Primary afferent projections from the upper respiratory tract in the muskrat

The central projections of the ethmoidal, glossopharyngeal, and superior laryngeal nerves were determined in the muskrat by use of the transganglionic transport of a mixture of horseradish peroxidase (HRP) and wheat germ agglutinin (WGA)‐HRP. The ethmoidal nerve projected to discrete areas in all subdivisions of the ipsilateral trigeminal sensory complex. Reaction product was focused in ventromedial portions of the principal nucleus, subnucleus oralis, and subnucleus interpolaris. The subnucleus oralis also contained sparse reaction product in its dorsomedial part. Projections were dense to ventrolateral parts of laminae I and II of the rostral medullary dorsal horn, with sparser projections to lamina V. Label in laminae I and V extended into the cervical dorsal horn. A few labeled fibers were followed to the contralateral dorsal horn. The interstitial neuropil of the ventral paratrigeminal nucleus was densely labeled. Extratrigeminal primary afferent projections in ethmoidal nerve cases involved the Kölliker‐Fuse nucleus and ventrolateral part of the parabrachial nucleus, the reticular formation surrounding the rostral ambiguus complex, and the dorsal reticular formation of the closed medulla. Retrograde labeling in the brain was observed in only the mesencephalic trigeminal nucleus in these cases. The cervical trunk of the glossopharyngeal and superior laryngeal nerves also projected to the trigeminal sensory complex, but almost exclusively to its caudal parts. These nerves terminated in the dorsal and ventral paratrigeminal nuclei as well as lamina I of the medullary and cervical dorsal horns. Lamina V received sparse projections. The glossopharyngeal and superior laryngeal nerves projected to the ipsilateral solitary complex at all levels extending from the caudal facial nucleus to the cervical spinal cord. At the level of the obex, these nerves projected densely to ipsilateral areas ventral and ventromedial to the solitary tract. Additional ipsilateral projections were observed along the dorsolateral border of the solitary complex. Near the obex and caudally, the commissural area was labeled bilaterally. Labeled fibers from the solitary tract projected into the caudal reticular formation bilaterally, especially when the cervical trunk of the glossopharyngeal nerve received tracer. Labeled fibers descending further in the solitary tract gradually shifted toward the base of the cervical dorsal horn. The labeled fibers left the solitary tract and entered the spinal trigeminal tract at these levels. Retrogradely labeled cells were observed in the ambiguus complex, especially rostrally, and in the rostral dorsal vagal nucleus after application of HRP and WGA‐HRP to either the glossopharyngeal or superior laryngeal nerves. In glossopharyngeal nerve cases, retrogradely labeled neurons also were seen in the inferior salivatory nucleus. These results suggest convergence of primary afferent fibers innervating the upper respiratory tract of the muskrat into the paratrigeminal nuclei and lamina I of the medullary dorsal horn. These data show that the cervical branch of the glossopharyngeal nerve and the superior laryngeal nerve have robust projections into the solitary complex but the ethmoidal nerve has no such projections.