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Chapter 6
enzymes for processing them, and the capability of generating
weak electrical responses. Thus, in addition to all their other
roles, it is speculated that astrocytes may take part in informa-
tion signaling in the brain.
A third type of glial cell, the
microglia,
is a specialized,
macrophage-like cell (Chapter 18) that performs immune
functions in the central nervous system. Lastly,
ependymal
cells
line the fl uid-fi lled cavities within the brain and spinal
cord and regulate the production and fl ow of cerebrospinal
fl uid, which will be described later.
Schwann cells, the glial cells of the peripheral nervous
system, have most of the properties of the central nervous
system glia. As mentioned earlier, Schwann cells produce the
myelin sheath of peripheral nerve fi bers.
Neural Growth and Regeneration
The elaborate networks of nerve cell processes that character-
ize the nervous system are remarkably similar in all human
beings and depend upon the outgrowth of specifi c axons to
specifi c targets.
Development of the nervous system in the embryo begins
with a series of divisions of undifferentiated precursor cells
(stem cells)
that can develop into neurons or glia. After the last
cell division, each neuronal daughter cell differentiates, migrates
to its fi nal location, and sends out processes that will become
its axon and dendrites. A specialized enlargement, the
growth
cone,
forms the tip of each extending axon and is involved in
fi nding the correct route and fi nal target for the process.
As the axon grows, it is guided along the surfaces of
other cells, most commonly glial cells. Which route the axon
follows depends largely on attracting, supporting, defl ecting,
or inhibiting infl uences exerted by several types of molecules.
Some of these molecules, such as cell adhesion molecules,
reside on the membranes of the glia and embryonic neurons.
Others are soluble
neurotrophic factors
(growth factors
for neural tissue) in the extracellular fl uid surrounding the
growth cone or its distant target.
Once the target of the advancing growth cone is reached,
synapses form. The synapses are active, however, before their
fi nal maturation. This early activity, in part, determines their
fi nal function. During these early stages of neural develop-
ment, which occur during all trimesters of pregnancy and into
infancy, alcohol and other drugs, radiation, malnutrition, and
viruses can exert effects that cause permanent damage to the
developing fetal nervous system.
A surprising aspect of development of the nervous sys-
tem occurs after growth and projection of the axons. Many of
the newly formed neurons and synapses degenerate. In fact, as
many as 50 to 70 percent of neurons undergo a programmed
self-destruction called
apoptosis
in the developing central
nervous system! Exactly why this seemingly wasteful process
occurs is unknown, although neuroscientists speculate that
this refi nes or fi ne-tunes connectivity in the nervous system.
The basic shapes and locations of existing neurons in the
mature central nervous system do not change. The creation
and removal of synaptic contacts begun during fetal develop-
ment continue, however, though at a slow pace throughout
life as part of normal growth, learning, and aging. Division of
neuron precursor stem cells is largely complete before birth.
If axons are severed, they can repair themselves and
restore signifi
cant function provided that the damage occurs
outside the central nervous system and does not affect the
neuron’s cell body. After such an injury, the axon segment that
is separated from the cell body degenerates. The part of the
axon still attached to the cell body then gives rise to a growth
Ependymal
cells
Cerebrospinal
fluid
Neurons
Astrocyte
Microglia
Oligodendrocyte
Capillary
Myelinated axons
Myelin (cut)
Figure 6–6
Glial cells of the central nervous system.
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