Responses of Hypoglossal and Phrenic Nerves to Decreased Respiratory Drive in Cats

Publisher Website
Google Scholar
Abstract
Agents which depress respiration, such as alcohol, seem to increase the occurrence of obstructive apneas during sleep. It has been proposed that upper airway obstruction can result from an imbalance in the activity (or forces) produced by the upper airway muscles versus the chest wall muscles so that upper airway passages might be blocked when a disproportionate decrease in upper airway muscle activity occurs. This study examines the hypothesis that depression of respiration affects the activity of the hypoglossal nerve (the motor nerve to the tongue) more than the activity of the phrenic nerve (the motor nerve to the diaphragm). In addition, we examined the role of the putative central chemoreceptor area on the ventrolateral medullary surface (VMS) in maintaining phrenic and hypoglossal discharge. In chloralose-anesthetized, artificially ventilated, paralyzed cats, three methods of reducing respiratory drive were studied: hyperoxic hypocapnia (produced by mechanical hyperventilation), the application to the intermediate area of the ventral medullary surface of the respiratory depressant GABA and its agonist muscimol, and cooling the same area of the VMS (using a water-cooled thermode). All these interventions decreased hypoglossal nerve activity more than phrenic nerve activity (range of p values: p < 0.001 to p < 0.01). Moreover, the reduction in hypoglossal activity was greater with GABA and muscimol than with the other two maneuvers; this was statistically significant for both GABA versus VMS cooling (p < 0.02) and muscimol versus VMS cooling (p < 0.01). These results show that respiratory depression can differentially affect hypoglossal and phrenic nerve activity. In addition, they suggest that structures near the VMS are influential in maintaining balanced activity of upper airway and chest wall muscles.