter. The
human nervous system contains about 100 billion neurons and perhaps 10 times as
many glial cells. No, no one has counted them all. Scientists have estimated the total
number by counting the cells in a small sample of brain tissue and then multiplying by
the brain’s volume (Figure 3-3).
How can we explain how 100 billion cells cooperate, make connections, and produce
behavior? Fortunately, examining how one cell works can be a source of insight
that we can generalize to other cells. Brain cells really are like robots built to a common
plan, depending on their particular type. As you learn to recognize some of the different
types of neurons and glial cells in your body, you will also see how their specialized
structures contribute to their functions.
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 77
Dendrites
Cell body
Axon
Biophoto Associates/Science Source/
Photo Researchers
Figure 3-1
Histological Preparation Tissue
preparation revealing human pyramidal
cells stained by using the Golgi
technique.
Figure 3-3
Estimating Cell Count The researcher
first obtains a thin slice of brain tissue
from a rat (left) and then selects a region
of the cortex for cell counting (middle).
Multiplying the number of cells in this
part of the tissue by the volume of the
cortex yields an estimate of the total
number of cells. The cells pictured at the
right are stained with cresyl violet, which
adheres to protein molecules in the cell
body, giving it a blue color.
Figure 3-2
Drawing of a Neuron by Cajal
Dendrites gather information from other
neurons, the soma (cell body) integrates
the information, and the axon sends the
information to other cells. Note that
there is only one axon, though it may
have branches, called collaterals. Adapted
from Histologie du système nerveux de l’homme
et des vertebres, by S. Ramón y Cajal, 1909–1911,
Paris: Maloine.
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Neurons
As the information-processing units of the nervous system, neurons must do many
things. They must acquire information from sensory receptors, pass that information
on to other neurons, and make muscles move to produce behaviors. They must encode
memories and produce our thoughts and emotions as well.At the same time, they must
regulate all the many body processes to which we seldom give a thought, such as breathing,
heartbeat, body temperature, and the sleep–wake cycle. This is a tall order but easily
accomplished by our microscopic neurons.
Some scientists think that a specific function is sometimes assigned to a single neuron.
For example, Fernando Nottebohm and his colleagues (1994) studied how birds
produce songs and proposed that a single neuron may be responsible for each note
sung. For most behavior in most species, however, scientists think that neurons work
together in groups of many hundreds to many thousands to produce some aspect of a
behavior.
According to this view, the loss of a neuron or two would be no more noticeable
than the loss of one or two voices from a cheering crowd. It is the crowd that produces
the overall sound, not each individual person. In much the same way, although neuroscientists
say that neurons are the information-processing units of the brain, they really
mean that large teams of neurons serve this function.
Scientists also speak informally about the structure of a particular neuron, as if
that structure never changes. If fresh brain tissue is kept alive in a dish of salty water
and viewed occasionally through a microscope, the neurons reveal themselves to be
surprisingly active, both producing new dendrite branches and losing old ones. In
fact, when they are watched over a period of time in the brain or in a dish, they seem
to be continuously growing and shrinking and
changing their shape.
For some neurons, these physical changes result
from coding and storing our experiences and
memories. Neural changes of all kinds are possible
because of a special property that neurons
possess. Even in a mature, fully grown neuron,
the cell’s genetic blueprints can be “reopened,”
allowing the neuron to alter its structure and
function by producing new proteins.
Another important property of neurons is their longevity. At a few locations
in the nervous system, the ongoing production of new neurons does
take place throughout life, and some behavior does depend on the production
of new neurons. But most of your neurons are with you for life and are
never replaced. If the brain or spinal cord is damaged, for example, the neurons
that are lost may not be replaced, and functional recovery is poor.
THE NEURON’S BASIC STRUCTURE AND FUNCTION
Figure 3-4 displays the basic features of neurons in detail. The surface area
of the cell is increased immensely by its extensions into dendrites and an
axon (Figure 3-4A and B). The dendritic area is further increased by many
small protrusions called dendritic spines (Figure 3-4C). A neuron may
have from 1 to 20 dendrites, each may have from one to many branches,
and the spines on the branches may number in the many thousands. Because dendrites
collect information from other cells, their surface area corresponds to how much information
the neuron can gather.
