Cation binding at the node of Ranvier: II. Redistribution of binding sites during electrical stimulation

Abstract

The nodal and paranodal areas of mature myelinated axons are known to bind cations. To examine whether the cation binding substance may play a role in saltatory conduction, a combined electrophysiological and histochemical study was undertaken. The sciatic nerve of anesthetized or unanesthetized adult C57Bl mice was exposed and not stimulated (control) or stimulated with constant square-wave pulses at one of the following rates: 10/sec, 30/sec, 100/sec or 500/sec. Phosphate-buffered 2.5% glutaraldehyde was either dropped onto the nerve during stimulation until cessation of the compound action potential or the nerve was fixed after discontinuing stimulation. The nerve was excised and processed for the histochemical reaction of copper sulfate/potassium ferrocyanide (which forms an electron dense precipitate at areas of cation binding), dehydrated and infiltrated with Spurr R epoxy resin. Individual nerve fibers were microdissected and counts made of the numbers of paranodal and nodal areas exhibiting the reaction product. The percentage of nodes stained, with respect to the total numbers of nodes and paranodes stained, was calculated. There was no significant difference in percent of nodes stained between the simultaneously fixed, non-stimulated, anesthetized (43.1%), the non-stimulated unanesthetized (45.3%), the animals stimulated at 10/sec (45.9%) and the animals stimulated at 30/sec (50.2%) and 100/sec (46.0%), and fixed post-stimulation. However, all values at the higher frequencies and fixed during stimulation were significantly different both from the control and from each other (30/sec-59.3%; 100/sec-70.5%; and 500/sec-76.4%). The location of cation binding appears to change in response to electrical stimulation and correlates with the increased frequency of the inward movement of sodium ions.