Terminal nervous plexus of the atrioventricular junction of the rat. Microanatomical aspects and regional specialization.

The paraformaldehyde-fixed hearts of adult rats were embedded in paraffin. Serial sections throughout the interatrial septum and the atrioventricular junction were examined in the optical microscope after having been silver stained. The density and the distribution of different nervous elements in the perinodal nerve plexus were studied and the results compared with the electron-microscopic data. These data confirm that at the level of the atrioventricular junction, a dense terminal network can be seen. It is composed of both terminal nerve fibers and ganglionic cells forming a small ganglion adjacent to the lower edge of the bundle of His and a thin ganglionic lamina in the interatrial septum. Apart from small neurons, previously described as cholinergic, large neurosecretory cells could also be demonstrated in some of the serial thin sections through the Hissian ganglion. The neuromuscular junctions between the terminal nerve fibers and specialized cells exhibit regional specialization. While the distal portion of the atrioventricular node and the bundle of His receive essentially efferent fibers, afferent coiled endings prevail in the reticular portion of the atrioventricular node and in the adjacent interatrial septum. Similar structures, larger in diameter, can also be seen around the muscular fibers and around the capillaries of the interventricular septum. The functional significance of the aforedescribed regional specialization of the terminal innervation of the atrioventricular junction is briefly discussed.