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 lung 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, tracheotomized and spontaneously breathing rats. Mean EMG activity during inspiration 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 muscle exceeding that of the intercostal muscles. Static elevations of end-expiratory lung volume evoked by 2 cmH₂O positive end-expiratory pressure (PEEP) had no effect on tongue muscle activity. 4. Despite increasing inspiratory flow, tidal volume and transpulmonary pressure, the inhibition of tongue muscle activity by lung inflation diminished 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 inflation. This suggests that hypoglossal and external intercostal motoneuron pools are controlled by different circuits or have different sensitivities to CO₂, lung inflation and/or anaesthetic agents. 6. We conclude that hypoglossal motoneuronal activity is more strongly influenced by chemoreceptor-mediated facilitation than by lung volume-mediated inhibition. Hypoglossal motoneurons driving tongue protrudor and retractor muscles respond identically to these stimuli.