Innervation principles of the sinoatrial and the atrioventricular nodes
In physiological experiments in canine and primates, removal of the epicardial ganglia located at the junction of the inferior vena cava and inferior wall of the LA eliminated the negative dromotropic effects of vagal nerve stimulation without suppressing vagally mediated reductions in heart rate (6, 7, 38, 39). On the contrary, surgical removal of the epicardial ganglia and nerves located around the right PVs attenuated the negative chronotropic response to vagal nerve stimulation without adversely affecting vagal inhibition of atrioventricular conduction (6, 7). In the human heart as well as in the hearts of some other mammalian species, both the VRAsGP and the DRAsGP supply epicardial nerves to the sinoatrial nodal neural network (Figure 2 and Online Supplemental Figure 1) (40, 41). Although the size and number of ganglia inside these sGPs vary from one heart to another, the largest number and density of epicardial ganglia are usually located at the junction of the SVC with the RA. It is noteworthy that some part of the postganglionated nerves of the VRAsGP passing onto the LA invariably extended via a remarkable crest of the ventral surface of the LA. This anatomical distribution should be kept in mind to understand why bi-atrial ablation might be needed for complete denervation of the sinoatrial node in some cases (Figure 4). Despite results of physiological experiments, successive anatomical attempts to determine nerves that coursed towards the canine or ovine atrioventricular node failed as those endocardial nerves are especially tiny (33-35). However, postganglionated nerves from the LDsGP , and the MDsGP extend towards the interatrial septum and presumably supply the atrioventricular nodal region (Figure 4) (33-35).