Do segmental reflexes play a role in the distribution of external intercostal EMG activity in the rabbit?

External intercostal muscle is considered as an inspiratory muscle. Its electromyographic (EMG) activity is exquisitely sensitive to proprioceptive afferents and is predominant in zones with the greatest respiratory effect. In the current series of studies, we assessed the contribution of segmental reflexes to this particular distribution of activity in anesthetized rabbits. We first established a parallel between gradients of activity and mechanical advantage. The sensitivity of external intercostal EMG activity to rib displacement was then assessed in the 3rd and 5th interspaces and in the dorsal, middle and ventral muscle bundles of the 3rd interspace. Finally, persistence of the dorso-ventral gradient was tested after a dorsal rhizotomy (interspaces 3-5). There were no differences in the sensitivity to rib motion in the studied positions. After rhizotomy, the dorso-ventral gradient of activity persisted at rest and with inspiratory resistive loading. We, therefore, conclude that the neuromechanical matching of respiratory drive in the external intercostal muscles is not the result of segmental reflexes.