Control of left ventricular contractility by the interventriculo-septal ganglion and circuitry with cardiac afferents projecting to the nucleus of the solitary tract

An interventriculo-septal ganglion (IVSg) has been identified as the major source of parasympathetic postganglionic projections to left and right ventricles in cats. Recent evidence from our laboratory demonstrated, using sonomicrometry, that the IVSg plays a regulatory role in regional control of right ventricular contractility. In order to determine whether the IVSg forms part of local or central control pathways influencing heart function, we investigated its afferent circuitry and role in left ventricular (LV) contractility. The rate of change in ventricular pressure over time (LV dp/dt) was used as the index for contractility. Sensory afferents from the IVSg terminating in the medulla were labeled transganglionically thru the sensory neurites of the nodose ganglia by cholera toxin beta subunit conjugated to horseradish peroxidase (CTB-HRP) injected into the IVSg area. The results demonstrate that the IVSg may not mediate right vagal control of LV contractility: this was highly surprising since the IVSg is the major source of innervation to the LV. In neuroanatomical studies, we observed that sensory afferents from the IVSg were primarily distributed to the dmNTS at a level 13.5 mm posterior to the interaural line, with 48 % of total on right and 35 % on left NTS (p < 0.05, compared to three other levels in the rostrocaudal extent of NTS). No labeled profiles were seen in NTS of cats receiving injections of tracer into the pericardial sac. Sections from the dmNTS were processed for substance P immunoreactivity (SP-IR) and further processed for electron microscopic analysis of synaptic profiles. At the ultrastructural level, tungstate crystals were present in terminals and unmyelinated axons throughout dmNTS. Multiple types of axo-axonic and axo-dendritic synapses were identified but none were between cardiac afferents from the IVSg area and SP-IR terminals.;These results demonstrate that nodose sensory afferent terminals originating in the IVSg and adjacent area terminate primarily in the dmNTS. These afferents are making axo-axonic synapses that are not SP-IR. Furthermore, these IVSg area afferents make axo-dendritic synapses with neurons within the NTS, but these do not involve SP.