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1 Physiological Science, University of California Los Angeles, Los Angeles, California, United States
2 Brain Research Institute, UCLA School of Medicine, Center for Health Sciences, 10833 Le Conte Avenue, Los Angeles, California, 90095-1761, United States; Physiological Science, University of California Los Angeles, Los Angeles, California, United States
3 Brain Research Institute, UCLA School of Medicine, Center for Health Sciences, Los Angeles, California, United States; Physiological Science, University of California Los Angeles, Los Angeles, California, United States
* To whom correspondence should be addressed. E-mail: vre{at}ucla.edu.
Changes in epidurally-induced (S1) spinal cord reflexes were studied as a function of the level of restoration of stepping ability following spinal cord transection (ST). Three types of responses were observed. The early response (ER) had a latency of 2.5 to 3 msec and resulted from direct stimulation of motoneurons. The middle response (MR) had a latency of 5 to 7 msec and was monosynaptic. The late response (LR) had a latency of 9 to 11 msec and was polysynaptic. After a complete mid-thoracic ST, the LR was abolished, whereas the MR was facilitated and progressively increased. The LR reappeared approximately three weeks after ST and increased during the following weeks. Restoration of stepping induced by epidural stimulation at 40 Hz coincided with changes in the LR. During the first two weeks post-ST, rats were unable to step and electrophysiological assessment failed to show any LR. Three weeks post-ST, epidural stimulation resulted in a few steps, and these coincided with reappearance of the LR. The ability of rats to step progressively improved from week three to week six post-ST. There was a continuously improved modulation of rhythmic EMG bursts that was correlated with restoration of the LR. These results suggest that restoration of polysynaptic spinal cord reflexes after complete ST coincides with restoration of stepping function when facilitated by epidural stimulation. Combined, these findings support the view that restoration of polysynaptic spinal cord reflexes induced epidurally may provide a measure of functional restoration of spinal cord locomotor networks after ST.
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