Projections from buffer nerves to the nucleus of the solitary tract: an anatomical and electrophysiological study in the cat.

The projections of aorta depressor (ADN) and carotid sinus (CSN) afferent fibers to the region of the nucleus of the solitary tract were studied in the cat with the anterograde transport of horseradish peroxidase (HRP) technique and by recording single unit activity during electrical stimulation of these nerves. In the first series of experiments, after application of crystalline HRP to the proximal cut end of either buffer nerve and a postoperative survival period of 24-120 h, brain stem sections were processed according to the tetramethyl benzidine method. ADN and CSN labeling were found bilaterally, with a predominant ipsilateral labeling, in the medial (Sm), lateral (Slt), commissural (Com) and dorsomedial aspect of the parvocellular solitary nuclei. Additional CSN labeling was found in the ventrolateral and intermediate (Int) solitary nuclei, in the reticular formation ventrolateral to the solitary complex and along the dorsal border of the dorsal motor nucleus of the vagus. In the second series of experiments these areas receiving primary afferent fibers were explored for single units responding to stimulation for the buffer nerves in chloralosed cats, paralyzed and artificially ventilated. Of 177 responsive units, 80 responded only to stimulation of the CSN, 44 only to the ADN and 53 to both nerves. Responsive units were found throughout the rostrocaudal extent of the solitary complex and areas adjacent to the solitary complex. However, most of the units were found in 3 regions: the Sm, Slt and adjacent areas. Units in the Slt and Com were found to respond to only one input, either the CSN or the ADN alone. On the other hand, units in the Int responded only to both buffer nerves and not selectively to one nerve. These results demonstrate that the CSN has a wider distribution in the solitary complex than the ADN and that second order neurons in the solitary complex receive inputs from either one or both buffer nerves, suggesting a degree of separation of central pathways carrying cardiovascular afferent information.