Projections of the carotid sinus nerve to the medulla in the dog.

The afferent and efferent projections of the carotid sinus nerve were examined within the medulla of the dog with axonal transport of horseradish peroxidase (HRP), and compared with the projections of the glossopharyngeal nerve. The carotid sinus nerve was identified electrophysiologically prior to injection of tracer. Carotid sinus nerve afferent fibers entered the medulla as part of the glossopharyngeal nerve root near the caudal limits of the cochlear nuclei. Labeled axons entered the solitary tract and ran caudally to about 3 mm anterior to the obex, where they began to enter the nucleus tractus solitarii (nTS). Carotid sinus afferent fibers and presumptive terminals were discretely localized within limited portions of the ipsilateral dorsal, medial, and lateral nTS as far as 3 mm caudal to the obex. A few fibers entered the dorsolateral area postrema ipsilateral to HRP injection. Labeled fibers in the commissural nTS crossed the midline and entered the contralateral medial nTS. Efferent neurons were observed only in half of the cases, and were limited to one to three labeled perikarya in the periphery of the retrofacial nucleus. Comparison of the carotid sinus distribution with the previously described vagal afferent projections to the canine nTS revealed partially overlapping, but clearly distinctive patterns, which support a viscerotopic organization of the nTS.