The dorsal motor nucleus of the vagus nerve of the hamster: ultrastructure of vagal neurons and their responses to vagotomy.

The dorsal motor nucleus of the vagus nerve of the hamster was studied electron microscopically. Two types of neuron distinguished by their ultrastructural features were identified. Type I cells, which had an average dimension of 20 X 12 microns, were rich in cytoplasmic organelles. The smaller Type II cells (15 X 9 microns) had scanty cytoplasm and a deeply indented nucleus. Type I cells were labelled by horseradish peroxidase (HRP) following its injection into the vagus nerve, whereas Type II cells were not. Seven days after cervical vagotomy, Type I neurons underwent typical retrograde degeneration. By the fourteenth postoperative day, the density of the cells was greatly enhanced, an early sign of cell death. Type II cells were not affected by vagotomy. With the electron microscope, three types of axon terminals were identified in the neuropil of the DMN: (1) boutons containing round agranular vesicles, (2) boutons containing round and elongated dense-cored vesicles, and (3) boutons containing pleomorphic agranular vesicles, a few of which were dense-cored. All of them showed synaptic contacts with HRP-labelled dendrites. Occasional boutons containing pleomorphic vesicles showed degenerative changes after vagotomy. Moreover, a few of them were seen to contain HRP granules. The observations made in the present study strongly suggest that the Type I neurons send efferent fibres to the vagus nerve. The Type II cells which are not labelled by HRP nor affected by vagotomy, probably serve as interneurons, or they may be cells projecting to other areas of the brain.