Excitability cycle of cardiac muscle examined by intracellular stimulation.

Several characteristic phenomena of excitability were found in the cardiac muscle during the relative refractory period when the excitability was studied by electrical stimulations with surface electrodes.According to BROOKS and his associates1,2,15)and subsequently by HOFFMAN et al.7,8),CRANEFIELD et al.3),and VAN Dum et al.20)the characteristic aspects of the excitability of cardiac muscles during the relative refractory period may be stated as follows:(1) formation of a distinct'dip'in the strength-interval curve,(2)presence of a definite supernormality in anodal excitability,and preferencial occurrence of excitation at the anode,and(3)existence of'no-response phe nomenon'and `vulnerable period to fibrillation, .It was also reported that the cardiac muscle