Strength-Duration Curves and the Theory of Electrical Excitation

Abstract

Measurements of the interrelations of current strength and time for a fixed magnitude of effect due to repetitive electrical excitation in nerve and muscle are described with excellent fidelity by a logarithmic probability integral. The same kind of relationship, in which magnitude of sensory effect is expressed as a function of intensity, appears in phenomena of stimulation by light and by sound. The value of time for which 100 C/C = 50%, where C is the asymptotic lower exciting current, is essentially the chronaxie. This parameter is not by itself sufficient for the description of the strength-duration curve; the standard deviation of the underlying frequency distribution is an independent parameter, being different in various kinds of nerves and according to conditions of excitation. The log-probability integral gives a readily applied means of comparing different forms and manifestations of excitability. When an exciting energy is applied to a tissue, it is to be expected that the thresholds for excitation of the elements open to stimulation, in terms of the resultant chosen, will form a logarithmic frequency distribution provided the excitability of each element intrinsically fluctuates.