Direct synaptic projections to the myenteric ganglion of the rat subdiaphragmatic esophagus from the dorsal motor nucleus of the vagus

We have examined the ultrastructure of the myenteric ganglion of the subdiaphragmatic esophagus and determined whether the ganglion neurons receive direct projections from the dorsal motor nucleus of the vagus (DMV) using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) as an anterograde tracer. The neurons (22.2 μm × 13.3 μm) of myenteric ganglion in the esophagus contained dark cytoplasm having many free ribosomes, mitochondria, and an oval nucleus, and received only a few axon terminals contacting somata. All axon terminals formed asymmetric synaptic contacts with dendrites or somata. Approximately 85% of the axon terminals contacting dendrites and about 50% of the axon terminals contacting somata contained pleomorphic vesicles, while the rest contained round synaptic vesicles. When WGA-HRP was injected into the DMV, anterogradely labeled fibers and terminals were found in the myenteric ganglia. The WGA-HRP labeled terminals were large (1.97 μm) and contained round clear vesicles and small granular vesicles. These labeled terminals contacted exclusively the small dendrites, but not the somata. These results suggest that the DMV neurons project directly to the myenteric ganglion neurons and regulate the esophageal muscles via the ganglion neurons.