Spinal DC stimulation in humans modulates post-activation depression of the H-reflex depending on current polarity

ObjectiveTranscranial direct current stimulation induces long-lasting changes in cortical excitability in humans depending on the current used. Further, transcutaneous spinal application of direct current (tsDCS) induces plastic changes in spinal conduction properties, tested by somatosensory evoked potentials. To verify this thesis on plastic changes in spinal circuitry, we investigated the effects of tsDCS on H-reflex size and post-activation depression.MethodsTen healthy subjects participated in the study. The Hmax/Mmax ratio and H-reflex post-activation depression were evaluated before, at current offset, and 15 min after anodal, cathodal or sham tsDCS. Stimulation of the spinal cord (2.5 mA, 0.063 mA/cm2, 0.056 C/cm2) was applied for 15 min at Th11 level.ResultsAnodal tsDCS induced a lasting decrease in H-reflex post-activation depression, while cathodal stimulation resulted in a sustained increase. Sham stimulation had no significant effects. The Hmax/Mmax ratio remained unchanged throughout all conditions.ConclusionAnodal and cathodal tsDCS is a non-invasive and painless method that is able to induce lasting changes in the efficacy of the Ia fibre-motoneurone synapse.SignificanceTranscutaneous spinal DC stimulation might be a valuable new tool in modulating spinal motor pathways.