Distribution of motor activity to expiratory muscles during sciatic nerve stimulation in the dog

The present study examined the effect of increasing sensory input from the lower limbs (assessed from the response to electrical stimulation of the sciatic nerve) on the distribution of electrical activity to the expiratory muscles. Expiratory muscle response to sciatic nerve stimulation (SNS) was compared to the response of the inspiratory muscles, and to their response to hypercapnia. In 16 anesthetized dogs afferent SNS increased ventilation and augmented the integrated EMG of all six expiratory muscles studied. Increases in abdominal muscle electrical activity were not uniform, being greater in the transversus abdominis and external oblique as compared to the internal oblique and the rectus abdominis. Increases in thoracic expiratory and inspiratory muscle activity during SNS were similar in magnitude. SNS performed while dogs were breathing 7% CO2, produced increased neural activity similar to those observed during O2 breathing. During CO2 rebreathing, at equal levels of minute ventilation, expiratory muscle responses to SNS and to CO2 were similar. In contrast, the rate of rise of the inspiratory muscle EMGs was greater during SNS. The present study indicates that the abdominal muscles participate in the respiratory response to afferent neural drive from skeletal muscles. The magnitude of their response is independent of their pre-stimulation level of activity and is similar to that observed during CO2 stimulated breathing.