Ultrastructural circuitry of cardiorespiratory reflexes: there is a monosynaptic path between the nucleus of the solitary tract and vagal preganglionic motoneurons controlling atrioventricular conduction in the cat.

We have tested the hypothesis: (1) that presumptive negative dromotropic vagal preganglionic neurons in the ventrolateral nucleus ambiguus (NA-VL) can be selectively labelled from the heart, by injecting one of two fluorescent tracers into the two intracardiac ganglia which independently control sino-atrial (SA) rate or atrioventricular (AV) conduction; i.e., the SA and AV ganglia, respectively. The NA-VL was examined for the presence of single and/or double labelled cells. Over 91% of vagal preganglionic neurons in the NA-VL projecting to either intracardiac ganglion did not project to the second ganglion. Consequently, we also tested the hypothesis: (2) that there is a monosynaptic connection between neurons of the medial, and/or dorsolateral nucleus of the solitary tract (NTS), rostral to obex, and negative dromotropic neurons in the NA-VL. An anterograde tracer was injected into the NTS, and a retrograde tracer into the AV ganglion. The anterograde marker was found in both myelinated and unmyelinated axons in the NA-VL, as well as in nerve terminals. Axo-somatic and axo-dendritic synapses were detected between terminals labelled from the NTS, and retrogradely labelled negative dromotropic neurons in the NA-VL. This is the first ultrastructural demonstration of a monosynaptic pathway between neurons in the NTS and functionally associated (negative dromotropic) cardioinhibitory neurons. The data are consistent with the hypothesis that the neuroanatomical circuitry mediating the vagal baroreflex control of AV conduction may be composed of as few as four neurons in series, although interneurons may also be interposed within the NTS.