Postinspiratory activity of the parasternal and external intercostal muscles in awake canines

Previous studies have shown in awake dogs that activity in the crural diaph ragm, but not in the costal diaphragm, usually persists after the end of in spiratory airflow. It has been suggested that this difference in postinspir atory activity results from greater muscle spindle content in the crural di aphragm. To evaluate the relationship between muscle spindles and postinspi ratory activity, we have studied the pattern of activation of the parastern al and external intercostal muscles in the second to fourth interspaces in eight chronically implanted animals. Recordings were made on 2 or 3 success ive days with the animals breathing quietly in the lateral decubitus positi on. The two muscles discharged in phase with inspiration, but parasternal i ntercostal activity usually terminated with the cessation of inspiratory fl ow, whereas external intercostal activity persisted for 24.7 +/- 12.3% of i nspiratory time (P < 0.05). Forelimb elevation in six animals did not affec t postinspiratory activity in the parasternal but prolonged postinspiratory activity in the external intercostal to 45.4 +/- 16.3% of inspiratory time (P < 0.05); in two animals, activity was still present at the onset of the next inspiratory burst. These observations support the concept that muscle spindles are an important determinant of postinspiratory activity. The abs ence of such activity in the parasternal intercostals and costal diaphragm also suggests that the mechanical impact of postinspiratory activity on the respiratory system is smaller than conventionally thought.