LATE ELECTRO-MYOGRAPHIC RESPONSE TO LIMB DISPLACEMENT IN MAN .1. EVIDENCE FOR SUPRASPINAL CONTRIBUTION

Recent findings in primate experiments suggest that the 'late' EMG response (termed here the FSR) to sudden limb displacement, generated with the intention to oppose the stimulus, might be mediated via a transcortical long-loop pathway. In the present investigation we tested this hypothesis in man by examining the EMG responses elicited in a variety of limb muscles in opposition to sudden limb displacements. These were generated either by a manually applied force or by means of a servo-controlled high performance electro-hydraulic actuator. Ten normal human subjects and 22 with clinically established spasticity (12 with a high probability of mid-thoracic transection, 10 with cerebral lesions) were studied. In normal subjects, the difference between tendon jerk and FSR latencies was found to increase with descending spinal level of muscle innervation, being 32.1, 47.2 and 73.1 msec respectively for biceps, quadriceps and gastrocnemius. Provided that this measure reflects the time required for central conduction, this finding is compatible with the long-loop hypothesis. Furthermore, no EMG response was ever observed in the affected muscles of the spastic subjects in the time bracket normally occupied by the FSR, despite the fact that the tendon jerk response was present and prolonged. This suggests that the segmental mechanisms responsible for hyper-reflexia cannot themselves generate the FSR. It is therefore concluded that the results are consistent with the current view that a long-loop pathway, probably involving the cortex, contributes to the FSR in response to sustained stretch in man. © 1979.