Ganglionated plexuses (GPs) are major constituents of the intrinsic cardiac nervous system, the final common integrator of regional cardiac control. We hypothesized that nicotinic stimulation of individual GPs exerts divergent regional influences, affecting atrial as well as ventricular functions. In 22 anesthetized canines, unipolar electrograms were recorded from 127 atrial and 127 ventricular epicardial loci during nicotine injection (100 mcg in 0.1 ml) into either the 1) right atrial (RA), 2) dorsal atrial, 3) left atrial, 4) inferior vena cava-inferior left atrial, 5) right ventricular, 6) ventral septal ventricular or 7) cranial medial ventricular (CMV) GP. In addition to sinus and AV nodal function, neural effects on atrial and ventricular repolarization were identified as changes in the area subtended by unipolar recordings under basal conditions and at maximum neurally-induced effects. Animals were studied with intact AV node or following ablation to achieve ventricular rate control. Atrial rate was affected in response to stimulation of all 7 GPs with an incidence of 50-95% of the animals among the different GPs. AV conduction was affected following stimulation of 6/7 GP with an incidence of 22-75% among GPs. Atrial and ventricular repolarization properties were affected by atrial as well as ventricular GP stimulation. Distinct regional patterns of repolarization changes were identified in response to stimulation of individual GPs. RAGP predominantly affected the RA and posterior right ventricular walls whereas CMVGP elicited biatrial and biventricular repolarization changes. Spatially divergent and overlapping cardiac regions are affected in response to nicotinic stimulation of neurons in individual GPs.

Our purpose was to identify and localize intrinsic cardiac ganglia innervating distinct regions of the heart using postmortem tracing of nerve projections with DiI, a method not previously used to study the intrinsic cardiac nervous system. We also investigated the possibility of collateral innervation of myocardium and intrinsic ganglia. In isolated paraformaldehyde-fixed guinea pig hearts, crystals of DiI (1,1′- dioctadecyl-3,3,3′,3′-tetramethylin-docarbocyanine perchlorate) were inserted into the posterior ventricular myocardium below the atrioventricular groove, the right atrium, or the left ventricular septum. Hearts were placed in the dark at 37°C for 2-14 weeks to allow DiI diffusion within neuronal membranes. Labeled neurons were observed in intracardiac ganglia after at least 4 weeks of dye exposure. Labeling was restricted to the inferior-most ganglia (those near the atrioventricular groove) when DiI was inserted into the posterior ventricular myocardium and to ganglia near the sinus node after right atrial DiI placement. Application of DiI to the left ventricular septum resulted in neuron labeling in ganglia primarily in the interatrial septum near the atrioventricular node. After 8 weeks, DiI-labeled nerve fibers and varicosities were seen surrounding unlabeled neurons in some ganglia, suggesting that axons terminating in or passing through the DiI application site in posterior ventricular tissue had collateral branches innervating these ganglia. These results indicate that intrinsic innervation of major cardiac subdivisions is accomplished by regionally segregated cardiac ganglia. Also, tracing with DiI has provided evidence for collateral nerve projections that could be the substrate for novel intracardiac regulatory circuits.

The origin of cardiac preganglionic neurons in the rat was investigated using the retrograde transport of horseradish peroxidase (HRP). A single injection of HRP was made into the right myocardium in either a sinoatrial or mid-ventricular location. Labeled cells were found in the mid- and lower medulla primarily in and around the nucleus ambiguus (NA) 600-1800 μm above the obex. The dorsal motor nucleus of X (DMN) was sparsely labeled and a few cells were found in an intermediate zone near the level of the obex. Labeling was bilateral with slightly heavier labeling found ipsilateral to the injection site than contralateral to it. Following a unilateral vagotomy, labeled cells were only found ipsilateral to the intact vagus. Atrial and midventricular injections yielded similar results. Occasionally only 1- cells in the NA were labeled per section. Inspection of serial sections revealed that in these sparsely labeled rats, the HRP was often in the same location within the NA forming a column of cells within the nucleus. The columns sometimes extended at least 240 μm in the rostral-caudal direction. The columnar organization was most apparent in rats with few labeled cells presumably because it was obscured in nuclei that were heavily labeled. In a second group of rats, the right vagus was cut at the cervical level and dipped in HRP to determine the extent of the NA and DMN in rats. In these animals, heavier labeling was found in the DMN than in the NA. Cells in the DMN were filled from the upper spinal cord to its most rostral extent 1200 μm above the obex. Thus, although the DMN and NA send projections in the vagus nerve, those axons terminating in the myocardium primarily originate in the NA.