Differential control of the inspiratory intercostal muscles during airway occlusion in the dog.

1. The effect of airway occlusion on the electrical activity of the three groups of inspiratory intercostal muscles (external intercostal, levator costae, parasternal intercostal) situated in the cranial portion of the rib-cage has been studied in thirty anaesthetized, spontaneously breathing dogs. 2. The three muscles were active during normal inspiration, and their activity was prolonged similarly during airway occlusion. However, a comparison of activity during occluded and unoccluded inspirations indicated that airway occlusion caused a facilitation of external intercostal and levator costae activities but an inhibition of parasternal intercostal activity. 3. The facilitation of external intercostal and levator costae activities was markedly reduced after section of the phrenic nerves and completely suppressed after section of the appropriate thoracic dorsal roots. 4. The inhibition of parasternal intercostal activity was not affected by section of the phrenic nerves or by section of the thoracic dorsal roots. This phenomenon, however, was abolished after bilateral cervical vagotomy. 5. Activation of the external intercostals and levator costae during inspiratory efforts are thus highly dependent on segmental reflexes arising in these muscles. In contrast, activation of the parasternal intercostals resembles that of the diaphragm in the sense that it depends primarily on the central respiratory drive.