ACTIVITY OF POTASSIUM MECHANISM IN SINGLE RANVIER NODE DURING EXCITATION

Abstract

When a single nerve fiber is immersed in a potassium rich solution and the membrane potential level maintained by an applied voltage, a striking change takes place in the shape of the action potential elicited by cathodal stimulation. The contour of the depolarization, normally spike-like in appearance, develops a prolonged depolarization or plateau phase following partial repolarization of the spike. If sodium is removed from the medium, the initial spike disappears, leaving only the slow developing, latent and prolonged depolarization. It has been shown (1) that the prolonged response duration is dependent upon the membrane potential level as set by an applied voltage, the early sodium spike is not, (2) that the spike can be initiated during the plateau phase by a second cathodal pulse, (3) that the pleateau may be prolonged, abolished, or reinitiated by small cathodal or anodal pulses applied during this depolarization phase, and (4) that a very definite dip or discontinuity occurs in the deflection between the spike and plateau phase. With the voltage clamp technique a second inward current was elicited in a node in high potassium. This current occurs with considerable latency, is very prolonged, and may be recorded quite separately from the initial familiar sodium inward current. This second current has been identified as an inward potassium ion current. Such a current may account for the prolonged depolarization and must result from the activation and inactiva-tion of a second ion carrier system in the membrane specific to potassium. The evidence supports the contention that two separate ion carrier systems exist in the excitable membrane, one which reacts rapidly with sodium, the other more slowly with potassium.