Freeze-fracture studies of the sinoatrial and atrioventricular nodes of the caprine heart, with special reference to the nexus.

The caprine sinoatrial node (SAN) and atrioventricular node (AVN) were studied by freeze-fracture techniques, and their nexus or gap junction structure were compared with that of ordinary atrial and ventricular muscle cells. The general features of the nexus in both the SAN and AVN were essentially identical. Approximately two-thirds of the nexuses observed in the nodal cells consisted of typical macular arrangements of nexal particles, and the remaining third, of atypical configurations of either circular arrangements or linear arrays of particles in continuity with the macular nexuses. Such atypical nexuses were never observed in the ordinary adult myocardial cells. Quantitative analysis revealed that all of the nexuses in the nodal cells measured, were less than 0.1 micron 2, whereas the majority of the nexuses in ordinary myocardial cells (64% in the atrium and 76% in the ventricle) were larger than 0.1 micron 2. No significant differences in diameter and center-to-center distance of nexal particle were found between the nodal cells and ordinary myocardial cells.