Page:Response in the Living and Non-Living.djvu/29

 CHAPTER II

ELECTRIC RESPONSE

Conditions for obtaining electric response— Method of injury —Current of injury—Injured end, cuproid: uninjured, zincoid—Current of response in nerve from more excited to less excited—Difﬁculties of present. nomenclature—Electric recorder—Two types of response, positive and negative-—Universal applicability of electric mode of response—Electric response a measure of physiological activity Electric response in plants.

UNLIKE muscle, a length of nerve, when mechanically or electrically excited, does not undergo any visible change. That it is thrown into an excitatory state, and that it conducts the excitatory disturbance, is shown however by the contraction produced in an attached piece of muscle, which serves as an indicator.

But the excitatory effect produced in the nerve by stimulus can also be detected by an electrical method. If an isolated piece of nerve be taken and two contacts be made on its surface by means of non—polarisable electrodes at A and B, connection being made with a galvanometer, no current will be observed, as both A and B are in the same physico—chemical condition. The two points, that is to say, are iso—electric.

If now the nerve be excited by stimulus, similar disturbances will be evoked at both A and B. If, further, these disturbances, reaching A and B almost simultaneously, cause any electrical change, then,