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1 Prince of Wales Medical Research Institute and University of New South Wales, Sydney, NSW, Australia
2 Division of Sleep Medicine, Brigham and Women's Hospital, and Harvard Medical School, Boston, Massachusetts, USA
3 Department of Psychology, University of Melbourne, Melbourne, Victoria, Australia
* To whom correspondence should be addressed. E-mail: s.gandevia{at}unsw.edu.au.
A tongue muscle, the genioglossus (GG), is important in maintaining pharyngeal airway patency. Previous recordings of multiunit EMG suggest it is activated during inspiration in humans with some tonic activity in expiration. We recorded from populations of single motor units in genioglossus in 7 subjects during quiet breathing when awake. Ultrasonography assisted electrode placement. The activity of single units was separated into 6 classes based on a step-wise analysis of the discharge pattern. Phasic and tonic activities were analysed statistically with the coefficient of determination (r2) between discharge frequency and lung volume. Of the 110 motor units, 29 % discharged tonically without phasic respiratory modulation (firing rate ~19 Hz). Further, 16 % of units increased their discharge during expiration (Expiratory Phasic and Expiratory Tonic units). Only half the units increased their discharge during inspiration (Inspiratory Phasic and Inspiratory Tonic units). Units firing tonically with an inspiratory increase had significantly higher discharge rates than those units that only fired phasically (peak rates 25 vs 16 Hz respectively). Simultaneous recordings of 2 or 3 motor units showed neighboring units with differing respiratory and tonic drives. Our results provide a classification and the first quantitative measures of human genioglossus motor unit behavior and suggest this activity results from a complex interaction of inspiratory, expiratory, and tonic drives at the hypoglossal motor nucleus. The presence of different drives to GG implies that complex premotor networks can differentially engage human hypoglossal motoneurones during respiration. This is unlike the ordered recruitment of motor units in limb and axial muscles.
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