ON THE INTERCOSTAL MUSCLE COMPENSATION FOR DIAPHRAGMATIC PARALYSIS INTHE DOG

1. Paralysis of the diaphragm in the dog is known to cause a compensat ory increase in activation of the inspiratory intercostal muscles (par asternal intercostals, external intercostals, and levator costae). The present studies were designed to assess the mechanism(s) of that comp ensation. 2. Complete, selective diaphragmatic paralysis was induced b y injecting local anaesthetic into small silicone cuffs placed around the phrenic nerve roots in the neck. 3. Paralysis produced a decrease in tidal volume and an increase in arterial P-CO2 (P-a,P- CO2). The in creased hypercapnic drive was a primary determinant of the increased i nspiratory intercostal activity. 4. However, paralysis also produced a n increased inspiratory cranial displacement of the ribs. When this in creased rib displacement was reduced to that seen before paralysis, it appeared that the increase in external intercostal and levator costae inspiratory activity was commonly greater than anticipated on the bas is of the increased P-a,P- CO2. 5. Diaphragmatic paralysis after bilat eral vagotomy also elicited disproportionate increases in inspiratory intercostal activity, thus indicating that these increases are not cau sed by vagal afferent inputs. 6. These observations are consistent wit h the idea that the intercostal muscle compensation for diaphragmatic paralysis is, in part, due to the release of an inhibition originating from the contracting diaphragm. This inhibition might arise in the di aphragmatic tendon organs.