OPERANT-CONDITIONING OF H-REFLEX IN SPINAL CORD-INJURED RATS

Operant conditioning of the spinal stretch reflex or its electrical an alog, the H-reflex, is a new model for exploring the mechanisms of sup raspinal control over spinal cord function, Both rats and primates can gradually increase (HRup conditioning mode) or decrease (HRdown condi tioning mode) soleus H-reflex magnitude when exposed to an operant con ditioning task. This study used H-reflex operant conditioning to asses s and modify spinal cord function after injury. Soleus H-reflexes were elicited and recorded with chronically implanted electrodes from rats that had been subjected to calibrated contusion injuries to the spina l cord at T8. From 18 to 140 days after injury, background EMG, M resp onse amplitude, and initial H-reflex amplitude were not significantly different from those of normal rats, HRdown conditioning was successfu l in some, but not all, spinal cord-injured rats. The H-reflex decreas e achieved by conditioning was inversely correlated with the severity of the injury as assessed histologically or by time to return of bladd er function. It was not correlated with the length of time between inj ury and the beginning of conditioning. The results confirm the importa nce of descending control from supraspinal structures in mediating ope rantly conditioned change in H-reflex amplitude. In conjunction with r ecent human studies, they suggest that H-reflex conditioning could pro vide a sensitive new means for assessing spinal cord function after in jury, and might also provide a method for initiating and guiding funct ional rehabilitation.