Vagus nerve and spinal cord projecting neurons demonstrated by horseradish peroxidase and different fluorescent dyes

Vagal projecting (VP) neurons were localized by intraneural injections of fluorescent dyes or cholera toxin conjugated horseradish peroxidase (CT-HRP) or by intraperitoneal injection of fluorescent dyes. Spinal projecting (SP) neurons were localized by injecting CT-HRP or contrasting dyes into the C4/C5 cord segments. No doubly labelled neurons were seen in the three nuclei known to project to both vagus nerve and spinal cord, viz., dorsal nucleus of the vagus (DNV), nucleus ambiguous complex (NAc) and the intermediate region (NI) between DNV and NAc. VP and SP neurons intermingled in the caudal parts of the NAc and DNV. In the middle part of the NAc, VP neurons congregated mostly dorsal to the SP neurons. In the rostral extremity of the NAc, SP neurons were rarely encountered. No SP neurons were seen in the rostral end of the DNV. In contradistinction to the few VP neurons in the NI, there were many SP neurons in this region. The ratios of VP to SP neurons in DNV were on the average 20 to 1 and those of VP to SP neurons in the NAc, 1.35 to 1.