Myelin ultrastructure

Myelin facilitates conduction 51 Myelin has a characteristic ultrastructure 52 Myelin is an extension of a glial plasma membrane 55 Myelin affects axonal structure 56... 

Liu, K. M. and Shen, C. L., Ultrastructural sequence of myelin breakdown during Wallerian degeneration in the rat optic nerve, Cell Tissue Res., 242, 245, 1985. 

The chronic effect of carbon disulfide and ethanol was examined by Opacka et al. (1984). Rats were exposed to 257 ppm of carbon disulfide for 5 hours a day, 6 days a week, for 11 months, and 10% ethanol (in water ad libitum for the last 3 months, control water ad libitum). Control rats were exposed to filtered air. The behavior, memory, and learning ability of the ethanol-fed rats were adversely affected compared to controls. Additional studies indicate biochemical alterations in the central nervous system and increased -glucuronidase activity ultrastructural studies show degeneration in the peripheral nervous system, particularly in the myelin sheath. These authors reported that the effects from combined exposures are greater than those from each substance alone. Wronska-Nofer et al. (1986) investigated the... 

Recent studies confirm the initial observations of a decade ago that the turnover of myelin in adults is extremely slow. Myelin as a whole can be considered relatively stable, but various components are metabolized at different rates. The turnover rates (as measured by metabolic half-lives) of phosphotidylcholine, phosphotidylethanolainine, and phosphotidylserine are at least one-half as fast in myelin as in microsomes. Other lipids most probably turn over more slowly. Most lipids have turnover times in myelin on the order of weeks or months. Proteins have similar turnover times. In summary, the original proposal that myelin has a long-term metabolic stability appears accurate. However, various components turn over at different rates, and each component undergoes two phases, one of slow and one of fast degradation. Further information on the biochemistry, ultrastructure, and metabolism of myelin can be found in Bunge (1968), Davison and Peters (1970), Morell (1977), and Norton (1975). 

FIGURE 5-3 Variation in biomembranes in different cell types, (a) A smooth, flexible membrane covers the surface of the discoid erythrocyte cell, (b) Tufts of cilia (Cl) project from the ependymal cells that line the brain ventricles, (c) Many nerve axons are enveloped in a myelin sheath composed of multiple layers of modified plasma membrane. The individual myelin layers can be seen in this electron micrograph of a cross section of an axon (AX). The myelin sheath is formed by an adjacent supportive (glial) cell (SC). [Parts (a) and (b) from R. G. Kessel and R. H. Kardon, 1979, Tissues and Organs A Text-Atlas oT Scanning Electron Microscopy, W. H. Freeman and Company. Part (c) from P C. Cross and K. L. Mercer, 1993, Cell and Tissue Ultrastructure A Functional Perspective, W. FI. Freeman and Company, p. 137]... 

CoggeshaU RE, Carlton SM. Ultrastructural analysis of NMDA, AMPA, and kainate receptors on unmyelinated and myelinated axons in the periphery. 

Certain aspects of neural anatomy and ultrastructure favor LA blockade of noxious impulses [2-5]. To reduce conduction safely in sensory and motor fibers, at least three myelin inter-nodes must be blocked. This rule of three favors blockade of thin unmyelinated fibers and also A-delta fibers which have small inter-nodal distances. Larger A-alpha and A-beta fibers are more resistant to blockade because of their size and large distance between nodes of Ranvier (Figure 64.3). Unmyelinated C fibers are easiest to block since LAs can impede Na+ conductance and action potential propagation at any single site along the course of the nerve fiber. 

Ultrastructural studies in acute cytotoxic oedema obtained by TET application [14, 15] reveal separation of myelin lamellae and splitting of the intraperiod line but no evidence of destruction of myelin. In contrast, in brain oedema induced by insertion of PPD (vasogenic type), [16] there is myelin damage, and macrophages with phagocytosed myelin are found. 

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