Pathway-specific plasticity in the human spinal cord

The aim of the present study was to artificially induce plasticity in the human spinal cord and evaluate whether this plasticity is pathway specific. For this purpose, a technique called paired associative stimulation (PAS) was applied. Volleys evoked by transcranial magnetic stimulation over the primary motor cortex and peripheral nerve stimulation of the nervus tibialis in the popliteal fossa were timed to coincide at the spinal level. The transmission of different corticospinal projections was assessed before and after PAS using conditioned H-reflexes. Different groups of healthy volunteers (28 +/- 5 years) were tested; intervention groups 1 (n = 9) and 2 (n = 8) received spinal PAS (360 paired stimuli) and the induced effects were evaluated using cortical (group 1) or cervicomedullary (group 2) conditioning of musculus soleus H-reflexes. After spinal PAS, the conditioned H-reflexes were significantly facilitated when tested with cortical and cervicomedullary stimulation. The effect of the latter technique is independent of changes in the excitability of cortical neurons. Therefore, the finding that conditioned H-reflexes were increased after spinal PAS when tested with both cortical and cervicomedullary stimulation suggests that neural plasticity was induced within the spinal cord. The facilitation could only be observed for specific inter-stimulus intervals between volleys induced by peripheral nerve stimulation and transcranial magnetic stimulation. As the specific inter-stimulus intervals were assumed to relate to transmission within specific motor pathways, it is argued that changes in the corticospinal transmission were pathway-specific. These findings may be helpful in inducing and assessing neural plasticity in pathological conditions like spinal cord injuries.