Effect of chemical stimuli on nerves supplying upper airway muscles.

Studies of upper airway resistance suggest that the activity of cranial nerves supplying upper airway muscles changes with chemical drive and that imbalances in the activation of these nerves as compared to the phrenic play a role in causing upper airway obstruction. We assessed the effect of hypoxia and hypercapnia on the activity of the hypoglossal nerve, the recurrent laryngeal nerve, and phrenic nerve in paralyzed anesthetized artificially ventilated dogs. Comparison of hypoglossal and phrenic nerves were also repeated after vagotomy. Both hypoglossal and recurrent laryngeal nerves exhibited increased activity with inspiration. Hypoxia and hypercapnia increased phrenic nerve activity as well as the activity of the two cranial nerves. While linear increases occurred in phrenic and recurrent laryngeal nerve activity with both chemical stimuli, the relationship between hypoglossal and phrenic nerve activity was curvilinear. At lower levels of chemical drive, changes in hypoglossal nerve were less than in the phrenic, and the reverse was true at higher levels of chemical stimulation. There were also differences in the response of both cranial nerves and the phrenic to changing vagal stimulation. The dissimilarities observed in the cranial response of the nerves (versus the phrenic) could potentially affect the forces developed during inspiration and lead to obstruction in the upper airway.