GAIN OF THE TRICEPS SURAE STRETCH REFLEX IN DECEREBRATE AND SPINAL CATS DURING POSTURAL AND LOCOMOTOR ACTIVITIES

1. The triceps surae (TS) stretch reflex was measured in decerebrate c ats during crossed extensor stimulation and after spinalization during rhythmic locomotor activity induced by clonidine and manual perineal stimulation. The TS force in response to sinusoidal stretch was measur ed at a given contraction level before and after deafferentation, and the 'reflex force' was computed by subtracting these two responses. Re flex 'gain' was computed as the ratio of the reflex and deafferented f orce responses (a unitless estimate of the open loop feedback gain). 2 . Prior to locomotion the spontaneous muscle activity was low (less th an 15% of maximum), but the reflex gain was relatively high (close to 1.0 with a 5 Hz stretch). When locomotion commenced the reflex gain wa s markedly lowered when measured at the same contraction level as befo re locomotion (25% of the gain prior to locomotion). At higher contrac tion levels the reflex gain was not significantly increased. The refle x force and EMG responses to stretch increased with the contraction le vel, but their effect on the total reflex gain was cancelled by an ass ociated increase in the intrinsic muscle stiffness. 3. In the decerebr ate cat, during weak tonic contractions (spontaneous), the reflex gain was high and comparable with the gain in the resting spinal cat. Howe ver, with increased tonic contractions produced by crossed extensor st imulation the reflex gain dropped. At higher contraction levels the ga in was not significantly different from the gain during spinal locomot ion. 4. When the frequency of stretch was increased from 3 to 20 Hz, E MG responses to stretch increased, but the reflex force decreased, sin ce a more fused contraction developed with the more frequent reflex ac tivations. Overall, the reflex gain decreased with frequency in both s pinal and decerebrate cats. The phase lag of the reflex force, relativ e to the intrinsic muscle force, increased with increasing frequency, due to reflex delays, with a 180 deg lag occurring between 12 and 18 H z (tremor frequencies). The mean gain was significantly lower and the phase lag was significantly greater during locomotion than during toni c crossed extensor contractions, suggesting different reflex mechanism s. 5. In conclusion, during locomotion in spinal cats afferent feedbac k from low frequency ankle movements, similar to those occurring durin g the normal step cycle, reflexly produces a small but significant fra ction of the extensor force (about a quarter of the stretch-related fo rce modulation). This fraction is remarkably constant at the different contraction levels of the step cycle. Afferent feedback during higher frequency movement is less effective, minimizing the chance of instab ility and tremor. In contrast during tonic contractions afferent feedb ack produces half of the total muscle force during perturbations, clea rly contributing to the maintenance of posture.