Myelin movements in mature mammalian peripheral nerve fibers

Abstract

Mature mouse and cat peripheral nerve fibers have been examined in vitro by time-lapse photography. Some Schmidt-Lanterman clefts which were open at the start closed later; others were seen to open and then to close, some of them more than once. The implications of these movements are considered, especially in regard to the question of the passage of materials from the endoneurial connective tissue spaces to the axon.Myelin movements other than those occurring at the Schmidt-Lanterman clefts consisted primarily of the development and frequent regression of indentations of the myelin sheath. A single evagination was seen to develop and then to recede. These myelin movements suggest that previously described invaginations and evaginations of the myelin sheath, including flaps of "redundant myelin", are not static but rather that they are in a state of movement, forming and regressing at intervals.The possible functional significance of the development and regression of myelin sheath indentations in relationship to axoplasmic flow is discussed.The mutual relationship between axons and their Schwann myelin sheaths is a subject of considerable interest. On the one hand, it is known that the myelin sheath is dependent for its integrity upon that of the axon; disintegration of the latter following nerve section or crush, or damage to the neuronal perikaryon, is accompanied by disruption of the myelin sheath, as seen in wallerian degeneration. It has been postulated that a neurotrophic factor emanates from the axon to maintain the morphology of the myelin sheath (Singer and Steinberg, '72). On the other hand, evidence has accumulated to indicate that substances of metabolic importance enter the axon not only from the neuronal cell body by axoplasmic flow (Lubinska, '64; Ochs, '66; Barondes and Samson, '67; Jacobson, '70; Lasek, '70) but also along its course, by passage through the axolemma (Singer and Salpeter, '66; Singer, '68; Singer and Green, '68; Caston and Singer, '69a,b; Jacobson, '70 the Schwann cell and the myelin sheath of the internode (see DISCUSSION). Metabolic products exuding from the axon, including the hypothetical trophic factor which maintains the intactness of the myelin (Singer and Steinberg, '72), could conceivably travel in the opposite direction by the same route or routes.The possibility that movements of the Schwann cell may be concerned in the transport of materials to and from the axolemma led Singer and Bryant ('69) to examine fresh, teased, excised sciatic nerve fibers of the amphibian Xenopus lamis in nutrient medium by means of time lapse photography. They observed movements of the myelin sheath in many of the specimens examined, involving especially the Schmidt-Lanterman clefts which were seen to open and to close. They suggested that a pulsatile flow of cytoplasm within the Schwann cell between its perikaryon and the adaxonal layer could serve as a pathway for metabolites.The present study is an extension of that of Singer and Bryant ('69), an at- MATERIALS AND METHODSFres...

Excitation block in a nerve fibre model owing to potassium-dependent changes in myelin resistance

“…There is a balance between the processes of aggregation and splitting of myelin layers, especially in the paranode -node -paranode regions, which together with the Schmidt -Lanterman incisures are the most labile and complex regions of the fibre [17]. Prolonged repetitive excitation leads to changes in the myelin structure even in normal fibres.…”

Functions and distribution of voltage‐gated sodium and potassium channels in mammalian schwann cells

“…While transport of proteins from a neuronal cell body to a peripheral axonal site may occur over a distance of several feet in large mammals, the maximum distance in transporting materials from a Schwann cell body to a site near the axon is only -8,000 km for a 15 p,m myelinated fibre when the longest route is used of looping around 200 concentric rings of myelin lamellae via the Schmidt Lanterman incisures. Time-lapse photography has shown pulsating movements along these incisures in adult nerves (Gitlin and Singer, 1974). Transport of RNA to maintain protein synthesis in the distal processes of a Schwann cell takes place at an extremely slow rate, namely, on the order of 0.1 mm per day along the outermost cytoplasm of an internode (Gould and Mattingly, 1990).…”

Effects of Phoneutria Nigriventer Spider Venom on Mouse Peripheral Nerve

“…Biological membranes are generally regarded as being fluid in nature with constituent proteins and lipids able to move relative to one another in the plane of the membrane (Singer & Nicolson, 1972). Fluidity in the myelin sheath has been demonstrated in vitro by Matus, de Petris & Raff (1973), who showed that concanavalin A receptors, glycoproteins evenly dispersed in the intraperiod lines of myelin, were redistributed into clusters by addition ofcross-linking antibodies, and by Gitlin & Singer (1974), who observed development and regression of indentations in the myelin sheath and opening and closure of Schmidt-Lanterman incisures. Present observations indicate that myelin components may undergo rapid reversible redistribution in vivo without loss of structural integrity.…”