Effects of acute hypocapnia on respiratory timing (inspiratory and expiratory times (Ti, TE)) and on inspiratory activities of the efferent superior laryngeal (Xsl), 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 C02

more »

... ion of end-tidal gas (FETco2), and disappeared simultaneously at a threshold FETCO, for apnea. The decrease in the peak inspiratory activity by hypocapnia was larger in the XII than in the Phr or Xsl nerve (XII>Phr>Xsl). The results suggest that the C02 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 Xsl motoneurons to variation in C02 stimulus differ from that of the Phr motoneurons (or of the Phr driving medullary neurons). A possible functional significance of these observations is discussed.