Vagal cardiac preganglionic neurons: distribution, cell types, and reflex discharges

In chloralose- and urethan-anesthetized rats, the cardiac branch (CB) of the vagus nerve was electrically stimulated, and antidromic responses of medullary cells were recorded. The cells identified as the vagal cardiac preganglionic neurons (VCPN) were localized in the dorsal motor nucleus (ND group, 8 cells), a region in and around the nucleus ambiguus (NA group, 7 cells) and an intermediary zone (IM group, 2 cells) lying in between. Latencies of the antidromic responses were distinctly different among the three groups, and calculated conduction velocities indicated that the VCPN of the ND group possess C-fiber axons whereas those of the NA group and probably of the IM group, B-fiber axons. In another series of experiments, the right carotid sinus nerve (CSN) or the left cervical vagus nerve was stimulated, and efferent fiber group(s) mediating reflexly evoked discharges to the CB was determined by means of two-point recordings. Among reflex discharges evoked by stimulation of the CSN the shortest latency reflex was proved to be mediated by B-efferent fibers. In contrast, among reflex discharges evoked by stimulation of the vagus nerve, the greatest reflex component was found to be conveyed by C-efferent fibers. It was concluded that the VCPN consist of two types of cells, each located in a different region of the medulla oblongata and contributing to vagal cardiac reflex mechanisms in a different manner.