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Myelin is an electrically insulating fatty layer that surrounds the axons of many neurons, especially those in the peripheral nervous system. It is an outgrowth of glial cells: Schwann cells supply the myelin for peripheral neurons while oligodendrocytes supply it to those of the central nervous system.

The main consequence of a myelin layer (or sheath) is an increase in the speed at which impulses propagate along the myelinated fiber. Along unmyelinated fibers impulses move continuously as waves, but in myelinated fibers they hop (or "propagate by saltation"). When a fiber is severed, the myelin sheath provides a track along which regrowth can occur. Unmyelinated fibers do not regenerate.

Demyelination is a loss of myelin and is the root cause of symptoms experienced by patients with diseases such as multiple sclerosis and transverse myelitis. When an axon's myelin degrades due to these diseases, conduction can be impaired or lost.\n