Discharges of bulbar respiratory neurons during rhythmic straining evoked by activation of pelvic afferent fibers in dogs.

Each cycle of rhythmic straining evoked through the reflex center in the Kölliker-Fuse nucleus by stimulation of pelvic afferents in decerebrate dogs usually began in early expiration. During the rhythmic straining cycle, postinspiratory discharges of the phrenic nerve increased simultaneously with a burst of discharges of the nerves innervating the rectus abdominis and adductors of the glottis. While about half of the bulbar expiratory units discharged concurrently with the rhythmic straining, almost none of the inspiratory units examined did so. Nearly all expiratory bulbospinal units discharged concurrently, but none of the inspiratory bulbospinal units did so. These results show that expiratory neurons in the caudal bulb relay commands for rhythmic straining from the pontine reflex center to motor neurons of expiratory muscles, but that bulbar inspiratory neurons do not relay the commands to inspiratory motor neurons. Discharges concurrent with rhythmic straining were also evoked in all 4 postinspiratory units of the ventral group, 3 very early onset expiratory units and all 9 inspiratory-expiratory units of the dorsal group. Possible roles played by these respiratory neurons in the organization of rhythmic straining were discussed.