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DEMYELINATING DISEASES
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| A myelin sheath
is the insulating coat of lipid-rich cell membranes that wraps around a
neuron's axon and giving a nerve its normal, glistening white appearance.
The sheath comes from supporting oligodendrocyte cells in the head
and Schwann cells in the trunk region of the body which are derived
from neural crest. Interruptions in the sheath, called nodes of Ranvier,
are where saltatory conduction (conduction by leaps) occurs. Only at these
nodes can sodium and potassium move into and out of the axon. The effect
is to substantially increase the velocity of the nerve impulse. For example,
the difference in speed between small, unmyelinated nerves and larger, myelinated
ones can be as much as that between a few millimeters per second and 200
meters per second (33, p.634). Therefore, myelin
is essential for efficient conduction of the impulse (33,
34). |
 (44) |
A further important consideration is that such a swift rate of conduction
allows the axon to have a reduced diameter, giving it the basic electrical
properties of a conducting "wire." This, in turn, allows for
the construction of a larger and more complex nervous system. Without
myelin (assuming that insulation - myelin's other important function -
is not a problem) the human head and spinal chord would have to be about
50 times bigger in order to accomodate the functional brain capacity we
now possess (44).
Demyelination is a hallmark of several diseases like multiple sclerosis
(MS) and leukodystophies in addition to being a common feature
of spinal cord injuries.
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 (1) |
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Focus
on Multiple Sclerosis
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| This chronic disease of the central nervous system
(CNS) interferes with the ability of the brain to control functions including
seeing, walking, and talking. "Multiple" indicates
the many and scattered areas of the brain and spinal chord which are affected.
"Sclerosis" refers to the "sclerosed,"
or scarred tissue in the damaged areas, resulting from a breakdown of myelin.
This tissue distorts, or even blocks nerve "messages." The cause
of MS remains a mystery, but there are three current hypotheses
(41): |
| 1) MS is the effect of a slow-acting
virus. |
| 2) MS results from an auto-immune reaction,
in which the body's defense system "backfires" and attacks the
body's own cells. |
| 3) MS results from a combination of the above
two hypotheses in which a virus takes over parts of the body's cells, confusing
the immune system, and causing it to attack both the virus and body cells
(41). |
 (40) |
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Caption: Sagittal image of a human brain. The
large round white spot in the right frontal region (top left) is a relatively
new lesion. With time, the lesion will enlarge, and a "halo"
of white (high) signal (not shown) will appear surrounding the lesion. This
"halo" probably represents the edema which forms in reaction to
the acute damage (42). |
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