ACCOMMODATION IN MAMMALIAN MOTOR NERVES

Abstract

Accommodation, that is, a change of threshold during the passage of current, was studied in circulated cat''s nerves by measuring the peak voltage for threshold stimulation by an exponentially rising current of variable time constant. The peak voltage-time constant curves were smooth or they had a break, thus indicating the existence of 2 segments. The responses plotted in the 1st segment differed from those plotted in the 2d. Both types of response, brief and protracted, could be seen with suprathreshold voltages and rapidly rising currents. Hyperventilation and veratrine caused a decrease of rheobase and a lowering of the curves; hypercapnia resulted in a rise of the curves. The curves do not fit the theoretical equation which was suggested by Hill. They are adequately described by a parabolic relationship between the increment of voltage and the corresponding time constant. This relationship implies that there is no limiting slope. The break in the curves denotes 2 different thresholds with different time constants of accommodation. From these 2 thresholds it is inferred that currents may stimulate nerve by 2 different independent processes. The measurement of accommodation is discussed. The existence of a critical lineal gradient for stimulation by direct current is rendered unlikely by the data.