Spontaneous and reflexly evoked activity in pharyngeal, laryngeal, and phrenic motoneurons of cat

Abstract

Experiments were performed on spontaneously breathing cats that were anesthetized with chloralose-urethane in order to study changing excitability in pharyngeal, laryngeal, and phrenic motoneurons. Pharyngeal constrictor motoneurons chiefly were spontaneously active during expiration; the stylopharyngeus and certain motoneurons serving the superior pharyngeal constrictors were inactive during either phase of tidal respiration. While confirming work of previous investigators that feeble glossopharyngeal (epipharyngeal branch) and superior laryngeal nerve stimulation transiently attenuated on-going phrenic nerve activity, such evoked afferent volleys excited laryngeal motoneurons independent of phases of respiration. On-going pharyngeal constrictor nerve activity (expiration) was attenuated for 20–50 msec by superior laryngeal nerve stimulation and for 100–300 msec by glosso-pharyngeal nerve stimulation. Intercostal nerve stimulation evoked occasional pharyngeal constrictor nerve discharges but more commonly attenuated its spontaneous nerve activity. Stylopharyngeus and an unknown portion of the superior pharyngeal constrictor motoneurons were excited by afferent volleys evoked in glossopharyngeal and superior laryngeal nerves. These aforementioned changes in excitability were unaffected by total neuromuscular paralysis while maintaining artificial respiration. It is concluded that certain pharyngeal constrictor motoneurons of the cat are spontaneously active during tidal expiration, are under central respiratory control, and that their intermittence along with those serving the diaphragm and larynx by modest afferent volleys in the ninth and tenth cranial nerves stage the act of swallow or expulsive reflexes, or both.