Inhibition of inspiratory upper airway motoneuron activity by phasic volume feedback

The effects of phasic volume feedback on efferent hypoglossal, recurrent laryngeal and phrenic nerve activity were studied in decerebrate, paralyzed intubated cats ventilated with a phrenic-driven servo-respirator. The gain of the respirator was altered for single inspirations, and the resulting changes in neural activities were quantified by comparison with respective neural activities without phasic volume feedback. The volume thresholds for suppression of hypoglossal and recurrent laryngeal activities were time independent. Above these two thresholds and extending over a substantial range, volume feedback caused graded inhibition of upper airway motoneuron outputs. At any particular time during inspiration the relationships between hypoglossal or recurrent laryngeal inhibition and volume were concave to the volume axis. Rate of airflow appeared to exert an effect on upper airway motoneuron activity independent of volume. These results indicate that for hypoglossal and recurrent laryngeal efferent activity 1) volume feedback can cause a sustained graded inhibition throughout inspiration; 2) the volume thresholds are time independent; and 3) partial inhibition decreases susceptibility to additional inhibition. These actions of volume feedback on upper airway motoneuron output differ from those on phrenic efferent discharge and show that phasic vagal volume feedback has a marked and differential effect on upper airway motoneuron activity. The vagus, in this preparation, appears to play a critical role in the regulation of upper airway motoneuron activity and therefore maintenance of upper airway patency.