Localization of motoneurons innervating the levator veli palatini muscle in the cat

Recent investigations of the nucleus ambiguus (NA) have attempted to identify motoneurons associated with the branchiomeric muscles of the larynx and pharynx. However, relatively little attention has been directed to the levator veli palatini muscle (LVP) which is critical in respiration, deglutition and eustachian tube function. Although the consensus is that cranial nerve X (vagus) innervates this muscle, some investigators have suggested that the LVP is innervated by either cranial nerve VII (facial) or IX (glossopharyngeal). The present study was designed to identify the specific location of LVP motoneurons within the brainstem. Horseradish peroxidase (HRP) was injected into the LVP of 18 cats. Following a survival period of 24–48 hours, animals were sacrificed and tissue processed according to Mesulam's TMB procedure. HRP labeled cells were located in the rostral NA both ipsilateral and contralateral to the side of injection and in the ipsilateral retrofacial nucleus (RFN). There were no labeled cells in the facial nucleus. Innervation of the LVP by cranial nerve VII would thus be excluded. This is the first report to definitively localize LVP motoneurons. Although the innervation of LVP by cranial nerve X is generally agreed upon in basic anatomy textbooks, identification of LVP motoneurons within the NA does not exclude innervation by cranial nerve IX.