Each neuron has but a single axon (Figure 3-4D). It begins at one end of the cell
body at an expansion known as the axon hillock (little hill). The axon may branch out
78 ! CHAPTER 3
(A) (B) (C)
Axon
hillock
Axon
Dendrite
Nucleus
Nucleolus
Cell body
Axon from another neuron
Axon
Cell body
(soma)
Nucleus
Axon
Teleodendria collateral
End foot
(terminal button)
Dendrites
Dendrites from
neighboring neuron
Synapse
Dendritic spine
End foot
(D)
Figure 3-4
Major Parts of a Neuron Note here
how different stains highlight different
aspects of the neuron. (A) A typical
neuron stained with the use of the
Golgi technique to reveal its dendrites
and cell body. (B) A drawing of a neuron
illustrates its basic structures. (C) An
electron micrograph captures the
synapse between an axon from another
neuron and a dendritic spine. (D) A highpower
light-microscopic view inside the
cell body.
Go to the Web links for Chapter 3 on
the Brain and Behavior Web site
(www.worthpublishers.com/kolb) to
read more about the history of the neuron
hypothesis.
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into one or many axon collaterals, which usually emerge from it at right angles (see
Figure 3-4B).
As shown in detail in Figure 3-4B, the lower tip of an axon may divide into a number
of smaller branches (teleodendria, or end branches). At the end of each teleodendrion
is a knob called an end foot or terminal button. The end foot is very close to a
dendritic spine or some other part of another neuron, although it does not touch it
(see Figure 3-4C). This “almost connection,”which includes the surfaces of the end foot
and the neighboring dendritic spine as well as the space between them, is called a
synapse.
Chapter 4 describes how neurons communicate; here we simply generalize about
function by examining shape. Imagine looking at a river system from an airplane. You
see many small streams merging to make creeks, which join to form tributaries, which
join to form the main river channel. As the river reaches its delta, it breaks up into a
number of smaller channels again before discharging its contents into the sea.
The general shape of a neuron is somewhat similar to such a river system, and the
neuron works in a broadly similar way. It collects information from many different
sources on its dendrites, channels that information onto its axon, and then sends the
information along its teleodendria to its end feet. At the end feet, the information is
released onto a target surface. This flow of information from the dendrites through
the cell body and then along the axon to the end feet is illustrated in Figure 3-5.
A neuron receives a great deal of information on its hundreds to thousands of dendritic
spines, but it has only one axon, and so it must also act as a decision-making device.
As described in Chapter 2, the message that it sends
must be an averaged, or summary, response to all
the incoming information. Because it produces a
summary response, the neuron is also a computational
device. Chapter 4 describes in detail how
these decision-making processes take place.
TYPES OF NEURONS
The nervous system contains neurons in an array
of shapes and sizes, structured differently because
of their specialized tasks. Some appear quite simple
and others very complex.With a little practice
in looking into a microscope, you can quickly
learn to recognize three neuron types by their
features and functions. Sensory neurons (Figure
3-6A) are designed to bring information into the
brain from sensory receptors, interneurons (Figure
3-6B) to process it within the brain, and motor
neurons (Figure 3-6C) to carry it out of the brain
to the body’s muscles.
Sensory Neurons These neurons are the simplest
structurally. A bipolar neuron found in the
retina of the eye has a single short dendrite on one
side of its cell body and a single short axon on the
other side. It brings sensory information from the
retina’s light receptors to the neurons that will
carry visual information into the visual centers of
the brain.
A bit more complicated is the somatosensory
neuron that brings sensory information from the
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 79
Figure 3-5
Information Flow in a
Neuron The sequence of
information processing
in a neuron is in
integrate out. Neurons
act as decision-making
devices by summing
incoming information
and incorporating this
summation into the
messages that they send.
