Membrane and Myelin

Abstract

Electron microscopy has radically altered our concepts of the structure of the myelin sheath. No longer is it considered as merely a lipid sheath enclosing the axon, but rather is now known to be an intricate lamellar structure that is a part of the sheath cell. Electron microscopy also has created new problems in both terminology and ideology. One of these is evident in the current philosophy that all axons are myelinated. Since myelin is a lamellated membranous structure, and since all axons have at least a single cell membrane about them, it is therefore reasoned that all axons are medullated, thus making a farce of the term myelin rather than leading to a correlation of structure and function. Still other problems remain unsolved. In spite of new information concerning fine structure of myelin, the stimulus or signal for myelin formation is yet unknown— whether it be a response to conduction in the axon, a response to some “neurohumor” produced by the axon, or a product of the glial or satellite cell. Thus the stimulus that incites the sheath cell to produce myelin membranes is as much an unanswered question as is the signal that halts myelin production. Another enigma concerns the “C” fibers with their multiple axons within a single Schwann cell, rather than the single axon within a Schwann cell of the myelinated axon. Does the fundamental difference between an unmyelinated and a myelinated axon, then, reside in the sheath cell or in the axon?