The main purpose of a myelin layer (or ''sheath'') is to 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."
Myelin increases electrical resistance across the cell membrane by a factor of 5,000 and decreases capacitance by a factor of 50. Thus, myelination helps prevent the electrical current from leaving the axon.
When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate.
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