Trans-spinal direct current stimulation modulates motor cortex-induced muscle contraction in mice.

The present study investigated the effect of trans-spinal direct current (tsDC) on the firing rate, pattern, and amplitude of spontaneous activity of the tibial nerve and on the magnitude of cortically elicited triceps surae (TS) muscle contractions. The effect of combined tsDC and repetitive cortical electrical stimulation (rCES) on the amplitude of cortically elicited TS twitches was also investigated. Stimulation was applied by two disk electrodes (0.79 cm(2)): one was located subcutaneously over the vertebral column (T(10)-L(1)) and was used to deliver anodal DC (a-tsDC) or cathodal DC (c-tsDC) (density range: ± 0.64 to ± 38.2 A/m(2)), whereas the other was located subcutaneously on the lateral aspect of the abdomen and served as a reference. While the application of a-tsDC significantly increased the spike frequency and amplitude of spontaneous discharges compared with c-tsDC, c-tsDC made the spontaneous discharges more rhythmic. Cortically elicited TS twitches were depressed during a-tsDC and potentiated after termination. Conversely, cortically elicited TS twitches were enhanced during c-tsDC and depressed after termination. While combined a-tsDC and rCES produced similar effects as a-tsDC alone, combined c-tsDC and rCES showed the greatest increase in cortically elicited TS twitches. tsDC appears to be a powerful neurostimulation tool that can differentially modulate spinal cord excitability and corticospinal transmission.