(a) M muscle; A uninjured, B injured ends. E E′ non-polarising electrodes connecting A and B with galvanometer G. Stimulus produces ‘negative variation’ of current of rest. Index connected with galvanometer needle records curve on travelling paper (in practice, moving galvanometer spot of light traces curve on photographic plate). Rising part of curve shows effect of stimulus; descending part, recovery.
(b) O is the zero position of the galvanometer; injury produces a deflection A B; stimulus diminishes this deflection to C; C D is the recovery.
Two types of response—positive and negative.—It may here be added that though stimulus in general produces a diminution of current of rest, or a negative variation (e.g. muscles and nerves), yet, in certain cases, there is an increase, or positive variation. This is seen in the response of the retina to light. Again, a tissue which normally gives a negative variation may undergo molecular changes, after which it gives a positive variation. Thus Dr. Waller finds that whereas fresh nerve always gives negative variation, stale nerve sometimes gives positive; and that retina, which when fresh gives positive, when stale, exhibits negative variation.
The following is a tabular statement of the two types of response:
I. Negative variation.—Action current from more excited to less excited—cuproid change in the excited—e.g. fresh muscle and nerve, stale retina.
II. Positive variation.—Action current from less excited to more excited—zincoid change in the excited—e.g. stale nerve, fresh retina.[4]
From this it will be seen that it is the fact of the electrical response of living substances to stimulus that is of essential importance, the sign plus or minus being a minor consideration.
Universal applicability of the electrical mode of response.—This mode of obtaining electrical response is applicable to all living tissues, and in cases like that of muscle, where mechanical response is also available, it is found that the electrical and mechanical records are practically identical.
The two response-curves seen in the accompanying diagram ([fig. 5]), and taken from the same muscle by the two methods simultaneously, clearly exhibit this. Thus we see that electrical response can not only take the place of the mechanical record, but has the further advantage of being applicable in cases where the latter cannot be used.
Electrical response: A measure of physiological activity.—These electrical changes are regarded as physiological, or characteristic of living tissue, for any conditions which enhance physiological activity also, pari passu, increase their intensity. Again, when the tissue is killed by poison, electrical response disappears, the tissue passing into an irresponsive condition. Anæsthetics, like chloroform, gradually diminish, and finally altogether abolish, electrical response.