Respiratory-related pharyngeal constrictor muscle activity in awake goats

Respiratory-related electromyogram (EMG) activities of the middle (MPC) and inferior (IPC) pharyngeal constrictor (PC) muscles were determined simultaneously with up to six additional upper airway abductor and adductor muscles in awake adult goats. Phasic PC activation began in late inspiration and persisted throughout expiration with a steady, an augmenting or a biphasic pattern of activity. Considerable differences were noted in the EMG responses of the MPC and IPC muscles to respiratory-related stimuli. During hypoxia and hypercapnia, phasic MPC activity decreased or was not recruited whereas phasic IPC activity was augmented with increased chemical drive. During spontaneous augmented breaths and peripheral chemoreceptor stimulation with sodium cyanide, the pattern of activation of the MPC was similar to that of the thyroarytenoid muscle (TA), a laryngeal adductor whereas IPC activity was strikingly similar to activity of the laryngeal and pharyngeal dilators. The expiratory portion of an augmented breath was associated with increased phasic MPC and TA but not IPC activities. Dopamine-induced apneas resulted in tonic activation of the MPC and TA at a level equal to or greater than control activity but no recruitment of IPC activity. The marked differences in MPC and IPC responses to respiratory-related stimuli suggests that these muscles may have different mechanical effects on pharyngeal airway caliber in the goat. The results suggest that the MPC may help brake expiratory flow thus helping to control expiratory timing and lung volume. In contrast, the IPC may promote pharyngeal airway patency by stiffening or dilating the pharyngeal airway. The results demonstrate that a variety of stimuli can influence respiratory-related PC activity and suggest that the PC muscles are important in the regulation of breathing and upper airway patency.