myelin sheath

A myelin sheath is a fatty wrapping that coats the long output cable of a nerve cell, the part called the axon, much the way plastic insulation coats a copper wire. It is not made by the nerve cell itself. Instead, helper cells called glia (support cells that surround and care for neurons) flatten themselves out and wind round and round the axon, layer upon layer, until the cable is sealed in a thick, pale, greasy jacket. Because this fatty coat is whitish, the parts of the brain and nerves that are heavily wrapped look pale, which is why they are called white matter.

The point of all that insulation is speed. An electrical signal traveling down a bare axon leaks and fades as it goes, so it crawls. Myelin plugs those leaks, but it does something cleverer too: the sheath is not one unbroken tube. It comes in segments with tiny bare gaps between them, called nodes, and the signal jumps from gap to gap instead of oozing along every inch. That hopping makes nerve messages race far faster, sometimes more than a hundred times quicker than they would in an unwrapped fiber, while using less energy. Two different glia do the wrapping: oligodendrocytes inside the brain and spinal cord, and Schwann cells out in the body's nerves.

When myelin is damaged, signals slow down, scramble, or stop, which is why losing it causes serious trouble. In multiple sclerosis, for example, the body's own immune system strips myelin from nerves, and the result can be blurred vision, numbness, weakness, or problems with balance, depending on which cables are affected. Healthy myelin, then, is one of the quiet reasons your nervous system can react in a blink rather than a sluggish moment.