OP 7. Anodal transcranial direct current stimulation of the motor cortex induces opposite modulations of reciprocal inhibition in wrist extensors and flexors

Abstract

Development of non-invasive brain stimulation techniques in humans allowed to change cortex excitability and thus to study physiological and physio-pathological mechanisms of cortical plasticity. Recently, it was shown that transcranial direct current stimulation (tDCS) applied over the motor cortex can also affect neural networks in the spinal cord. This opens the way to use tDCS to explore cortical descending control onto spinal neural networks. In the present study, tDCS (in anodal polarity) was applied over the hand motor area so as to explore effects of increased motor cortex excitability onto reciprocal inhibition pathways innervating wrist flexors and wrist extensors. For the first time, tDCS effects were tested in parallel on reciprocal inhibition directed from wrist flexors to wrist extensors and in reverse situation i.e reciprocal inhibition directed from wrist extensors to wrist flexors. We showed that modulations of reciprocal inhibition between flexors and extensors during anodal tDCS application are opposite: tDCS depresses reciprocal inhibition directed from flexors to extensors but increases reciprocal inhibition from extensors to flexors. This asymmetry but to a lesser extent was also observed when ipsilateral motor cortex was stimulated by tDCS. Our results suggest that reciprocal inhibition between flexors and extensors at wrist level is not symmetrical. The functional significance is that reciprocal inhibition is arranged to favor the extensor contraction which is required in a large sample of wrist and finger movements in humans. We will study the effect of tDCS in patients with central nervous system lesion to explore if anodal tDCS applied over the hand motor cortex area may be used as a tool to favor extensor contraction.