Effects of a midsagittal lesion of the rabbit medulla. I. Respiratory motoneuronal outputs.

The effects of a midsagittal lesion of the medulla on integrated efferent activities of the phrenic (Phr) and vagus nerves (Eff. Vag.) and on external intercostal EMG (EI EMG) activity were studied in 23 rabbits, anaesthetized with chloralose and urethane. To support depressed respiratory movements, controlled ventilation was applied after the lesion. No muscle relaxant was administered. Bilateral recordings of respiratory activities showed that the lesion produced a desynchronization of inspiratory volleys recorded from the left and right Phr and EI EMG. Following the lesion, the phasic activities of EI EMG and Eff. Vag. were reduced more than that of Phr. Both EI EMG and Phr activities were more sensitive to changes in lung volume than in intact animals. We suggest that the lesion raises the motoneuronal threshold for inspiratory firing differentially for Phr, EI EMG and Eff. Vag. activities. Midsagittal lesion of the medulla enhances spinal reflexes.