1. A test was performed of the hypothesis that the motion of the ribs during inspiration modulates, via changes in spindle afferent activity, the activation of the inspiratory intercostal muscles. The electrical activity of the parasternal intercostal, external intercostal, and levator costae muscles in anaesthetized spontaneously breathing dogs was thus recorded during manipulation of the inspiratory displacement of the ribs over a wide range of rib motion. 2. In agreement with the hypothesis, the external intercostal and levator costae muscles lengthened and showed increased inspiratory activities when the normal inspiratory cranial motion of the lower rib was reduced or reversed into an inspiratory caudal motion. Conversely, the inspiratory activities decreased when the inspiratory cranial motion of the rib and the inspiratory shortening of the muscles was augmented. The inspiratory activity of the parasternal intercostal remained unchanged throughout. 3. However, when the two ribs making up the interspace were linked together so that the external intercostal muscle was constant in length, the relationship of muscle activity to rib motion was maintained. 4. In addition, when the upper rather than the lower rib of the interspace was manipulated, the relationship between the change in muscle length and inspiratory activity was reversed, so that activity decreased when the muscle was lengthened and increased when the muscle was shortened. The relationship of muscle activity to lower rib motion, however, was still maintained. 5. These observations thus indicate that rib motion triggers proprioceptive reflexes which, regardless of the changes in length of the individual muscles, make the external intercostal inspiratory activity exquisitely sensitive to the direction of rib displacement.
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