The central distribution of the afferent and efferent components of the superior laryngeal nerve (SLN), which in the rat is ramified into the three branches of the rostral branch (R.Br), middle branch (M.Br), and caudal branch (C.Br), was examined after application of horseradish peroxidase conjugated with wheat germ agglutinin (HRP-WGA) to the proximal cut end of each branch. In addition, the afferent and efferent neural activities of each branch were recorded to investigate the functional properties. The present study provided several new findings as to the distribution of each branch and the functional properties of the SLN. The following conclusions were drawn: 1) the R.Br, containing only afferent fibers projecting to the ipsilateral lateral region of the nucleus of the solitary tract (NST), extends between slightly below the obex and the region approximately 0.6 mm rostral from the obex, and it corresponds to the interstitial subnucleus of the NST; 2) the M.Br, innervating the cricothyroid muscle, contains only efferent fibers originating ipsilaterally from the motoneurons localized within the ambiguus nucleus (Amb) and in the area ventrolateral to the Amb; and 3) the C.Br, which innervates the inferior pharyngeal constrictor muscle, contains both efferent and afferent fibers. HRP-WGA-labeled cells are distributed within both the Amb and the dorsal motor nucleus of the vagus nerve, ipsilateral to the injection site. Afferent proprioceptive fibers project to the ipsilateral interstitial subnucleus of the NST. The present results provide evidence that each branch of the SLN has distinctive functional properties and contributes to the laryngeal functions.

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The levator veli palatini muscle is innervated by motoneurons of the glossopharyngeal nerve, which are located within the ambiguus nucleus; however, little is known about the afferent fibers of this muscle. A horseradish peroxidase study was conducted in rats following injection into the levator veli palatini muscle branch to reveal the location and the distribution of dendrites of the afferent fibers of the muscle. Terminal labels were densely distributed in the lateral region of the solitary nucleus, which receives afferents of the glossopharyngeal nerve, ipsilateral and contralateral to the injection site. The relationship of the levator veli palatini muscle with respiration was suggested by the localization of labeled terminals at sites where the afferents from the respiratory organs project densely, and by the demonstrated proprioceptive role of the afferent fibers passing through the muscle spindles contained in the levator veli palatini muscle.

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