Differential autonomic control of SAN and AVN regions of the canine heart: structure and function

Both anatomical and physiologic evidence for relatively rich autonomic innervation of sinoatrial (SAN) and atrioventricular (AVN) regions of the canine heart exist, with indication that SAN is especially responsive to parasympathetic, while AVN is preferentially sensitive to sympathetic regulation. The distribution of autonomic pathways are sufficiently separate and discrete that careful surgical intervention can selectively delete either parasympathetic or sympathetic nerve supplies to either (or both) SAN and AVN regions. Selective blockade by restricted injections of lidocaine (general neuronal blocker) or hexamethonium (ganglionic blocker) indicate that the vast majority (perhaps all) of vagal ganglia supplying SAN reside in the pulmonary vein fat pad and associated adipose tissues. In contrast, the vagal ganglia supplying AVN are found within a smaller fat pad overlying epicardium at the junction of inferior vena cava-inferior left atrium. These vagal pathways to either automatic cells of SAN or conductile tissues of AVN can be selectively interrupted without interfering with vagal regulation of the remaining intact system. Electroneurograms from large neurons situated within PVFP of the anesthetized, open-chest animal, reveal vigorous, phasic electrical activity associated with the cardiac and respiratory cycles, as well as with sensory stimulation of the heart, great vessels, and lungs. Spontaneous electrical activity of presently unknown origin is also observed. Direct neuronal stimulation, plus retrograde transport of fluorescent markers suggest that highly selective postganglionic intracardiac pathways may regulate discharge patterns of the sinus automatic cells.