Function of the atrioventricular node considered on the basis of observed histology and fine structure

From the hearts of 20 young dogs, the region of the atrioventricular (AV) node was studied in vitro utilizing direct perfusion of the AV node artery. Intracellular impalement with microelectrodes provided records of local transmembrane action potentials in all 20 dogs. These were correlated with serial section histologic studies in 7 of the 20 dogs to characterize a smaller region that served as an anatomic guide for electron microscopic examination in 4 other dog hearts. This report describes the variety of specific cells found, including their intracellular content and organization, as well as the nature of their intercellular junctions. On the basis of these findings, AV nodal cells were arbitrarily divided into two types, transitional cells and P cells, although three somewhat different groups of transitional cells were identified. The first group, found principally at the outer margin of the AV node, has long and slender cells that exhibit large profiles of gap junctions or nexuses. The second and third groups of transitional cells, which constitute most of the body of the AV node, are oblong or oval and contain fewer and smaller gap junctions. P cells of the AV node resemble those more abundantly present in the sinus node; they are found principally at the junction of the AV node and His bundle. On the basis of these fine structural features and the histologic organization and transmembrane action potentials observed, clinical and experimental aspects of the local electrophysiologic events are discussed.