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Early reactive changes at myelin sheaths gaps (nodes of Ranvier) of nerve fibers (a supravital study)

Neuroscience and Behavioral Physiology, ISSN: 1573-899X, Vol: 42, Issue: 7, Page: 775-779
2012
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An inverted phase contrast microscope was used to perform supravital studies of the structural dynamics of individual gaps (nodes of Ranvier) in the isolated myelin sheaths of frog nerve fibers subjected to mechanical injury and in medium with a decreased ionic strength in conditions in which electrophysiological experiments lead to expression of K channels in the axolemma in the paranodal region. Video recordings showed that delamination of the myelin sheath occurs in this region in association with swelling of the Schwann cell cytoplasm in the terminal membranous loops of myelin. Increases in the extent of delamination of myelin lamellar complexes makes them invisible under the light microscope, so movement of the myelin sheath from the gap, as has been proposed by many authors, is not seen, but rather the displacement only of the visible margin of the compact, still non-delaminated myelin sheath. Thus, removal of myelin (demyelination) does not occur, such that the electrophysiological effect can be explained in terms of a significant decrease in electrical resistance in the paranodal area due to swelling of the terminal membranous loops and delamination of the myelin sheath. Preparations also showed decreases in axon diameter which were proportional to the swelling of the terminal section of the myelin sheath. As the external fiber diameter did not change, it can be concluded that swelling of the Schwann cell cytoplasm occurred as a result of displacement of the aqueous fraction of the axoplasm, which may be a characteristic feature of axoglial interactions. © 2012 Springer Science+Business Media, Inc.

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