Dendritic spine. Protrusion from a
dendrite that greatly increases its surface
area and is the usual point of dendritic
contact with axons.
Axon hillock. Juncture of soma and
axon where the action potential begins.
Axon collateral. Branch of an axon.
End foot. Knob at the tip of an axon that
conveys information to other neurons;
also called a terminal button.
Synapse. Gap between one neuron and
another neuron, usually between an end
foot of the axon of one neuron and a
dendritic spine of the other neuron.
Bipolar neuron. Neuron with one axon
and one dendrite.
Somatosensory neuron. Brain cell that
brings sensory information from the body
into the spinal cord.
Collecting
information
Integrating
information
Flow of
information
Sending
information
Axons from
other neurons
Axon
Cell
body
End feet
Dendrites
of target
neuron
Dendrites
…passed on
to the axon,…
…then to the end feet,
where it is passed on to
a target neuron.
Information
from other
neurons is
collected
at dendrites,…
…processed
in the cell
body,…
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80 ! CHAPTER 3
body into the spinal cord. Structurally, the somatosensory
dendrite connects directly to its
axon, and so the cell body sits to one side of this
long pathway.
Interneurons Generally called association cells
because they link up sensory and motor activity,
interneurons include pyramidal cells and Purkinje
cells. A specific association cell, the stellate
cell (meaning star shaped), is characteristically
small, with many dendrites extending around
the cell body. Its axon is difficult to see among
the maze of dendrites.
A pyramidal cell has a long axon, a pyramid-
shaped cell body, and two sets of dendrites,
one set projecting from the apex of the cell body
and the other from its sides. Pyramidal cells
carry information from the cortex to the rest of
the brain and spinal cord.
A Purkinje cell (named for its discoverer)
is a distinctive pyramidal cell with extremely
branched dendrites that form a fan shape. It carries
information from the cerebellum to the rest
of the brain and spinal cord. A major difference
between animals with small brains and animals
with large brains is that large-brained animals
have more interneurons.
Motor Neurons To collect information from
many sources, motor neurons have extensive
networks of dendrites, large cell bodies, and
long axons that connect to muscles.Motor neurons are located in the lower brainstem
and spinal cord. All outgoing neural information must pass through them to reach the
muscles.
NEURAL CONNECTIONS
Neurons are “networkers” that produce behavior, and the appearance of each neuron
tells us something about the connections that it must make. Figure 3-6 illustrates the
relation between form and function of neurons but does not illustrate actual size.
Generally, neurons that project for long distances, such as somatosensory neurons,
pyramidal neurons, and motor neurons, are very large relative to other neurons. In
general, neurons with large cell bodies have extensions that are very long, whereas
neurons with small cell bodies have short extensions.
Long extensions carry information to distant parts of the nervous system; short
extensions are engaged in local processing. For example, the tips of the dendrites
of some sensory neurons are located in your big toe, whereas the target of their
axons is at the base of your brain. These sensory neurons send information over a
distance as long as 2 meters, or more. The axons of some pyramidal neurons must
reach from the cortex as far as the lower spinal cord, a distance that can be as long
as a meter. The imposing size of this pyramidal cell body therefore is in accord with
the work that it must do in providing nutrients and other supplies for its axons and
dendrites.
(A) Sensory neurons
Bipolar neuron
(retina)
Somatosensory neuron
(skin, muscle)
Dendrite
Dendrites
Axon
Axon
Stellate cell
(thalamus)
Pyramidal cell
(cortex)
Motor neuron
(spinal cord)
Purkinje cell
(cerebellum)
(C) Motor neurons
(B) Interneurons
Dendrites
Dendrites
Axon
Axon
Bring information to the
central nervous system
Associate sensory
and motor activity
in the central
nervous system
Send signals from
the brain and spinal
cord to muscles
Figure 3-6
Neuron Shape and
Function The appearance
of different kinds of
neurons is distinctive (note
that these cells are not
drawn to scale). Sensory
neurons (A) collect
information from a source
and pass it on to an
interneuron (B). The many
branches of interneurons
collect information
from many sources.
