Myelin is a fatty white substance that surrounds parts of some nerve cells and assists in the transmission of nerve signals. Damaged or defective myelin is a symptom of several diseases, including multiple sclerosis.
Myelin consists mostly of lipid (fat) along with some protein. Sheaths of myelin surround the axon, a tubelike extension of the nerve cell. A nerve signal can “jump” between these sheaths, increasing the speed at which the signal travels along the axon. Parts of the brain and spinal cord called white matter consist primarily of myelinated (myelin-covered) axons.
Specialized nerve-supporting cells called glia produce myelin. In the central nervous system, which consists of the brain and spinal cord, myelin is produced by glia called oligodendrocytes. In the peripheral nervous system, which connects the central nervous system to the muscles and internal organs, glia called Schwann cells make myelin.
Myelin covers axons in segments about 1/26 inch (1 millimeter) in length. Small gaps, called nodes of Ranvier, separate myelin segments. Nerve impulses travel as electrical signals through axons. Nerve cells generate these signals, called action potentials, by exchanging ions (electrically charged atoms) of sodium and potassium across their membranes. Myelin blocks this exchange except at the nodes of Ranvier. As a result, the action potential hops from node to node. This effect, called saltatory conduction, transmits the signal much faster than it could travel along an axon without myelin.
See Multiple sclerosis; Nervous system (The axon) (How neurons carry impulses).
