Effects of a midsagittal lesion of the rabbit medulla. II. Vagal modulation of respiratory activity.

Vagal reflexes were studied in 12 anaesthetized rabbits after a midsagittal lesion of the medulla oblongata. The rabbits were either breathing spontaneously or were ventilated by a phrenic nerve-controlled servorespirator without the use of muscle relaxants. After splitting the medulla, asynchronous respiratory activity of the left and right phrenic nerves was frequently seen. The vagal reflexes evoked by lung inflation and deflation, cigarette smoke and respiratory loading persisted after the lesion. After unilateral vagotomy, all reflexes were transmitted ipsilaterally to the applied stimulus. On the contralateral side, only reinforcement of inspiration was observed when both phrenic nerves fired in phase. In comparison with the nonlesioned rabbits, vagal input in the split-brainstem preparation has a stronger excitatory effect on inspiratory generation. The existence of self-excitatory neuronal connections in spinal motoneuronal pools is also suggested.