Direct-current stimulation of posterior tibial nerve modulates the Soleus H-reflex amplitude

Introduction: Several studies demonstrated that transcranial direct current stimulation (tDCs) is a promising non-invasive tool able to modulate the excitability of several CNS structures. Its effect is usually facilitatory when using anodal polarity and inhibitory for the cathodal one. In most studies, DC stimulation was applied on cortical or spinal structures, while little is known about its effect on peripheral nerves fibres. This research aims at highlighting such effect. Methods: In twenty subjects, electrical stimulation of the posterior tibial nerve (1 ms current pulses, 1 shock every 9 s) was used to elicit the H-reflex in the Soleus muscle. Once the H-reflex amplitude was stable for at least 15 min, DCs (either cathodal or anodal) was applied proximally to the same nerve for 10 min, looking for changes in reflex amplitude. Then, the H-reflex was measured for 30 further minutes, looking for after-effects. Results: Cathodal DCs induced a significant increase of the H-reflex amplitude (about +35%) with respect to the control value. In this configuration the after-effect lasted about 25 min. Anodal DCs induced instead a significant decrease (about -25%) of the reflex amplitude. A significant after-effect was observed for just about 5 min. Discussion: This study shows that DCs applied to a peripheral nerve is able to elicit neuromodulation. Its polarity dependence suggests a local change in the excitability of nerve fibres rather than a central modulation of the spinal reflex circuit. Moreover it is worth to note that the polarity dependence was opposite to what found for tDCS.