[Innervation of the atrioventricular portion of the interatrial septum in the rat].

A quantitative electron microscopical investigation has revealed reliable differences in contents of muscular, connective tissue and neural elements in the conductive system, on the one hand (in the atrioventricular node and in the internodal specialized pathways) and in the working myocardium of the inferior third of the interatrial septum, on the other hand. Morphometrical analysis of the neuromuscular interrelations, performed separately for every type of specialized conductive myocytes (II, III) and of working cardiomyocytes of the interatrial septum, reveals some differences in size of the neural fibers and their terminals and in distance between them and myocytes. The synaptic vesicles of the efferent terminals are qualitatively different in all types of the myocytes. The afferent terminals situating near specialized cardiac cells are characterized. Criteria for distinguishing small and large granular vesicles in the terminals are discussed. A suggestion is made that the innervational peculiarities revealed reflect certain specificity in regulation the conduction in the atrioventricular area of the heart.