Tonically Discharging Genioglossus Motor Units Show No Evidence of 1 Rate Coding with Hypercapnia

28 The genioglossus (GG) is considered the principle protrudor muscle of the human tongue. 29 Unlike most skeletal muscles, GG electromyographic (EMG) activities are robustly preserved in 30 sleep and thus may fulfill a critical role in preserving airway patency. Previous studies in human 31 subjects also confirm that the GG EMG increases in response to chemoreceptor and 32 mechanoreceptor stimulation. This increase occurs secondary to the recruitment of previously 33 inactive motor units (MUs) and/or an increase in firing rate of already active MUs (Burke 1981). 34 Which strategy the nervous system uses when the synaptic drive onto GG motoneurons increases 35 is not known. Here we report on GG whole muscle and tonic MU activities under conditions that 36 mimic sleep i.e., mild-moderate elevations in CO2 (3% inspired CO2, or the addition of a 1.0 L 37 dead space) and elevated airway resistance. Based upon previous work in rat (John et al. 2005), 38 we hypothesized that mild hypercapnia would increase the firing rates of tonic MUs and that 39 these effects would be further potentiated by a modest increase in airway resistance. Fine wire 40 and tungsten microelectrodes were inserted into the GG to record whole muscle and single MU 41 activities in 21 subjects (13 women, 8 men; 20-55 years). Either 3% inspired CO2 or added dead 42 space resulted in a 200-300% increase in the amplitude of both tonic and phasic components of 43 the whole muscle GG EMG and a doubling of minute ventilation. Despite these changes, 44 recordings obtained from a total of 84 tonically discharging GG single MUs provide no evidence 45