VAGAL DEPRESSION OF THE TURTLE ATRIUM

Abstract

Vagal stimulation in the turtle was studied with respect to the time course of the resulting depression of atrial contraction. When a single effective shock was applied to the vagus, the inotropic effect in the atrium followed a deF1nite intensity-time course. The plotted curves have the general form of skew distribution curves. They may, however, be satisfactorily F1tted from an equation deF1ning the concentration of a substance under conditions of both diffusion and chemical change. Increase in strength (between threshold and maximal) of single shocks applied to the vagus resulted in an increase in atrial depression (between threshold and a maximum for single shocks) with but little change in the time functions of the curve of depression. The times of achievement of and recovery from depression are increased as the temperature is lowered. Driying the atrium by electrical shocks at rates of about twice that of the normal rhythm does not affect the time course of the inotropic depression resulting from a single shock to the vagus. If, with a given shock strength, the depression following a single shock be compared with that resulting from several shocks, the summated effect from the latter is in accordance with that which would be expected from the former. This is true both for the degree of maximal depression achieved and the time required for its achievement. If repeated stimulation be continued "indefinitely," the depression develops to a certain level which is then maintained with little or no change. Within physiological limits, this depression is increased either by keeping the shock strength constant and increasing the frequency, or by keeping the frequency constant and increasing the shock strength. It is concluded that in the preparation used, summation of nerve impulse effects is largely determined by the intensity-time course of the effect produced by a single stimulation of a single nerve unit. Conditions of refractoriness in the system determine the minimum time which must elapse between successive stimuli if each is to be effective. The duration of the change produced by a nerve impulse determines the maximum time for summation. Within the limits of such conditions, repeated stimulation produces sum-mated effects.