Brain stem genesis of automatic ventilatory patterns independent of spinal mechanisms

Efferent activities of the phrenic and recurrent laryngeal (RLN) nerves were monitored during eupnea, apneusis, and gasping in decerebrate, paralyzed, and ventilated cats before and after spinal cord transection at the first cervical level. The vagi were sectioned caudal to the RLN being studied and at the midcervical level contralaterally. Before spinal transection, the onset of RLN inspiratory activity preceded that of the phrenic nerve during eupnea and apneusis; in gasping, phrenic activity began before the RLN. These results were the same in normocapnia, hypercapnia, and hypoxia. After spinal transection, no phasic phrenic activity was observed at normoxia or hyperoxia, whereas the RLN exhibited discharge patterns similar to those before transection. Upon end-tidal O₂ partial pressure diminutions below 50 Torr, one or more 'bursts' of phrenic activity were recorded; these bursts were not synchronized with the phasic RLN discharge. It is concluded that automatic activity may be generated by inherent brain stem mechanisms. These results further imply that processes underlying gasping neurogenesis may differ fundamentally from those of eupnea or apneusis.