title: Motor unit recruitment in human genioglossus muscle in response to hypercapnia.
abstract: OBJECTIVE
  single motor unit recordings of the genioglossus (GG) muscle indicate that GG motor units have a variety of discharge patterns, including units that have higher discharge rates during inspiration (inspiratory phasic and inspiratory tonic), or expiration (expiratory phasic and expiratory tonic), or do not modify their rate with respiration (tonic). Previous studies have shown that an increase in GG muscle activity is a consequence of increased activity in inspiratory units. However, there are differences between studies as to whether this increase is primarily due to recruitment of new motor units (motor unit recruitment) or to increased discharge rate of already active units (rate coding). Sleep-wake state studies in humans have suggested the former, while hypercapnia experiments in rats have suggested the latter. In this study, we investigated the effect of hypercapnia on GG motor unit activity in humans during wakefulness.
  
  
  METHODS
  sleep research laboratory.
  
  
  METHODS
  sixteen healthy men.
  
  
  RESULTS
  each participant was administered at least 6 trials with P(et)CO(2) being elevated 8.4 (SD = 1.96) mm Hg over 2 min following a 30-s baseline. Subjects were instrumented for GG EMG and respiratory measurements with 4 fine wire electrodes inserted subcutaneously into the muscle. One hundred forty-one motor units were identified during the baseline: 47% were inspiratory modulated, 29% expiratory modulated, and 24% showed no respiratory related modulation. Sixty-two new units were recruited during hypercapnia. The distribution of recruited units was significantly different from the baseline distribution, with 84% being inspiratory modulated (P < 0.001). Neither units active during baseline, nor new units recruited during hypercapnia, increased their discharge rate as P(et)CO(2) increased (P > 0.05 for all comparisons).
  
  
  CONCLUSIONS
  increased GG muscle activity in humans occurs because of recruitment of previously inactive inspiratory modulated units.
authors.affiliation: School of Psychological Science, University of melbourne, Parkville, Australia.
authors.forename: Christian L
authors.initials: CL
authors.lastname: Nicholas
authors.forename: Bei
authors.initials: B
authors.lastname: Bei
authors.forename: Christopher
authors.initials: C
authors.lastname: Worsnop
authors.forename: Atul
authors.initials: A
authors.lastname: Malhotra
authors.forename: Amy S
authors.initials: AS
authors.lastname: Jordan
authors.forename: Julian P
authors.initials: JP
authors.lastname: Saboisky
authors.forename: Julia K M
authors.initials: JK
authors.lastname: Chan
authors.forename: Ella
authors.initials: E
authors.lastname: Duckworth
authors.forename: David P
authors.initials: DP
authors.lastname: White
authors.forename: John
authors.initials: J
authors.lastname: Trinder
mesh_terms: D000328:Adult; D004567:Electrodes, Implanted / Q000706:statistics & numerical data; D004576:Electromyography / Q000379:methods / Q000706:statistics & numerical data; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology*; D007002:Hypoglossal Nerve / Q000503:physiopathology; D008297:Male; D009046:Motor Neurons; D011999:Recruitment, Neurophysiological; D012016:Reference Values; D015656:Respiratory Mechanics; D012132:Respiratory Muscles / Q000503:physiopathology*; D014851:Wakefulness