Myelin

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 (a 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"). On the other hand, myelin reduces the capability of neurons to heal themselves.

If an axon's myelin degrades, due to a disease such as multiple sclerosis for example, conduction can be impaired or lost.