Motor neurons (C) are
distinctively large and pass
this information on to
command muscles to move.
To learn more about synapses, visit
the section on the structure of a neuron in
the neural communications module on
the Foundations CD. You can also view
an animation showing a simple neural
network in the section on neural
integration.
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WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 81
Interneuron. Association neuron
interposed between a sensory neuron
and a motor neuron; thus, in mammals,
interneurons constitute most of the
neurons of the brain.
Pyramidal cell. Distinctive neuron
found in the cerebral cortex.
Purkinje cell. Distinctive neuron found
in the cerebellum.
Microphones
Cricket
Cricket
robot
(A) (B)
Motor
neuron
Wheel
Photoreceptor
cells
+
+
Microphones
+ +
– –
+ +
+
+ +
+
Figure 3-7
Excitation and Inhibition
(A) Excitatory inputs from the chirping
of a male cricket, picked up by the
cricket robot’s microphones, activate the
robot’s wheels to orient toward the
chirp. (B) In a slightly more complex
cricket robot, sensory neurons from the
speaker excite motor neurons, but
inhibitory input from photoreceptors
turn the motor neurons off.
THE LANGUAGE OF NEURONS: EXCITATION
AND INHIBITION
Neurons are in constant communication. Their basic language may remind you of how
digital devices such as computers work. That is, neurons either excite other neurons
(turn them on) or inhibit other neurons (turn them off). Like computers, neurons send
“yes” or “no” signals to one another; the “yes” signals are the excitatory signals, and the
“no” signals are the inhibitory signals. Recall that excitation and inhibition are subjects
of one of the principles discussed in Chapter 2. Each neuron receives thousands of excitatory
and inhibitory signals every second.
The neuron’s response to all those inputs is democratic: it sums them. A neuron
is spurred into action only if its excitatory inputs exceed its inhibitory inputs. If the
reverse is true and inhibitory inputs exceed excitatory inputs, the neuron does not
activate.
We can apply the principle of the excitation and inhibition of neuron action to the
workings of the cricket robot described at the beginning of this chapter. Suppose we
could insert a neuron between the microphone for sound detection on each side of this
robot and the motor on the opposite side. Figure 3-7A shows how the two neurons
would be connected. It would take only two rules to instruct the robot to seek out a
chirping male cricket:
Rule 1: Each time that a microphone detects a male cricket’s song, an excitatory message
is sent to the opposite wheel’s motor, activating it. This rule ensures that the robot
turns toward the cricket each time that it hears a chirp.
Rule 2: The message sent should be proportional to the intensity of the sound. This
rule means that, if the chirp is coming from the robot’s left side, it will be detected as
being louder by the microphone on the left, which will make the right wheel turn a little
faster, swinging the robot to the left. The opposite would happen if the sound came
from the right. If the sound comes from straight ahead, both microphones will detect
it equally, and the robot will move directly forward. This rule ensures that the robot
travels in the correct direction.
To make the robot in a more “intelligent” way requires more neurons. Figure
3-7B shows how we could mimic the idea of sensory and motor neurons. The robot
now has two sound-detecting sensory neurons receiving input from its microphones.
When activated, each of these sensory neurons excites a motor neuron that turns on
one of the two wheel motors. But now we add sensory neurons coming from photoreceptors
on the robot that detect light. These light-detecting sensory neurons,
when activated, inhibit the motor neurons leading to the wheels and so prevent the
robot from moving toward a male until it is dark and “safe.”
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This arrangement gives the robot some interesting properties. For example, at dusk
the excitatory signals from sound and weak inhibitory signals from the dim light might
conflict. The robot might make small “intention”movements that orient it to the male
while not actually searching for it. A researcher might want to examine the behavior of
a real female cricket to see if it acts in the same way under these conditions.
This arrangement illustrates the function of sensory and motor neurons and the
principle of excitation and inhibition, but bear in mind that it contains only six neurons
and each neuron has only one connection with another neuron.We have not even
placed interneurons in the robot. Imagine how infinitely more complex a human nervous
system is with its hundred billion neurons, most of which are interneurons, each
with thousands of connections.
