The cerebellum in maintenance of a motor skill: A hierarchy of brain and spinal cord plasticity underlies H-reflex conditioning

Operant conditioning of the H-reflex, the electrical analog of the spinal stretch reflex, is a simple model of skill acquisition and involves plasticity in the spinal cord. Previous work showed that the cerebellum is essential for down-conditioning the H-reflex. This study asks whether the cerebellum is also essential for maintaining clown-conditioning. After rats decreased the soleus H-reflex over 50 d in response to the down-conditioning protocol, the cerebellar Output nuclei dentate and interpositus (DIN) were ablated, and down-conditioning continued for 50-100 more days. In naive (i.e., unconditioned) rats, DIN ablation itself has no significant long-term effect on H-reflex size. During down-conditioning prior to DIN ablation, eight Sprague-Dawley rats decreased the H-reflex to 57% ( 4 SEM) of control. It rose after ablation, stabilizing within 2 d at about 75% and remaining there until similar to 40 d after ablation. It then rose to similar to 130%, where it remained through the end of study 100 d after ablation. Thus, DIN ablation in clown-conditioned rats caused an immediate increase and a delayed increase in the H-reflex. The final result was an H-reflex significantly larger than that prior to down-conditioning. Combined with previous work, these remarkable results Suggest that the spinal cord plasticity directly responsible for down-conditioning, which Survives only 5-10 d on its own, is maintained by supraspinal plasticity that survives similar to 40 d after loss of cerebellar output. Thus, H-reflex conditioning seems to depend oil a hierarchy of brain and spinal cord plasticity to which the cerebellum makes an essential contribution.