Interaction of upper airway reflexes on protective laryngeal closure

Reflex responses evoked in the adductor branches of recurrent laryngeal nerve (RLN) by stimulation of the afferent nerves in the upper respiratory tracty were studied on anesthetized cats for the purpose of elucidating the mechanisms of the protective laryngeal closure reflex. A single pulse stimulation to each of the infraorbital branch of the trigeminal nerve (ION), the glossopharyngeal nerve (GPN) and the superior laryngeal nerve (SLN) elicited reflex responses in RLNs on both sides. The frequency-dependent attenuation of reflex responses in right RLN elicited by repetitive stimulation of ipsilateral SLN began at frequencies of 40-50/sec; on the other hand, that elicited by repetitive stimulation of ipsilateral GPN or ION was attenuated at frequencies of 5-10/sec. Furthermore, interactions among the reflexes in right RLN elicited by stimulation of ipsilateral ION, GPN and SLN were observed by using the technique of double shock stimulation (the "conditioning-testing" technique). The effect of conditioning stimulation of ION was a short facilitation followed by a long-lasting inhibition of RLN reflex responses elicited by test stimulation of SLN and GPN. conditioning stimulation of GPN elicited a similar effect on RLN reflex responses evoked by test stimulation of SLN. These results indicate an important role of the interactions among upper airway afferent activities for the performance of laryngeal defense action.