SOLITARIAL PREMOTOR NEURON PROJECTIONS TO THE RAT ESOPHAGUS AND PHARYNX - IMPLICATIONS FOR CONTROL OF SWALLOWING

Background & Aims: The buccopharyngeal and esophageal phases of swallo wing are controlled by distinct networks of premotor neurons localized in the nucleus tractus solitarius. The neuronal circuitry coordinatin g the two phases was investigated using a combination of central and p eripheral tracing techniques. Methods: Using pseudorabies virus, a tra nssynaptic tracer, in anesthetized rats, third-order esophageal neuron s (neurons projecting to premotor neurons) were identified. In a separ ate protocol that combined transsynaptic and retrograde fluorescent tr acing, third-order esophageal neurons projecting to pharyngeal motoneu rons (buccopharyngeal premotor neurons) were then identified. Results: Third-order esophageal neurons were identified in the interstitial an d intermediate subnuclei of the nucleus tractus solitarius and in othe r medullary, pontine, midbrain, and forebrain nuclei. A subpopulation of these neurons (double labeled) in the interstitial and intermediate subnuclei were found to project to pharyngeal motoneurons (buccophary ngeal premotor neurons) and to be linked synaptically to esophageal pr emotor neurons. Conclusions: The synaptic link between buccopharyngeal and esophageal premotor neurons provides an anatomic pathway for the central initiation of esophageal peristalsis and its coordination with the pharyngeal phase of swallowing. This neural circuitry within the nucleus tractus solitarius is consistent with a complex central contro l mechanism for the swallowing motor sequence that can function indepe ndently of afferent feedback.