Nerve excitation by alternating current

Abstract

When an alternating current is passed through a nerve a relation exists between the intensity I necessary for steady threshold excitation, once to each cycle at the electrode considered, and the frequencyn. For a pure sine-wave current applied to a nerve lying on two non-polarizable electrodes, and taking account of both time-constants (kof the local excitatory disturbance,λof accommodation), this relation (Hill, 1936a, p. 343) should be:— I/I0= √—(1 + 4π2k2n2) (1 + 1/4π2λ2n2), (1) where I0is the “ true rheobase ” for steady excitation by a regular series of constant-current pulses, each of sufficient duration to attain the minimum threshold. The theory, in its present form, takes no account of the phenomena of absolute and relative refractoriness after an effective stimulus, so that equation (1) need not be expected to apply rigorously at frequencies so high that successive waves fall within the refractory periods, absolute and relative, of their predecessors. Nor can it allow for the fact that, owing to effects of electrical capacity at surfaces or membranes in the nerve, the current distribution in the nerve must change with change of frequency. It was important, however, to examine the relation between I andnover the wider range, and experiments were made at frequencies from nearly zero to 10,000 cycles per second. Those at the higher frequencies are referred to in § XI below; in the rest of the paper we deal only with the lower range (up to 300-1000 cycles per second, according to the tempera­ture).