Length changes of intercostal muscles during respiration in the cat

Sonomicrometry was used to measure the length changes of intercostal muscles throughout the rib cage of anesthetized cats. The motor unit discharge in the vicinity of the length measurement was monitored with bipolar electrodes. The external intercostal muscles in the rostral spaces and the parasternals actively shortened during inspiration. The external intercostals in the caudal spaces, which were inactive, either shortened or lengthened passively due to the forces imposed on the lower rib cage by the contraction of the diaphragm. The length changes of external intercostals of the mid-thoracic spaces, which were recruited during inspiration, were variable. The internal intercostals were usually inactive in our preparation and therefore their length excursions passively followed those of the external intercostals. The information regarding the length changes is correlated with that previously gathered for the role of muscle spindles in the intercostal muscles. It is suggested that gamma motoneurons are recruited to prevent the muscle spindles from being unloaded when the intercostal muscle shortens and to increase the sensitivity of the receptors during the phase of the respiratory cycle when the muscle is active.