Effects of DC stimulation on the excitability of posterior tibial nerve

Several studies demonstrated that transcutaneous direct current stimulation (DCs) may non-invasively modulate the excitability of several CNS structures. Its effect is typically facilitatory when using anodal polarity and inhibitory when using the cathodal one. In most studies, DCs has been applied on cortical or spinal structures, while little is known about its effects on peripheral nerves fibres. This research aims at highlighting such effects. In twenty subjects, square pulses electrical stimulation of the posterior tibial nerve (1 ms duration, 1 shock every 9 seconds) at the popliteal fossa was used to elicit Soleus H-reflexes before (15 min), during (10 min) and after (30 min) DCs. Cathodal or anodal current (1 to 3 mA) was applied just proximally to the same nerve. Cathodal DCs induced a significant increase of the H-reflex amplitude (about +35%) with respect to the control value (before). The excitatory effect lasted about 25 min after DCs end. Anodal DCs induced instead a significant decrease (about -25%) of the reflex amplitude, which lasted just about 5 min after DCs end. In conclusion, DCs is able to elicit acute and short-term polarity-dependent effects on peripheral nerve excitability. This neuromodulation, which is likely due to a local change in nerve excitability, is however opposite in sign to the effects usually found on cortical or spinal structures.