Hypercapnia and resistance modulate human genioglossus motor unit activities

Abstract

The genioglossus (GG) is considered the principle protrudor muscle of the human tongue and plays an important role in preserving airway patency during sleep. Here we report on GG whole muscle and single motor unit activities (SMU) under conditions that mimic sleep i.e., mild‐moderate elevations in CO2 (3% inspired CO2, or 1.0 L dead space) or increased airway resistance (< 10cm H2O). Fine wire and tungsten microelectrodes were inserted into the GG to record whole muscle and SMU activities in 21 subjects (18 women, 10 men; 20–55 years). The addition of 3% inspired CO2 or dead space resulted in a 200–300% increase in whole muscle GG EMG and a doubling of minute ventilation. Importantly, recordings obtained from 102 tonically discharging SMUs show no evidence of change in firing rate under any of the conditions. By comparison, phasically active SMUs (N= 10) switched their discharge pattern from inspiratory‐related to phase‐spanning in response to increased airway resistance but were not otherwise modulated by increasing respiratory drive. On this basis we conclude that, in healthy adults, the increase in the amplitude of the whole muscle GG EMG secondary to respiratory stimulation is attributable to alterations in the firing patterns of phasic motor units and to the recruitment of previously inactive tonic motor units