Denervation of the parasternal intercostal muscles in the dog is known to cause a substantial reduction in the inspiratory cranial displacement of the ribs and a compensatory increase in the activation of the other inspiratory intercostal muscles, namely the external intercostals and the levator costae. The present studies were designed to assess the mechanism(s) of that compensation. 2. Denervating the parasternal intercostals bilaterally caused a reduction in tidal volume and an increase in arterial PCO2 (Pa, CO2). Severing the parasternal intercostals selectively produced similar changes. The concomitant increases in external intercostal and levator costae activity, however, were much greater than predicted on the basis of the increased Pa, CO2. 3. Denervating the parasternal intercostals on one side of the chest produced large increases in ipsilateral, but not contralateral external intercostal activity. 4. Manipulating the ribs after the parasternal intercostals were inactivated so as to reproduce the normal inspiratory cranial displacement of the ribs elicited immediate, clear-cut reductions in external intercostal and levator costae activities. 5. The increases in external intercostal and levator costae activities that occur after inactivation of the parasternal intercostals thus result partly from the increased hypercapnic drive but mostly from proprioceptive reflexes, presumably muscle spindle reflexes.
