Some facts about myelination
1.
It has been observed that myelin sheaths are present only
around axons having a diameter more than 1.5 μm in peripheral nerves, and over
1 μm within the central nervous system. However, many axons of these or greater
diameter may remain unmyelinated.
2.
In general, the larger the axon diameter, the thicker the
myelin sheath, and the greater the internodal distance.
3.
All Schwann cells associated with a particular axon are
believed to be present in relation to it before myelination begins, there being
no division of Schwann cells thereafter. Axon diameter and internodal length
are also probably determined before myelination begins. However, these
dimensions increase with growth.
4.
In peripheral nerves Schwann cells accompany nerve fibres
as the latter grow towards their destinations. In contrast, in the CNS the
axons extend to their destination before oligodendrocytes become associated
with them.
5.
Myelination does not occur simultaneously in all axons. A
myelinated tract becomes fully functional only after its fibres have acquired
myelin sheaths. Nerve fibres are not fully myelinated at birth. Myelination is
rapid during the first year of life and becomes much slower thereafter. This is
to be correlated with the gradual ability of an infant to perform more
complicated actions.
6.
At their exit from the CNS axons in peripheral nerves
pass through a central-peripheral transition region where Schwann cells come
into relationship with glial cells. This junction normally lies at a node of
Ranvier that is called the PNS-CNS compound node (PNS = Peripheral nervous
system; CNS=Central nervous system).
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