Still, this simple example serves a valuable purpose. It shows the great versatility of
function possible from the dual principles of excitation and inhibition. From the simple
yes-or-no language of neurons emerges enormous possibilities for behavior.
Glial Cells
Imagine how much more efficient your robot would be if you could add components
that enhance the functioning of your simulated neurons. Some could attach the neurons
to the appropriate parts of the robot; others could insulate the neurons to prevent
them from short-circuiting one another. The insulating components might also increase
the speed of signals along the robot’s wired
pathways. Still other auxiliary components could lubricate
moving parts or eliminate debris, keeping
your robot clean and shiny. Does all this sound too
good to be true? Not really. All these functions are
served by glial cells in your nervous system.
Glial cells are often described as the support cells
of the nervous system. Although they do not transmit
much information themselves, they help neurons carry
out this task.Most neurons form only early in life, but
glial cells are constantly replacing themselves. (“Brain
Tumors” describes the results of uncontrolled glial cell
growth.) Table 3-1 lists the five major classes of glia.
Each has a characteristic structure and function.
EPENDYMAL CELLS
On the walls of the ventricles, the cavities inside your
brain, are ependymal cells that produce and secrete
the cerebrospinal fluid that fills the ventricles. Cerebral
spinal fluid is constantly being formed and flows
through the ventricles toward the base of the brain,
where it is absorbed into the blood vessels. Cerebrospinal
fluid serves several purposes. It acts as a
shock absorber when the brain is jarred; it provides a
medium through which waste products are eliminated;
it may play a role in brain cooling; and it may
be a source of nutrients for certain parts of the brain
located adjacent to the ventricles.
As CSF flows through the ventricles, it passes
through some narrow passages, especially the fourth
82 ! CHAPTER 3
Glial cell. Nervous system cell that
provides insulation, nutrients, and
support, as well as aiding in the repair of
neurons.
Tumor. Mass of new tissue that grows
uncontrolled and independent of
surrounding structures.
Ependymal cell. Glial cell that makes
and secretes cerebral spinal fluid; found
on the walls of the ventricles of the brain.
Types of Glial Cells
Type Appearance Features and function
Ependymal cell Small, ovoid; secretes
cerebrospinal fluid (CSF)
Astrocyte Star shaped, symmetrical;
nutritive and support function
Microglial cell Small, mesodermally derived;
defensive function
Oligodendroglial cell Asymmetrical; forms myelin
around axons in brain and
spinal cord
Schwann cell Asymmetrical; wraps around
peripheral nerves for form
myelin
Table 3-1
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WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 83
Brain Tumors
Focus on Disorders
One day while she was watching a movie in a neuropsychology
class, R. J., a 19-year-old college sophomore,
collapsed on the floor and began twitching, displaying
symptoms of a brain seizure. The instructor helped her to the
university clinic, where she recovered, except for a severe
headache. She reported that she had suffered from severe
headaches on a number of occasions.
A few days later, computer tomography (CT) was used to
scan her brain; the scan showed a tumor over her left frontal
lobe. She underwent surgery to have the tumor removed and
returned to classes after an uneventful recovery. She successfully
completed her studies, finished law school, and has
been practicing law for more than 15 years without any further
symptoms.
A tumor is a mass of new tissue that undergoes growth
that is uncontrolled and independent of surrounding structures.
No region of the body is immune, but the brain is a
common site. Brain tumors do not grow from neurons but
rather from glia or other supporting cells. The rate of growth
depends on the type of cell affected.
Some tumors, such as R. J.’s, are benign and not likely to
recur after removal; others are malignant, likely to progress,
and apt to recur after removal. Both kinds of tumors can pose
a risk to life if they develop in sites from which they are difficult
to remove.
