Contribution of spindle reflexes to post-inspiratory activity in the canine external intercostal muscles

1. The external intercostal muscles have greater post-inspiratory activity than the parasternal intercostal muscles and are more abundantly supplied w ith muscle spindles. In the present study, the hypothesis was tested that s pindle afferent inputs play a major role in determining this activity. 2. The electrical activity of the external and parasternal intercostal musc les in the rostral interspaces was recorded in anaesthetized spontaneously breathing clogs, and the ribs were manipulated so as to alter their normal caudal displacement and the normal lengthening of the muscles in early expi ration. 3. Post-inspiratory activity in the external intercostal muscles showed a r eflex decrease when the caudal motion of the ribs and the lengthening of th e muscles was impeded, and it showed a reflex increase when the rate of cau dal rill motion and muscle lengthening was increased. In contrast, the smal l post-inspiratory activity in the parasternal intercostal muscles remained unchanged. 4. When the two ribs snaking up the interspace investigated were locked to keep muscle length constant, post-inspiratory activity in the external inte rcostal muscles leas reduced and no longer responded to cranial rib manipul ation. 5. These observations confirm that, afferent inputs from muscle receptors, presumably muscle spindles, are a primary determinant of post-inspiratory a ctivity in the canine external intercostal muscles. In anaesthetized animal s, the contribution of central control mechanisms to this activity is small .