A light and electron microscopic study of the dorsal motor nucleus of the vagus nerve in the cat.

The morphology of the dorsal motor nucleus of the vagus nerve (DMV) in the cat was studied with the aid of light and electron microscopy. In frozen sections stained by the Kluver-Barrera method or stained to show retrograde labeling in the DMV following injections of horseradish peroxidase (HRP) in the cervical vagus nerve and the stomach wall a range of sizes of DMV neurons was observed but it was observed but it was not possible to distinguish separate types. In contrast, two distinct types of neurons, one medium-sized and the other small, were identified with the light microscope in Golgi-Cox and 1-micrometer Epon sections and with the electron microscope in ultrathin sections. The medium-sized neurons had a range of sizes but generally measured 18 X 25 micrometers and possessed three to four proximal dendrites which branched two or three times. Spines were observed occasionally on the soma and on dendrites. These neurons contained a well-developed cytoplasm and a noninvaginated round to oval nucleus. The small neurons generally measured 9 X 14 micrometers and were round or slightly elongated in shape. Their dendritic processes were fewer and thinner than those of the medium-sized neurons and extended for shorter lengths. Their soma contained scanty cytoplasm and an invaginated nucleus. The medium-sized neurons outnumbered the small neurons by more than three to one but both neuronal types were distributed evenly throughout the nucleus. The medium-sized neurons seemed to correspond in size to the parasympathetic efferent neurons of the viscera as indicated by the HRP studies. Axosomatic synapses on both types of neurons and axodendritic synapses were observed in the DMV. Terminals containing mainly small clear round vesicles and making asymmetrical contract with the postsynaptic membrane were involved in the majority of synapses on both the soma and dendrites. Terminals containing predominantly pleomorphic vesicles and making symmetrical contact with the postsynaptic membrane were also common, comprising up to one-third of all synapses observed. Serial sections revealed that most synaptic terminals contained varying numbers of large (75--110 nm) dense-cored vesicles. Smaller dense-cored vesicles (45--55 nm) were sometimes observed, often close to the area of synaptic contact. Terminals 1--2 micrometers in diameter which contacted dendrites 1--3 micrometers in diameter formed the most common synaptic combination throughout the rostral to caudal extent of the DMV. No distinct regional differences were observed with respect to distribution of synaptic types.