Interactions Between Axons and their Sheath Cells

The demonstration in the early 1950s that myelin is formed by the surface membranes of sheath cells was a turning point in the understanding of axon-sheath cell relationships. The subsequent development of ultrastructu­ ral, electrophysiologic, biochemical, and immunologic techniques has made possible the identification of many facets of the complex interdependencies in fibers of both the central and peripheral nervous systems. Furthermore, it is now recognized that nerve fibers both influence and are influenced by cellular and extracellular components of their immediate environment. In this selective review, we discuss structural and functional relationships of axons and sheath cells in normal and abnormal nerves. Our concern is primarily with axons and Schwann cells in the peripheral nervous system but we also touch on the more complicated relationships involving axons, oligodendrocytes, and other neuroglia in the central nervous system.