359
CHAPTER TEN
Nervous System I
Myelin begins to form on axons during the fourteenth week of prena-
tal development. By the time of birth, many axons are not completely
myelinated. All myelinated axons have begun to develop sheaths by the
time a child starts to walk, and myelination continues into adolescence.
Excess myelin seriously impairs nervous system functioning. In
Tay-Sachs disease, an inherited defect in a lysosomal enzyme causes
myelin to accumulate, burying neurons in fat. The affected child
begins to show symptoms by six months of age, gradually losing
sight, hearing, and muscle function until death occurs by age four.
Thanks to genetic screening among people of eastern European
descent who are most likely to carry this gene, Tay-Sachs disease is
extremely rare.
10.4
CLASSIFICATION OF CELLS
OF THE NERVOUS SYSTEM
Neurons and neuroglia are intimately related. They descend
from the same neural stem cells and remain associated
throughout their existence.
Classif
cation oF Neurons
Neurons vary in size and shape and may differ in the lengths
and sizes of their axons and dendrites and in the number of
dendrites. Based on
structural differences,
neurons can be
classiF ed into three major groups, as
f gure 10.6
shows. Each
Myelinated
axon
Myelin
sheath
Schwann
cell cytoplasm
Unmyelinated
axon
FIGURE 10.5
A falsely colored transmission electron micrograph of
myelinated and unmyelinated axons in cross section (30,000×).
(b)
Bipolar
(c)
Unipolar
(a)
Multipolar
Dendrites
Axon
Axon
Axon
Direction
of impulse
Peripheral
process
Central
process
FIGURE 10.6
Structural types of neurons include
(
a
) the multipolar neuron, (
b
) the bipolar neuron, and
(
c
) the unipolar neuron.
PRACTICE
1
List the general functions of the nervous system.
2
Describe a neuron.
3
Explain how an axon in the peripheral nervous system becomes
myelinated.
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