Axonal projections of medullary swallowing neurons in guinea pigs.

A central pattern generator (CPG) for swallowing in the medulla oblongata generates spatially and temporally coordinated movements of the upper airway and alimentary tract. To reveal the medullary neuronal network of the swallowing CPG, we examined the cytoarchitecture of the swallowing CPG and axonal projections of its individual neurons by extracellular recording and juxtacellular labeling of swallowing-related neurons (SRNs) in the medulla in urethane-anesthetized and paralyzed guinea pigs. Three major types of neuronal discharge patterns were identified during fictive swallowing induced by stimulation of the superior laryngeal nerve: early (burst-like activation during the pharyngeal stage), late (activation after the pharyngeal stage), and inhibited (inhibition during the pharyngeal stage) types. Sixteen neurons were successfully labeled in the nucleus tractus solitarii (NTS) and in the medullary reticular formation (RF). No motoneuron was labeled. The SRNs in the NTS had axons projecting to the NTS, RF, nucleus ambiguus, nucleus hypoglossus, and dorsal motor nucleus of the vagus on the ipsilateral side. Some NTS SRNs projected only within the NTS. The axons of SRNs in the RF projected also to the NTS, RF, motor nuclei on the ipsilateral side, and to the other side RF. These findings show anatomic substrates for the neuronal network of the CPG for swallowing, which consists of complex neuronal connections among SRNs in the NTS, RF, and motor nuclei.