Myelin synthesis in the peripheral nervous system

By imposing saltatory conduction on the nervous impulse, the principal role of the myelin sheath is to allow the faster propagation of action potentia ls along the axons which it surrounds. Peripheral nervous system (PNS) myel in is formed by the differentiation of the plasma membrane of Schwann cells . One of the biochemical characteristics that distinguishes myelin from oth er biological membranes is its high lipid-to-protein ratio, All the major l ipid classes are represented in the myelin membrane, while several myelin-s pecific proteins have been identified. During development, the presence of axons is required for the initiation of myelination, but the nature of the axonal signal is still unknown. The only certainties are that this signal i s synthesized by axons whose diameter is greater than 0.7 mu m, and that th e signal(s) include(s) a diffusible molecule. Morphological studies have pr ovided us with information concerning the timing of myelination, the mechan ism by which immature Schwann cells differentiate into a myelinating phenot ype and lay down the myelin sheath around the axon, and the accumulation an d the structure of the myelin membrane, The last 20 years have seen the ide ntification and the cDNA and gene cloning of the major PNS myelin proteins, which signalled the beginning of the knock-out decade: transgenic null-mut ant mice have been created for almost every protein gene. The study of thes e animals shows that the formation of myelin is considerably less sensitive to molecular alterations than the maintenance of myelin. During the same p eriod, important data has been gathered concerning the synthesis and functi on of lipids in PNS myelin, although this field has received relatively lit tle attention compared with that of their protein counterparts. (C) 2000 El sevier Science Ltd. All rights reserved.