The earliest symptoms are usually due to increased
pressure on surrounding brain structures and can include
headaches, vomiting, mental dullness, changes in sensory
and motor abilities, and seizures such as that experienced by
R. J. Many symptoms depend on the precise location of the
tumor. The three major types of brain tumors are classified
according to how they originate:
1. Gliomas arise from glial cells. They constitute roughly
half of all brain tumors. Gliomas that arise from astrocytes
are usually slow growing, not often malignant, and relatively
easy to treat. In contrast, gliomas that arise from
blast or germinal cells (precursor cells that grow into glial
cells; see Chapter 7) are much more often malignant,
grow more quickly, and often recur after treatment.
2. Meningiomas, the type of tumor that R. J. had, attach to
the meninges and so grow entirely outside the brain, as
shown in the accompanying CT scan. These tumors are
usually well encapsulated, and, if located in places that
are accessible, recovery after surgery is good.
3. The metatastic tumor becomes established by a transfer of
tumor cells from one region of the body to another (which
is what the term metatastic means). Typically, metatastic
tumors are present in multiple locations, making treatment
difficult. Symptoms of the underlying condition
often first appear when the tumor cells reach the brain.
Treatment for a brain tumor is usually surgery, which
also is one of the main means of diagnosing the type of
tumor. If possible, the entire tumor is removed. Radiotherapy
(treatment with X-rays) is useful for destroying developing
tumor cells. Chemotherapy, although common for treating
tumors in other parts of the body, is less successful in the
treatment of brain tumors because getting the chemicals
across the blood–brain barrier is difficult.
The red area in this colored CT scan is a meningioma, a
noncancerous tumor arising from the arachnoid membrane
covering the brain. A meningioma may grow large enough to
compress the brain but usually does not invade brain tissue. Dept. of Clinical Radiology, Salisbury District Hospital/
Science Photo Library/Photo Researchers
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ventricle, which runs through the brainstem (see Figure 2-10). If the fourth ventricle
is fully or partly blocked, the fluid flow is restricted. Because CSF is continuously being
produced, this blockage causes a buildup of pressure that begins to expand the ventricles,
which in turn push on the surrounding brain.
If such a blockage develops in a newborn infant, before the skull bones are fused,
the pressure on the brain is conveyed to the skull and the baby’s head consequently
swells. This condition, called hydrocephalus (literally, water brain), can cause severe
mental retardation and even death. To treat it, doctors insert one end of a tube, called
a shunt, into the blocked ventricle and the other end into a vein. The shunt allows the
CSF to drain into the bloodstream.
ASTROGLIA
Astrocytes (star-shaped glia shown in Figure 2-12), also called astroglia, provide structural
support within the central nervous system. Their extensions attach to blood vessels
and to the brain’s lining, creating scaffolding that holds neurons in place. These
same extensions provide pathways for the movement of certain nutrients between
blood vessels and neurons. Astrocytes also secrete chemicals that keep neurons healthy
and help them heal if injured.
At the same time, astrocytes play an important role in contributing to a protective
partition between blood vessels and the brain, the blood–brain barrier. As shown in
Figure 3-8, the end feet of astrocytes attach to the
cells of blood vessels, causing the blood-vessel
cells to bind tightly together. These tight junctions
prevent an array of substances, including
many toxins, from entering the brain through the
blood-vessel walls.
The molecules (smallest units) of these substances
are too large to pass between the bloodvessel
cells unless the blood–brain barrier is
somehow compromised. But the downside to the
blood–brain barrier is that many useful drugs, including
antibiotics such as penicillin, cannot pass
through to the brain either. As a result, brain infections
are very difficult to treat.
Yet another important function of astrocytes is to enhance brain activity. When
you engage a part of your brain for some behavior, the brain cells of that area require
more oxygen and glucose. In response, the blood vessels in the area dilate, allowing
greater oxygen- and glucose-carrying blood flow. But what triggers the blood vessels to
dilate? This is where the astrocytes come in. They convey signals from the neurons to
the blood vessels, stimulating them to expand and so provide more fuel.
