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Running head : Reflex wind-up before and after spinal transection

51 In this study, we used a novel technique to study reflex wind-up when the spinal cord is intact 52 and following an acute spinal transection. Specifically, we evaluated reflex responses evoked 53 by a series of ten electrical pulses to the tibial or superficial peroneal nerves in 9 decerebrate 54 adult cats, before and after an acute spinal transection. Electromyograms were recorded in 55 four hindlimb muscles (lateral gastrocnemius, tibialis anterior, semitendinosus, and sartorius) 56 to evaluate reflex amplitude, duration, and the temporal summation of reflex responses, so57 called wind-up. We identified two distinct reflex responses evoked by electrical stimulation 58 of the tibial or superficial peroneal nerves based on their pattern of change following acute 59 spinal transection, a short-latency (~10 ms) compound action potential (CAP) that was 60 followed by a burst of sustained activity (SA). Wind-up of CAP and SA amplitudes was 61 clearly present when the spinal cord was intact but was drastically reduced after acute 62 spinalization in some muscles. Moreover, CAP and SA reflex responses were differentially 63 modified by the acute spinalization. When the effects of acute spinal transection were 64 significant, CAP responses were increased after acute spinalization, whereas SA responses 65 were reduced, suggesting that the two signals are regulated by different neuronal 66 mechanisms. The present results provide the first assessment of reflex wind-up before and 67 after an acute spinal transection in the same animals and indicate that different reflex 68 components must be considered separately when evaluating changes in neuronal excitability 69