Effects of anodal transcranial direct current stimulation over the leg motor area on lumbar spinal network excitability in healthy subjects

Non-technical summary  Transcranial direct current stimulation (tDCS) induces modifications of motor cortex excitability depending on the polarity. However, the impact of tDCS applied to lower limb motor cortex on lumbar spinal network excitability has been unknown up to now. This study was performed in order to assess the effects of anodal tDCS compared to sham stimulation on three lumbar spinal circuits, namely reciprocal Ia inhibition, homonymous recurrent inhibition and presynaptic Ia inhibition, in healthy subjects. The results indicate that anodal tDCS modifies the behaviour of 2 of the 3 spinal circuits studied. Effects of anodal tDCS when applied over lower limb motor cortex should be considered with regard not only to cortical circuits but also to spinal motor circuits. The results also suggest that the effects of anodal tDCS on lumbar spinal circuits are similar to those observed during voluntary muscle co-contractions. Abstract  In recent years, two techniques have become available for the non-invasive stimulation of human motor cortex: transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). The effects of TMS and tDCS when applied over motor cortex should be considered with regard not only to cortical circuits but also to spinal motor circuits. The different modes of action and specificity of TMS and tDCS suggest that their effects on spinal network excitability may be different from that in the cortex. Until now, the effects of tDCS on lumbar spinal network excitability have never been studied. In this series of experiments, on healthy subjects, we studied the effects of anodal tDCS over the lower limb motor cortex on (i) reciprocal Ia inhibition projecting from the tibialis anterior muscle (TA) to the soleus (SOL), (ii) presynaptic inhibition of SOL Ia terminals, (iii) homonymous SOL recurrent inhibition, and (iv) SOL H-reflex recruitment curves. The results show that anodal tDCS decreases reciprocal Ia inhibition, increases recurrent inhibition and induces no modification of presynaptic inhibition of SOL Ia terminals and of SOL-H reflex recruitment curves. Our results indicate therefore that the effects of tDCS are the opposite of those previously described for TMS on spinal network excitability. They also indicate that anodal tDCS induces effects on spinal network excitability similar to those observed during co-contraction suggesting that anodal tDCS activates descending corticospinal projections mainly involved in co-contractions.