Electric Stimulation and the Excitatory Process in the Nerve Fiber

In various experiments on the single nerve fiber, we find that an isolated nerve fiber behaves in a grossly different manner from an ordinary nerve trunk in many respects. Sometimes we received the impression that we were dealing with an entirely new excitable tissue that had never been studied before. Especially in studying the action of narcotics, the distinction between the nerve fiber and the nerve trunk is striking. When a narcotizing solution of an appreciable concentration is applied to these tissues, the nerve fiber loses its conductivity almost instantaneously at the moment of application (Tasaki, 1936), while the nerve trunk continues to conduct an impulse for many minutes. Further, narcotics affect the conductivity, in the nerve fiber, only at the nodes of Ranvier (Tasaki, 1936 and 1938). Kubo and One's experiment (1934), which revealed remarkable variation in the threshold strength along the nerve fiber, has led me, through mathematical computations, to the conclusion that the myelin sheath must be an electric insulator which covers the axis-cylinder perfectly except at the nodes (1934). This conclusion is quite consistent with the results obtained by Erlanger and Blair (1934). The present investigation was first designed to substantiate, by applying the method of multipolar stimulation (Rushton, 1928) to the isolated single nerve fiber, my previous conclusion that the nerve impulse was set up only at the nodes of Ranvier. The range of observation was then extended to include the problems of the strength-duration relation and of the excitatory process in the nerve fiber. Though I have succeeded in confirming my previous conclusion, the results obtained in this investigation were again very different from what might be expected from our previous knowledge of the nerve trunk. Further, it appears that the strength-duration relation, and with it the chronaxie, varies with the distribution of potential along the nerve fiber. The phenomenon first observed by Davis (1923) that the chronaxie varies with the size of the electrode is explained by new experimental data. METHODS.