Astrocytes also contribute to the process of healing damaged brain tissue. If the
brain is injured by a blow to the head or penetrated by some sharp object, astrocytes
form a scar to seal off the damaged area. Although the scar tissue is beneficial in healing
the injury, it can unfortunately act as a barrier to the regrowth of damaged neurons.
Some experimental approaches to repairing brain tissue seek to get the axons and
dendrites of CNS neurons to grow around or through a glial scar.
MICROGLIA
Unlike other glial cells, which originate in the brain, microglia originate in the blood
as an offshoot of the immune system and migrate throughout the brain. Microglia
monitor the health of brain tissue. When brain cells are damaged, microglia invade
84 ! CHAPTER 3
Hydrocephalus. Buildup of pressure in
the brain and swelling of the head caused
if the flow of CSF is blocked; can result in
retardation.
Astrocyte. Glial cell with a star-shaped
appearance that provides structural
support to neurons in the central nervous
system and transports substances between
neurons and capillaries.
Microglial cell. Form of glial cell that
scavenges debris in the nervous system.
Astrocyte
Neuron
Blood vessel
Astrocyte
end feet
Myelinated axon
Tight
junctions
Blood-vessel
cells
Figure 3-8
Function of Astrocytes
Astrocyte processes attach
to neurons and to blood
vessels to provide support
between different
structures in the brain,
stimulate the cells on
blood vessels to form tight
junctions and so form the
blood–brain barrier, and
transport chemicals
excreted by neurons to
blood vessels.
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WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 85
Figure 3-9
Detecting Brain Damage
Arrows on the micrograph
at the left indicate a brain
area called the red nucleus
in a rat. (A) Closeup of
cresyl violet–stained
neurons in the healthy red
nucleus. (B) After exposure
to a neurotoxin, only
microglia remain.
Myelin. Glial coating that surrounds
axons in the central and peripheral
nervous systems.
Oligodendroglial cell. Glial cell in the
central nervous system that myelinates
axons.
Schwann cell. Glial cell in the
peripheral nervous system that forms the
myelin on sensory and motor axons.
Multiple sclerosis (MS). Nervous
system disorder that results from the loss
of myelin (glial-cell covering) around
neurons.
Paralysis. Loss of sensation and
movement due to nervous system injury.
(A) (B)
the area to provide growth factors that aid in repair and to engulf and remove foreign
matter and debris, an immune process called phagocytosis. Damage to the brain can be
detected in a postmortem examination because, as illustrated in Figure 3-9, microglia
will be left where neurons were once located.
OLIGODENDROGLIA AND SCHWANN CELLS
Two kinds of glial cells insulate the axons of neurons. Like the rubber insulation on
electrical wires, this myelin prevents adjacent neurons from short-circuiting each
other’s activity.Oligodendrogliamyelinate axons in the brain and spinal cord by sending
out large, flat branches that enclose and separate adjacent axons (the prefix oligo
means “few,” referring to the fact that these glia have few branches in comparison with
astrocytes; see Table 3-1).
Schwann cells provide myelin to axons in the peripheral nervous system. Each
Schwann cell wraps itself repeatedly around a part of an axon, forming a structure
somewhat like a bead on a string. In addition to the myelination, Schwann cells and
oligodendroglia contribute to a neuron’s nutrition and function by absorbing chemicals
that the neuron releases and releasing chemicals that the neuron absorbs.
In Chapter 4 you will learn how myelin speeds up the flow of information along
a neuron. Neurons that are heavily myelinated send information much faster than
neurons having little or no myelin. Most neurons that must send messages over long
distances, including sensory and motor neurons, are heavily myelinated.
If myelin is damaged, a neuron may be unable to send any messages over its axons.
In multiple sclerosis (MS),myelin is damaged, and the functions of the neurons whose
axons it encases are disrupted. “Multiple Sclerosis” on page 86 describes the course of
the disease.
