Effect of pulmonary stretch receptor feedback and CO2 on upper airway and respiratory pump muscle activity in the rat

1.Our purpose was to examine the effects of chemoreceptor stimulation and l ung inflation on neural drive to tongue protrudor and retractor muscles in the rat. 2. Inspiratory flow, tidal volume, transpulmonary pressure, compliance and electromyographic (EMG) activity of genioglossus (GG), hyoglossus (HG) and inspiratory intercostal (IIC) muscles were studied in 11 anaesthetized, tra cheotomized and spontaneously breathing rats. Mean EMG activity during insp iration was compared with mean EMG- activity during an occluded inspiration , at each of five levels of inspired CO2 (0, 3, 6, 9 and 12%). 3. Lung inflation suppressed EMG: activity in all muscles, with the effect on both tongue muscles exceeding that of the intercostal muscles. Static el evations of end-expiratory lung volume evoked by 2 cmH(2)O positive end-exp iratory pressure (PEEP) had no effect on tongue muscle activity. 4. Despite increasing inspiratory flow, tidal volume and transpulmonary pre ssure, the inhibition of tongue muscle activity by lung inflation diminishe d as arterial P-CO2 (P-a,(CO2)) increased. 5. The onset of tongue muscle activity relative to the onset of IIC muscle activity advanced with increases in P-a,(CO2), but was unaffected by lung i nflation. This suggests that hypoglossal and external intercostal motoneuro n pools are controlled by different circuits or have different sensitivitie s to CO2, lung inflation and/or anaesthetic agents. 6. We conclude that hypoglossal motoneuronal activity is more strongly infl uenced by chemoreceptor-mediated facilitation than by lung volume-mediated inhibition. Hypoglossal motoneurons driving tongue protrudor and retractor muscles respond identically to these stimuli.