Both intracellular and surface electrodes were employed to record electrical activity from embryonic chick hearts between the ages of 3 and 20 days. Cells from the sinus venosus, sinoatrial (SA) valves, atrium, atrioventricular (AV) ring, and ventricle were localized and characterized on the basis of shape, amplitude, rise time, and duration of transmembrane potentials. The differences in transmembrane potentials from these various regions provided the basis for a hypothesis concerned with the distribution of pacemaker potentiality and one related to the origin of the His-Purkinje system. Action potentials recorded along the entire embryonic AV ring were comparable to those of the adult rabbit AV nodal cells in both configuration and sequence of activation and were thus categorized into three functional regions (AN, N, NH). Histological sections of 7 and 14 day hearts demonstrated muscular continuity between the right atrium and ventricle across the muscular AV valve.

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