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" SCHWANN CELLS & THE MYELIN SHEATH "

All axons in the peripheral nervous system are surrounded by Schwann cells, and the cover produced by these cells is often referred to as the sheath of Schwann. Schwann cells that surround large diameter axons (= A and B fibers; 2 um or larger) undergo a wrapping process called myelination. The process begins when one part of the Schwann cell begins to move along the surface of the axon. As it moves, this leading edge slides underneath the outer portion of the Schwann cell, pushing it out of the way. The leading edge continues to wrap itself around the outside of the axon. During this repeated wrapping, more and more cytoplasm is pushed to the periphery of the Schwann cell. When the myelinatiion process is complete, the Schwann cell covers the axon with many layers of plasma membranes, consisting mostly of lipids. This covering is called a myelin sheath, and the axon is said to be myelinated. The Schwann cell cytoplasm outside the myelin sheath contains the cell nucleus and is referred to as the neurilemma. Schwann cells are only 0.3 mm to 1.5 mm in length, thus many are required to myelinate the length of a single axon.