Influence of ionic current on Na⁺ channel gating in crayfish giant axon

Abstract

In studies of nerve excitation it is important to examine the possible interaction of voltage-sensitive gating mechanisms with components forming the pathway of ionic channels. There is evidence that permeant ions and gating are independent in Na⁺ channels^1,2 of nerve fibres, but not in K⁺ channels³. Of interest in this and other studies of excitability is the determination of the physical basis for the resistance in series with the axon membrane (R_s)⁴. In an attempt to determine whether an appreciable R_s is associated with individual Na⁺ channels⁵, we have compared gating kinetics in conditions in which the ionic current through individual channels is varied while the total membrane current remains constant. The number of conducting Na⁺ channels, and thus their contribution to R_s, was lowered by addition of tetrodotoxin⁶. We report here that no more than 10% of the resistance in ‘series’ with activation gates is associated with the Na⁺ channel, and give some preliminary estimates for minimum distances between gates and ion-binding sites within the Na⁺ channel.