Electrical activity of single fibers of the atrioventricular node.

The mechanism of atrioventricular delay has been studied in isolated rabbit hearts. Multiple intracellular microelectrodes have been employed to obtain simultaneous records from single fibers of atrium, A-V node and His bundle. An appreciable delay in the transmission of excitation has been found only in the atrial portion of the A-V node. Action potentials recorded from single fibers in this area show a low resting potential, slow diastolic depolarization, slow upstroke and low amplitude. These action potentials frequently show one or more notches or steps on the rising phase. Action potentials recorded from fibers of the His bundle are similar in shape and amplitude to those of peripheral Purkinje fibers. Records obtained at several sites between the atrial portion of the node and the His bundle show a gradual transition in action potential shape. The mechanism of slow transmission across the A-V node is discussed in relation to the electrical activity of fibers at the atrial end of this structure.