Current-voltage relations in the isolated giant axon of the cockroach under voltage-clamp conditions.

Our understanding of the ionic exchanges taking place at the nerve membrane during electrical activity has been greatly increased by the application of the voltageclamp technique. This experimental method consists of the application of a stepwise change in potential to a given area of membrane and the simultaneous measurement of the current flow produced by this given potential gradient. The voltage-clamp technique was first introduced by Cole (1949) and Marmont (1949) and was subsequently developed by Hodgkin & Huxley (1952) and Hodgkin, Huxley & Katz (1952) using the squid giant axon. It has also been successfully applied to other excitable structures, including, for example, the isolated frog nerve fibre (Dodge & Frankenhaeuser, 1958, 1959) and the isolated lobster giant axon (Julian, Moore & Goldman, 19626). There has, however, been no application of this technique to the relatively small insect axons, and previous work has been confined to the measurement of the changes in membrane potential induced by alteration of the ionic composition of the bathing medium. Thus, Boistel & Coraboeuf (1958) have shown that partial replacement of sodium chloride in the Ringer bathing the nerve cord of the cockroach, Periplaneta americana, by choline chloride results in a small increase in resting potential and an appreciable decrease in the height of the action potential. Similar experiments were also carried out by Yamasaki & Narahashi (1959a) who observed that a 8emilogarithmic plot of the overshoot of the action potential against sodium concentration resulted in a straight line, which only diverged slightly from the theoretical relation predicted by the Nernst equation. Similarly, the resting potential was shown to be exponentially related to the external potassium concentration, for values higher than 20 mM/1. (Yamasaki & Narahashi, 1959a), although in this species, and also in Carausius morosus (Treherne & Maddrell, 1967), the line departed appreciably from the 58 mV. slope predicted by the Nernst equation. This departure was postulated to result from the involvement of other ions than potassium in determining the resting potential. The above results were interpreted as strong evidence for the validity of the sodium theory in cockroach giant axons. More recently, this evidence has been strengthened by the observation of Narahashi (1965) that the puffer fish poison, tetrodotoxin, blocked the excitability of cockroach axons without significantly changing the resting