Effects of hypocapnia on respiratory timing and inspiratory activities of the superior laryngeal, hypoglossal, and phrenic nerves in the vagotomized rat

Effects of acute hypocapnia on respiratory timing (inspiratory and expiratory times (TI, TE) ) and on inspiratory activities of the efferent superior laryngeal (Xs1), hypoglossal (XII), and phrenic (Phr) nerves were studied in artificially ventilated vagotomized, and anesthetized rats. Hyperventilation induced a decrease in respiratory frequency exclusively due to prolongation of TE and resulted in expiratory apnea. Inspiratory activities of three nerves decreased with reduction in CO2 concentration of end-tidal gas (FETCO2), and disappeared simultaneously at a threshold FETCO2 for apnea. The decrease in the peak inspiratory activity by hypocapnia was larger in the XII than in the Phr or Xs1 nerve (XII greater than Phr greater than Xs1). The results suggest that the CO2 stimulus (mainly via a central chemosensor) plays an important role in the process of terminating expiration or of expiratory-inspiratory phase switching and that the responses of the XII or Xs1 motoneurons to variation in CO2 stimulus differ from that of the Phr motoneurons (or of the Phr driving medullary neurons). A possible functional significance of these observations is discussed.