Inspiratory activity in the canine external intercostal muscles is exquisitely sensitive to the direction and amplitude of the inspiratory displacement of the ribs. This study was designed to investigate the role of muscle receptors, in particular the muscle spindles, in mediating this phenomenon. 2. External intercostal inspiratory activity showed a reflex increase when the normal cranial motion of the ribs and the normal shortening of the muscles was reduced, and showed a reflex decrease when the cranial motion of the ribs and the shortening of the muscles was augmented. However, clamping the two ribs making up the interspace and maintaining muscle length constant only moderately attenuated these responses. 3. These persistent responses remained unchanged after section of the levator costae muscles. 4. The responses were attenuated but still present after section of the external intercostals in the contiguous segments and denervation of the internal intercostals. 5. These reflex responses are therefore mediated in part by non-muscular receptors, which most likely lie within the costovertebral joints. These joint receptors might be a primary determinant of the load-compensating reflex.
