Response of the inspiratory intercostal [correction of intercoastal] muscles to increased inertial loads.

To test the hypothesis that the external intercostals and levator costae constitute an inspiratory reserve system, we have examined the response of these muscles to increased inertial loads. Weights were t hus attached sequentially to the ribs in ten lightly anesthetized, spontaneously breathing dogs. As weights were attached, the ribs were progressively displaced caudally at end-expiration, so that the external intercostal muscles were lengthened. In addition, the cranial motion of the ribs during inspiration was gradually reduced, the inspiratory shortening of external intercostal disappeared, and the external intercostal and levator costae inspiratory EMG activities increased. The parasternal intercostal inspiratory activity, however, remained unchanged. Studies also showed that: (1) the increases in external intercostal activity appeared with the first loaded breath and disappeared as soon as the load was removed; (2) these increases were related to the suppression of the inspiratory muscle shortening, rather than to the increase in precontraction muscle length or to vagal inputs; and (3) denervation of the external intercostal caused inspiratory muscle lengthening but had little effect on the inspiratory motion of the ribs. These observations thus indicate that increased inertial loads on the ribs trigger reflexes, possibly spindle reflexes, which cause selective increases in external intercostal and levator costae inspiratory EMG activities. In that sense, the present findings are consistent with the idea that these two muscles constitute an inspiratory reserve system. However, it appears that the major effect of these increased activities is simply to prevent the muscles from lengthening during inspiration.