Heterogeneity within geniohyoid motor unit subpopulations in firing patterns during breathing

Respiratory motor units (MU) segregate into subpopulations, which differ in firing patterns during resting and stimulated breathing. For phrenic/diaph ragm MUs, diversity also exists within subpopulations, and is greater for l ate than early-onset MUs. The present study characterized the extent of div ersity within upper airway respiratory MU subpopulations by recording genio hyoid MUs in anesthetized cats. Inspiratory MUs (I-MU, n = 21) had a wide r ange of firing durations (coefficient of variation (CV) = 42%). In contrast , inspiratory-expiratory MUs (I/E-MU, n = 19) had a narrow range of firing durations during inspiration (CV = 13%), but a wide range of firing duratio ns during expiration (CV = 36%). Mean firing frequency had similar degrees of diversity among units for I-MU and I/E-MU (CV = 31-40%). For I-MU firing duration correlated with mean firing frequency, whereas no such relationsh ip was apparent for I/E-MU. Single-breath end-expiratory airway occlusion d ecreased heterogeneity in firing duration during inspiration and increased it during expiration, whereas end-inspiratory airway occlusion decreased he terogeneity during expiration. In conclusion, (a) there is considerable div ersity within geniohyoid MU subpopulations receiving respiratory drive; (b) the degree of diversity within subpopulations differs for I-MU and I/E-MU; and (c) diversity within subpopulations in timing of activity is modulated by single-breath airway occlusion. (C) 2000 Elsevier Science B.V. All righ ts reserved.