A combined morphological and electrophysiological study of the atrioventricular node of the rabbit heart.

Thirteen rabbit atrioventricular nodes were studied both morphologically and electrophysiologically. Two of these nodes were subsequently reconstructed in three-dimensional fashion. From the morphological standpoint, it was shown that the atrioventricular node was divided into a smaller enclosed portion and a larger open portion by a fibrous collar derived from the central fibrous body. Three distinct nodal cell types were identified within these nodal segments. The open node was mostly occupied by transitional cells which merged proximalry with the atrial myocardium. As they entered the enclosed node, some of these transitional cells merged into a knot of midnodal cells. Others passed circumferentially round this knot and together with the midnodal cells, joined with a tract of lower nodal cells. The latter cells were continuous with the His bundle anteriorly, but they also extended into the open node posteriorly. It was possible to correlate the activation sequence of the node accurately with the disposition of these cells. Using both reconstructions and techniques to mark cells from which action potentials had been recorded, it was shown that the transitional cells correlated with the AN zone of the node. Cells with N potentials were located in the environs of the midnodal cell knot. The anterior portion of the lower nodal cells correlated with the NH nodal zone. The posterior extension of the lower nodal cells and the overlay fibers of the anterior transitional cells both functioned as dead-end pathways. Histologically distinct tracts were not identified within the internodal atrial myocardium.