MRI study of pharyngeal airway changes during stimulation of the hypoglossal nerve branches in rats.

The medial branch (Med) of the hypoglossal nerve innervates the tongue protrudor muscles, whereas the lateral branch (Lat) innervates tongue retractor muscles. Our previous finding that pharyngeal airflow increased during either selective Med stimulation or whole hypoglossal nerve (WHL) stimulation (coactivation of protrudor and retractor muscles) led us to examine how WHL, Med, or Lat stimulation affected tongue movements and nasopharyngeal (NP) and oropharyngeal (OP) airway volume. Electrical stimulation of either WHL, Med, or Lat nerves was performed in anesthetized, tracheotomized rats while magnetic resonance images of the NP and OP were acquired (slice thickness 0.5 mm, in-plane resolution 0.25 mm). NP and OP volume was greater during WHL and Med stimulation vs. no stimulation (P < 0.05). Ventral tongue depression (measured in the midsagittal images) and OP volume were greater during Med stimulation than during WHL stimulation (P < 0.05). Lat stimulation did not alter NP volume (P = 0.39). Our finding that either WHL or Med stimulation dilates the NP and OP airways sheds new light on the control of pharyngeal airway caliber by extrinsic tongue muscles and may lead to new treatments for patients with obstructive sleep apnea.