The adrenergic cardiac nerves of the cat

A fluorescence method was used for study of the adrenergic cardiac nerves of the cat. Sixteen regions of the heart were examined. The sinoatrial and atrioventricular nodes and the interatrial septum were the most richly innervated regions of the heart. There was little difference between the density of innervation in sections of atrial and ventricular portions of the myocardium. Most of the adrenergic nerves supplying the ventricles were derived from large bundles in the atrioventricular sulcus and were distributed through the subepicardial plexus. Each coronary artery was sparsely innervated in its initial elastic segment, but the vasa vasorum were well innervated. The main trunks of the coronary arteries were surrounded by plexuses composed only of preterminal axons; adrenergic vasomotor terminals were plentiful on the external surface of the muscular media of major and minor myocardial branches. Dense plexuses of adrenergic nerves were seen in both atrioventricular valves. The pulmonary valve was well innervated and contained more adrenergic nerves than the aortic valve. Perivascular plexuses and freely ending adrenergic terminals were observed in the pericardium. Small intensely fluorescent cells were grouped around small blood vessels in the interatrial septum. Most were in the vicinity of the atrioventricular node. It is suggested that these cells, under neural control may secrete catecholamines into the microcirculation of the interatrial septum. They may exert local humoral control over the atrioventricular node or the atrial parasympathetic ganglia.