Influence of lung volume on phrenic, hypoglossal and mylohyoid nerve activities

Abstract

In decerebrate, paralyzed cats, ventilated by a servo-respirator in accordance with phrenic nerve activity, we examined the influence of lung volume on the activities of the phrenic, hypoglossal and mylohyoid nerves. When lung inflation was briefly withheld, the durations of inspiration (TI) and expiration (TE) and the activities of all three nerves increased. The relative increase in hypoglossal activity greatly exceeded that of phrenic activity and was apparent earlier in the course o f inspiration. This hypoglossal response was enhanced by hypercapnia and isocapnic hypoxia. The responses of mylohyoid activity were quite variable: withholding lung inflation augmented insipiratory activity in some cats, but expiratory discharge in others. Sustained increases in end-expiratory lung volume were induced by application of 3–4 cm H₂O of positive end-expiratory pressure (PEEP). Steady-state PEEP did not influence nerve activities or the breathing pattern. Bilateral vagotomy increased TI, TE and the activities of all three nerves. No response to withholding lung inflation could be discerned after vagal section. The results provide further definition of the influence of vagally mediated, lung volume dependent reflexes on the control of upper airway muscles. These reflexes are well suited to relieve or prevent upper airway obstruction.