Intrinsic neurosecretory neurons of the rat heart atrioventricular junction: possibility of local neuromuscular feed back loops.

The microanatomy and ultrastructure of rat heart atrioventricular junction were studied on serial sections of both paraffin and epoxy embedded samples. The animals were pretreated with 5-hydroxy-dopamine in order to differentiate the adrenergic component of the terminal nerve plexus. Two kinds of ganglia could be identified throughout the interatrial septum by the optical microscope. The first was composed of large pale cells with voluminous nuclei. The other resembled acini-like clusters of osmophilic cells. Another small ganglion was invariably present between right and left branches of the bundle of His. The electron microscopic examination of this structure revealed, apart from small pale neurons (10 microns in diameter), several large 5-hydroxy-dopamine contrasted neurosecretory cells (up to 30 microns) with abundant electron dense vesicles typical for sympathetic neurons. Numerous glomeruli with dendrodendritic and axodendritic connections, some of which exhibit the same 5-hydroxy-dopamine loaded vesicles, could be found in the vicinity of the specialized conducting tissue. The functional implications of the above morphological data are not clear. However, the intrinsic neurosecretory component can be expected to modulate both specialized cells and neighbouring neurons. The heterogeneity of neuro-neuronal and neuro-muscular relations, such as perinodal mechanoreceptors, may suggest that the terminal nerve plexus not only distributes the efferent stimuli, but that it also has an integrative function which is the necessary prerequisite for a local feed back autoregulation.