nerve-fibre defined in 1951 yearnerve-fibre - nerve-fibre;
nerve-fibre - The axon of a nerve-cell, or the similar process (but carrying impulses towards the cell body) of many peripheral sensory nerve cells, together with the various special membranes which may surround it. Many nerve-fibres of vertebrates and some of Crustacea and annelid worms are myelinated (medullated): i.e. the axon is covered by a relatively thick fatty sheath of myelin except at periodically-spaced nodes of Ranvier. Fibres which lack a myelin sheath are called unmyelinated or non-medullated; their axons are bounded merely by the usual plasma-membrane of all cells. Surrounding the axons of fibres of the vertebrate peripheral nervous system lie the Schwann cells, of which the myelin is really a part; and each single myelinated fibre is further enclosed in a reticulin neurilemmal tube. In the central nervous system Schwann cells and neurilemmal tubes are absent. The diameter of nerve fibres varies in any vertebrate animal from i to aoum; in some invertebrates giant nerve fibres occur and may reach i mm. diameter in squids and burrowing worms. A nerve-fibre commonly branches into many small-diameter twigs at its termination. An increased diameter gives greater conduction velocity; the same velocity is also produced in smaller space by a myelin layer, the impulses then apparently leaping from node to node along a fibre instead of travelling continuously along it. A nerve may be nearly as long as the whole animal, so that in large vertebrates fibres may occur up to a million times longer than they are thick.
near nerve-fibre in Knolik
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