Afferent projections to pharynx and soft palate motoneurons: a light and electron microscopical tracing study in the cat

Pharynx and soft palate are muscles for respiration, vocalization, swallowing, and vomiting. In cat, motoneurons innervating pharynx/soft palate are located in the dorsal group of the nucleus ambiguus (dgNA) in the medulla oblongata. In cat, dgNA is the only part of nucleus ambiguus that can be distinguished as a separate cell group, which makes it possible to study its afferent input. In two cats, WGA-HRP injections in dgNA and surrounding tegmentum resulted in retrogradely labeled cells at several levels of the neuraxis. In 170 cases anterograde tracers were injected in areas in which the cells of origin were identified. Results demonstrate that dgNA afferents originate from the tegmentum dorsolateral to the superior olivary complex, medullary ventromedial tegmentum, caudal raphe nuclei, medullary lateral tegmental field, nucleus retroambiguus (NRA), and adjoining tegmentum, extending into the first cervical segment of the spinal cord. In order to determine whether periaqueductal gray (PAG) and parabrachial nuclei (PB) make synaptic contacts with dgNA, ultrastructural studies combined anterograde tracing from PAG, PB, and NRA with retrograde tracing of pharyngeal and soft palate motoneurons. The results showed that PB, but not PAG, projects to the dgNA and that NRA afferent synapses are three times as numerous as those from PB. The morphology of PB and NRA synapses is consistent with excitatory input. In conclusion, pharyngeal and soft palate motoneurons receive their afferents almost exclusively from the pontine and medullary tegmentum and first cervical spinal segment.