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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