Observations on stretch reflexes in lumbar back muscles of the cat.

Reflex responses to brief muscle stretch and to electrical stimulation of dorsal roots were studied in cat longissimus and iliocostalis muscles by myographic and electromyographic recordings in spinal and anesthetized preparations. Brief stretch applied simultaneously to both muscles, by pulling at an isolated segment of the iliac bone, elicited contractions only in the central region of longissimus, composed of slowly contracting fibers, the time to peak tension varying between 70 and 100 ms. No reflex responses were observed in the faster contracting parts of this muscle or in the iliocostalis under the stimulation conditions used. The total reflex time usually varied from 4.5 to 7 ms which is shown to correspond to intra-spinal conduction times, approximately between 2 and 5 ms. Only exceptionally was a central reflex delay corresponding to a monosynaptic transmission observed. Reflexes evoked by dorsal root simulation (L4, L5) have a central conduction time similar to those elicited by adequate stimulation. The possibility that the central pathway for stretch reflexes in longissimus may involve more than two neurons is considered. In decerebrate preparations the central region of longissimus displays a prominent tonic stretch reflex. With the longissimus in situ the magnitude of the reflex is highly dependent on the position of the lumbar spine relative to the pelvic girdle.