An analysis of reflex changes in excitability of phrenic,laryngeal, and intercostal motoneurons

An investigation was carried out in anesthetized cats to ascertain whether self-excitation of phrenic motoneurons is a specific or generalized reflex mechanism for motoneurons allied to respiration. Whereas stimulation of only caudal intercostal nerves evoked discharge of phrenic motoneurons (intercostal-to-phrenic reflex), stimulation of all intercostal nerves elicited discharges in the recurrent laryngeal nerve (intercostal-to-recurrent laryngeal reflex). Weak superior laryngeal nerve stimulation provoked short-latency discharges in the recurrent laryngeal nerve but inhibited on-going inspiratory activity in phrenic and external intercostal motoneurons. In the presence of self-excitation of phrenic motoneurons (phrenophrenic system), there was concomitant excitation of laryngeal motoneurons. In contrast, when self-excitation of laryngeal motoneurons occurred (laryngolaryngeal system) there was concomitant inhibition of inspiratory activity (phrenic and external intercostal motoneurons). Paired shocks delivered to superior laryngeal and intercostal nerves while recording from phrenic, recurrent laryngeal, and intercostal nerves failed to reveal convergent interaction. It is concluded that self-excitation is a generalized reflex mechanism for certain motoneurons allied to respiration.