Excitability Cycle of Cardiac Muscle Examined by Intracellular Stimulation

Recovery of the excitability of the terminal Purkinje fibers of the dog ventricle was studied by means of intracellular stimulation and recording of membrane potential.2. The strength-interval curve for the cathodal stimulation was a uniform hyperboloid with a sharp inflection. Their course was closely related to the membrane repolarization, and the relative refractory period corresponded to the phase of repolarization from-60 to-80mV.3. The strength-interval curve for anodal stimulation had a shape of an inverted spike located about 15 to 20 mSec.earlier than the end of the absolute refractory period to the cathodal stimulation.The intact fibers were unresponsive to any anodal stimulation during the diastolic phase.4. Break of anodal current produced an isolated spike response during phase 2 and in the first half of phase 3, but the response did not conduct. At a somewhat later stage, the anodal stimulation produced a response consisting of a spike and slow potential. In this case the response was conductive.5. In view of the excitability curve of a single fiber, it is concluded that the excitation elicited by the extracellular anodal stimulation in the diastolic phase may be due to the virtually cathodal effect of the current around the anode, and the ‘dip’ or some of the irregularities observed in the extracellular cathodal strength-interval curves may largely be ascribable to the virtually anodal effect of the stimulating cathode.The expenses for this work were defrayed, in part, by a grant from the Ministry of Education.