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Formation of Myelin Sheath
A many-layered white, lipid and protein covering produced by the neuroglia that is called the myelin sheath surround the axons of most neurons. The sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction. Axons with such a covering are said to be myelinated whereas those without it are said to be unmyelinated.
Two types of neuroglia produce myelin sheaths: neurolemmocytes and oligodendrocytes. In the PNS, neurolemmocytes form myelin sheaths around the axons during fetal development. In forming a sheath, a neurolemmocyte wraps around the axon in a spiral fashion in such a way that its nucleus and cytoplasm end up in the outside layer. The inner portion, up to 100 layers of neurolemmocyte membrane, is the myelin sheath.
The outer nucleated cytoplasmic layer of the neurolemmocyte, which encloses the myelin sheath, is called the neurolemma (sheath of Schwann). A neurolemma is found only around the axons in the PNS. When an axon is injured, the neurolemma aids in the regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon. At intervals along an axon, the myelin sheath has gaps called neurofibral nodes (nodes of Ranvier).
In the CNS, an oligodendrocyte myelinates many axons in somewhat the same manner that neurolemmocytes myelinate PNS axons. Oligodendrocytes merely deposit a myelin sheath, however, without forming a neurolemma. Neurofibral nodes are present, but they are fewer in number. Axons in the CNS display little regrowth after injury. This is thought to be due, in part, to the absence of a neurolemma.
The amount of myelin increases from birth to maturity, and its presence greatly increases the speed of nerve impulse conduction. Because myelination is still in progress during infancy, an infant's responses to stimuli are not as rapid or coordinated as those of an older child or adult. Certain diseases such as multiple sclerosis or Tay-Sachs disease involve destruction of myelin sheaths.
On to the Grouping of Neural Tissues...
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