Schwann Cells

Larger axons passing through peripheral nerves commonly are enclosed in sheaths called "Schwann cells." These are tightly wound around the axons, somewhat like insulation on a wire. The smallest axons also are enclosed in Schwann cells, but they are not wound around the axons. These membranes are composed largely of lipid-protein that has a higher proportion of lipid (fat) than other cell surface membranes. This lipid-protein is called myelin, and it forms a "myelin sheath" on the outside of an axon. Axons that have myelin sheaths are called "myelinated" (or medullated) nerve fibers, while those that lack these sheaths are "unmyelinated" nerve fibers. Myelin serves as an insulator by preventing almost all flow of ions through the membrane. Considering this, it might seem that the myelin sheath would prevent the conduction of a nerve impulse altogether, and this would be true if the sheath were continuous. It is, however, interrupted by some constrictions called "nodes of Ranier," which occur between adjoining Schwann cells. At these nodes, the fiber membrane is especially permeable to sodium and potassium ions.