The sheath of fatty tissue composed of 70-80% lipid and 20-30% protein, derived from glial cells, that wraps around the axons of neurons in the central and peripheral nervous system, whose purpose is to increase transmission speed and reliability. The combined length of myelinated nerve fibers in the adult brain has been estimated to achieve the incredible figure of 150,000 to 180,000 km. Another estimation puts the number of glial cells in the cerebral cortex at 39 billion for young adults and at 36 billion for older adults. Myelin is formed from one of two types of glial cells. In the central nervous system, they are olgiodendrocytes and in the peripheral nervous system Schwann cells. There are evenly-spaced gaps in the sheath of myelin known as the nodes of Ranvier that facilitate the rapid, and jump-like, propagation of the action potential, which is referred to as saltation or saltatory conduction. At the nodes, about one micrometer in length, the membrane of the axon is exposed to the surrounding liquid, and depolarisation takes place that is sufficient to increase voltage at the next node to the threshold required to re-initiate the axon potential. Without myelin, the impulse slowly spreads in the manner of a burning fuse; with myelin, the impulse seemingly jumps from one node to the next (saltatory conduction), achieving conduction speeds a hundred-fold greater than seen in unmyelinated conduction, top speeds being about 150 mm/msec. They were named after the histologist Louis Antoine Ranvier (1835-1922) who also discovered myelin in 1878. In axons lacking myelin, the action potential is propagated smoothly. When myelin in peripheral axons degenerates, it leads to multiple sclerosis, an autoimmune disease characterised by fatigue, muscle wasting, disrupted vision, loss of balance, and other problems.
See Action potential, Axon, Axon collateral, Brain (or encephalon), Central nervous system (CNS), Cholesterol, Diffusion anisotropy, Diffusion tensor imaging, Dorsal roots, Glial cells, Gray matter, Granule cells, Myelination, Nodes of Ranvier, Olgiodendrocytes, Optic nerve, Peripheral nervous system, Schwann cells, White matter