GLIAL CELLS AND NEURON REPAIR
A deep cut on your body, on your arm or leg for instance, may cut the axons connecting
your spinal cord to muscles and to sensory receptors as well. Severed motorneuron
axons will render you unable to move the affected part of your body, whereas
severed sensory fibers will result in loss of sensation from that body part. Cessation
of both movement and sensation is paralysis. Over a period of weeks to months
after motor and sensory axons are severed, movement and sensation will return.
The human body can repair this kind of nerve damage, and so the paralysis is not
permanent.
Both microglia and Schwann cells play a part in repairing damage to the peripheral
nervous system.When a PNS axon is cut, it dies. Microglia remove all the debris
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86 ! CHAPTER 3
left by the dying axon. Meanwhile, the Schwann cells that provided its myelin first
shrink and then divide, forming numerous smaller glial cells along the path that the
axon formerly took. The neuron then sends out axon sprouts that search for and follow
the path made by the Schwann cells.
Multiple Sclerosis
Focus on Disorders
One day J. O., who had just finished university requirements
to begin work as an accountant, noticed a slight cloudiness
in her right eye; the cloudiness did not go away when she
wiped her eye. The area of cloudiness grew over the next few
days. Her optometrist suggested that she see a neurologist,
who diagnosed optic neuritis, a symptom that could be a flag
for multiple sclerosis.
Although we do not yet understand what causes MS, we
do know that it is characterized by a loss of myelin, both on
pathways bringing sensory information to the brain and on
pathways taking commands to muscles. This loss of myelin
occurs in patches, and scarring is frequently left in the affected
areas.
Eventually, a hard scar, or plaque, may form in the
affected areas, which is why the disease is called sclerosis
(from the Greek word meaning “hardness”). Associated with
the loss of myelin is impairment in neuron function, causing
characteristic MS symptoms of sensory loss and difficulty in
moving. Fatigue, pain, and depression are common related
symptoms. Bladder dysfunction, constipation, and sexual
dysfunction all complicate the condition. Multiple sclerosis
greatly affects a person’s emotional, social, and vocational
functioning. As yet, it has no cure.
J. O.’s eye cleared over the next few months, and she
had no further symptoms until after the birth of her first child
3 years later, when she felt a tingling in her right hand that
spread up her arm, until gradually she lost movement in the
arm. Movement was restored 5 months later. Then 21/2 years
later, after her second child was born, she felt a tingling in
her left big toe that spread along the sole of her foot and then
up to her leg, eventually leading again to loss of movement.
J. O. received corticosteroid treatment, which helped, but
the condition rebounded when she stopped treatment. Then
it subsided and eventually disappeared.
Since then, J. O. has had no major outbreaks of motor
impairment, but she still feels occasional tingling in her
trunk, some weakness in her left leg, and brief periods of
tingling and numbness in different body parts that last a
couple of weeks before clearing. The feeling is very similar
to the numbness in the face after a dentist gives a local
anesthetic.
Although she suffers no depression, J. O. reports enormous
fatigue, takes daily long naps, and is ready for bed
early in the evening. Her sister and a female cousin have experienced
similar symptoms. Computer tomographic scans
on both J. O. and her sister revealed scarring in the spinal
cord, a condition that helped confirm an MS diagnosis for
them. One of J. O.’s grandmothers had been confined to a
wheelchair, although the source of her problem was never
diagnosed.
J. O. occasionally wears a brace to support her left knee
and sometimes wears a collar to support her neck. She
makes every effort to reduce stress to a minimum, but otherwise
she lives a normal life that includes exercise and even
vigorous sports such as water skiing.
J. O.’s extremely strange symptoms, which are often difficult
to diagnose, are typical of multiple sclerosis. The first
symptoms usually appear in adulthood, and their onset is
quite sudden and swift. These initial symptoms may be loss
of sensation in the face, limbs, or body or loss of control over
movements or loss of both sensation and control. Motor
symptoms usually appear first in the hands or feet.
Often there is remission of early symptoms, after which
they may not appear again for years. In some of its forms,
however, the disease may progress rapidly over a period of
just a few years until the person is reduced to bed care. In
cases in which the disease is fatal, the average age of death
is between 65 and 84.