Myelination

In the PNS, the myelin sheath is an insulation formed by Schwann cells around axons (refer to Figure 1 ). Each Schwann cell tightly wraps around the axon numerous times to form a multilayered insulation. The last wrapping of the plasma membrane of the Schwann cell, the neurilemma, is loose and contains the nucleus, cytoplasm, and organelles of the Schwann cell.

The myelin sheath consists of numerous Schwann cell wrappings along the length of the axon. Spaces occur between adjacent Schwann cells, leaving uninsulated areas, or neurofibral nodes (nodes of Ranvier), along the axons. As an insulator, the Schwann cells interrupt the continuous conduction of a nerve impulse along the axon. In response, the nerve impulse jumps across each Schwann cell, from one neurofibral node to the next (saltatory conduction). In this fashion, the myelin sheath provides insulation between adjacent nerve fibers, preventing the crossover of one nerve impulse to an adjacent axon.

In the CNS, the axons of neurons are insulated by oligodendrocytes. The myelin sheath is formed when a process extended by the oligodendrocyte wraps around an axon. Many axons may be myelinated by multiple processes from a single oligodendrocyte. The oligodendrocyte does not form a neurilemma because its cell body does not wrap around any axons.

The white and gray matter of the brain and spinal cord are distinguished by the presence or absence of myelin sheaths.

• White matter contains the myelinated axons of neurons. (The white color is from the myelin sheaths.)

• Gray matter contains the unmyelinated portions of neurons (cell bodies, dendrites, and axon terminals), unmyelinated neurons, and neuroglia.

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