The measurement of “accommodation” in nerve

Abstract

When an electric current, insufficiently strong to excite, is removed from a nerve the “excitatory disturbance” (Lucas, 1910) or “local potential” (Hill, 1936) reverts to its resting value. The time-constant of this decay, assumed exponential, is the time- constant of excitation, k . If this assumption is correct, the chronaxie is 0·693 k . When an electric current passing through a nerve lasts for a very short time the critical value of the “local potential” required for excitation, that is the threshold, is constant. The previous history of the nerve is not involved and the only time-constant is k . If the current lasts for a longer time, as with constant currents near the rheobase, with linearly or exponentially increasing currents, or with low frequency alternating currents, the threshold rises: the threshold for a slowly increasing current is well-known to be higher than for a rapidly increasing one. This change in threshold is called “accommodation” (Nernst, 1908). If the rise in threshold occurs because of the rise in “local potential,” the threshold will revert to its resting value when the “local potential” reverts to its resting value. The time-constant of the return, assumed exponential, of the threshold to its resting level is λ, the time-constant of “accommodation.” If this assumption is correct, the time to half return is 0·693 λ.