Schwann cells are glia of the peripheral nervous system named after a person. They are divided into myelinating and nonmyelinating types. For myelinating Schwann cells, which are in contact with thick axons, almost their entire membrane is myelin that is wrapped thinly around the axon many times, with the same structure and function as a myelin segments produced by oligodendrocytes.
Unlike oligodendrocytes, each Schwann cell forms only one segment of myelin on one axon. On the outside of these cell is a small lump that contains the nucleus and most of the cytoplasm.
Nonmyelinating Schwann cells are variably shaped, and in contact with thin axons. Instead of wrapping their membrane around these axons, though, multiple thin axons are often indenting part way into troughs or grooves around the periphery of the Schwann cell, perhaps to provide some structural support to thin axons. In addition to these functions, Schwann cells also appear to influence neurons, and vice versa, through exchange of a variety of substances.
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