Cardiac vagal preganglionic neurones are found in two locations in the medulla oblongata. The neurones of the ventral group near the nucleus ambiguus (NA) have a discharge pattern which reflects strong respiratory and baroreceptor inputs. In contrast the neuronal discharge of the dorsal group near the dorsal vagal motor nucleus (DVMN) is not modulated by either of these inputs. Electrophysiological analysis of these groups has shown that the DVMN group possesses C-fibre axons (conduction velocity, < 2 m s-1) whilst the NA group has B-fibre axons (conduction velocity, 10-30 m s-1). Therefore, selective stimulation of preganglionic inputs to cardiac ganglia is possible using anodal block techniques. The results of these kinds of experiments have shown that both populations have similar functions (related to cardiac chronotropy, dromotropy and inotropy), although the magnitude and time course of the effects differ substantially. Anterograde tracing experiments from brainstem to heart have confirmed these striking differences, as the potent NA group of neurones has a greater divergence of innervation. Both populations project to similar clusters of ganglion cells on the atrial epicardium suggesting functional overlap. On-going levels of sinus arrhythmia produced by vagus activity may depend on the recruitment and relative combination of these two populations of neurones. Much work remains to be done to investigate preganglionic convergence from neurones of the NA and the DVMN to their postganglionic targets on the heart.

Experimental Physiology (2001) 86.6, 797-801.