CHRONAXIE, a term introduced by Louis Lapicque in 19o9 to define the character of the stimulus which is required to excite various types of living tissue, particularly muscle and nerve. The most convenient form of stimulus is an electric current, since this can be made to excite (i.e., to cause the tissue to display its characteristic activity) without doing any damage. It is found that a current must fulfil three conditions if it is to excite-0) it must flow for more than a certain minimal duration ; the final strength must exceed a certain value ; and (3) the rate of increase of the current from zero must exceed a definite velocity. The need for a minimal duration is shown by the fact that a cur rent alternating at a million cycles a second is powerless to excite any tissue in the body; and the modern surgical technique of diathermy is based on this.
The curve relating minimal duration and minimal strength has the form shown in the figure, and as a first approximation it obeys the formula of Weiss, i= , where i is the current, t its duration and a and b are constants. Lapicque has shown that curves of the same form are obtained from the most diverse tissues, ranging from the human nerve, where the minimal dura tion is less than •000'sec., to the cells of a plant where it may be longer than 'sec. Thus the character of the stimulus for a given tissue can be defined at once if we know the constants a and t for that tissue. The constant a (the "Rheobase") is given by the minimal current strength at long durations. The other constant b is the more important, for it determines the slope of the curve, it can be fixed by measuring the "Chronaxie," which is equal to b and is the least duration required when the current strength a is 2a.
The true form of the curve differs somewhat from that given by Weiss's equation, and it has been used as a basis from which to deduce the mechanism of excita tion. In Lapicque's hands the measurement of the Chronaxie has also been used to investigate the passage of the state of exci tation from one tissue to another (e.g., from nerve to muscle).
(E. D. A.)