Brainstem distribution of neurons with efferent projections in the cervical vagus of the dog.

The distribution within the brainstem of cell bodies and efferent fibers projecting in the cervical vagus was studied with retrograde transport of horseradish peroxidase (HRP). Five to eight days after multiple microinjections of HRP into either the cervical vagosympathetic trunk or the nodose ganglion the brainstems and nodose ganglia were perfused and processed by the tetramethyl benzidine method. HRP-positive neurons were found in three brainstem regions: a dorsal cell column comprising the dorsal motor nucleus of the vagus (dmnX), a ventrolateral group in the region of nucleus ambiguus (nA), and scattered cells along a line between these columns. The density of labeled neurons was greatest within dmnX. Axons from cells of the ventrolateral column projected dorsomedially; just ventral to dmnX they turned laterally to exit the medulla in multiple rootlets. Within nA labelled neurons were distributed according to size, with larger cells more medial and smaller ones more lateral. Caudal to nA in nucleus retroambigualis and nucleus dorsalis medialis cell bodies appeared segregated into clusters.