The origins of innervation of the canine caudal pharyngeal muscles: an HRP study.

In deglutition, movements of the tongue and oropharynx direct a bolus to the laryngopharynx. The major muscles of this region, which includes the 'cricopharyngeal sphincter', must undergo sequential relaxation and contraction for correct swallowing action. The innervation of the caudal pharyngeal muscles involved in this action in the dog have not been determined previously by sensitive neuroanatomical techniques. In this study, the location of efferent and afferent neurons innervating the left cricopharyngeus, thyropharyngeus and hyopharyngeus muscles was determined by horseradish peroxidase (HRP) histochemistry in 7 puppies. Labeled cells were found ipsilaterally in the supraspinal nucleus, nucleus ambiguus (including nucleus retrofacialis) and nucleus intercalatus of all animals, in the parasympathetic nucleus of X (dorsal vagal efferent nucleus) of 6 animals, and in the hypoglossal nucleus of 4 animals. Small numbers of HRP-labeled cells were found contralaterally in the supraspinal nucleus of all animals, and in the rostral nucleus ambiguus, in the nucleus intercalatus and the parasympathetic nucleus of X of fewer animals. This defines a more extensive source of efferent neurons for these muscles than had been reported for the cat. Labeled postganglionic sympathetic neurons were found bilaterally in the cranial (superior) cervical, middle cervical and cervicothoracic (stellate) ganglia. Labeled afferent neurons were seen bilaterally in the proximal vagal (jugular) and distal vagal (nodose) ganglia and in the C1-C4 spinal ganglia. The location of sympathetic and sensory nerve cell bodies of the muscles of the laryngopharynx has not been previously reported.