Hyperpolarization of the Quiescent Heart and Inhibition of the Heart Beat by Vagal Impulses

Hyperpolarization by the vagus stimulation of the quiescent sinus of bullfrog's heart was recorded. The slope and magnitude of hyperpolarization varied with the pattern of vagal impulses. The hyperpolarization became larger with increase in the number of shocks, but after the magnitude of hyperpolarization attained its maximum the potential fell slowly toward the initial level during the stimulation. The optimal interval for the slope and magnitude of hyper-polarization was about 50 to 100 msec. Outside these intervals, the slope and magnitude of hyperpolarization were slow and small. Secondly, the electrogram was recorded from the tortoise's heart and the relation between the pattern of vagus stimulation and the quiescent period after the stimulation was investigated, and similar results were obtained. Of course, the phenomenon of escape occurred on continued vagus stimulation. The results were explained on the assumption of accumulation of the transmitter, its diffusion or destruction and the failure of nervous impulses, but the reduction of potential as well as the vagus escape in the later period of stimulation were interpreted as suggesting a development of the desensitization to the transmitter. Moreover, delayed response, change of firing potential, shift of pacemaker and rebound excitation were described.