Response of respiratory motoneurons to release from vagal inhibition in the rabbit.

Long-lasting effects of vagal input on phrenic (Phr) and external intercostal (EI) motoneuronal output were studied on 16 anesthetized rabbits breathing spontaneously or paralysed and ventilated by a phrenic nerve-driven pump. Withholding of ventilation by tracheal occlusion or by switching off the servorespirator maintained for seven breaths evoked a progressive increase of Phr and EI from breath to breath. This effect was more evident in animals ventilated by servo- respirator. The higher was the gain of the pump (volume-to-phrenic signal ratio) before the maneuvre, the bigger was the rate of increase of the tidal phrenic amplitude from breath to breath at all CO2, levels tested. Vagotomy strongly depressed or eliminated this effect. We conclude that with intact vagus nerve the increase of respiratory motoneuronal output was only partialy due to the gradual increase in chemical drive when ventilation was stopped. The character of the response indicates the existence of a long-lasting component of the Breuer-Hering reflex. Splitting the medulla abolished this prolonged response while preserving the inspiratory vagal inhibition indicates that the neuronal pathways crossing the midline of .the medulla are important for the effect.