Myelin Structure and Biochemistry

Publisher Summary The myelin sheath is an extended and modified plasma membrane that is wrapped around the nerve axon in a spiral fashion. Myelin is an electrical insulator, although its function of facilitating conduction in axons has no exact analogy in elec­trical circuitry. The morphological distinction between white matter and gray matter is the one that is useful for the neurochemist. White matter is com­posed of myelinated axons, glial cells, and blood vessels. Gray matter contains, in addition, the nerve cell bodies with their extensive dendritic arborizations. The predominant element of white matter is the myelin sheath that comprises about 50% of its total dry weight and is responsible for the gross chemical differences between white and gray matter. In unmyelinated fibers, impulse conduction is propagated by local circuits of ion current that flows into the active region of the axonal mem­brane, through the axon, and out through adjacent sections of the membrane. These local circuits depolarize the adjacent piece of membrane in a continuous sequential fashion. In myelinated axons, the excitable axonal membrane is exposed to the extracellular space only at the nodes of Ranvier that is known as the location of sodium channels. Information concerning myelin structure is also available from electron microscope studies that visualize myelin as a series of alternating dark and less-dark lines (protein layers) separated by unstained zones (the lipid hydrocarbon chains).