Role of vagal and spinal sensory pathways in diaphragmatic response to resistive loading

The effects of severe inspiratory (I) or expiratory (E) resistive loads on diaphragmatic activity were studied in two groups of cats anaesthetized with sodium pentobarbital or ethylcarbamate-chloralose. In intact cats, I or E loading never changed the amplitude of integrated diaphragmatic electric myogram (EMG) measured at 1.0 s (Edi 1.0); only I loading, prolonged the duration of diaphragmatic activity (Tdi). After selective procaine block of non-volume related vagal sensory inputs, I or E loading markedly increased Edi 1.0 and changes in Tdi due to I loading persisted. After bivagotomy, which also suppressed volume related vagal feed back, Edi 1.0 increased during I or E loading but change in Tdi disappeared. Initial spinal section at C8 level only reduced changes in Tdi with inspiratory loading. Bivagotomy plus spinal section abolished all load induced changes in diaphragmatic activity. These results suggest that all vagal information from the lungs participate in the mechanism of load compensation but that spinal sensory pathways play a minor role in this response in anaesthetized cats.