Transcutaneous spinal direct current stimulation modulates human corticospinal system excitability

Abstract

This study aimed to assess the effects of thoracic anodal and cathodal transcutaneous spinal direct current stimulation (tsDCS) on upper and lower limb corticospinal excitability. Although there have been studies assessing how thoracic tsDCS influences the spinal ascending tract and reflexes, none has assessed the effects of this technique over upper and lower limb corticomotor neuronal connections. In 14 healthy subjects we recorded motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) from abductor hallucis (AH) and hand abductor digiti minimi (ADM) muscles before (baseline) and at different time points (0 and 30 min) after anodal or cathodal tsDCS (2.5 mA, 20 min, T9-T11 level). In 8 of the 14 subjects we also tested the soleus H reflex and the F waves from AH and ADM before and after tsDCS. Both anodal and cathodal tsDCS left the upper limb MEPs and F wave unchanged. Conversely, while leaving lower limb H reflex unchanged, they oppositely affected lower limb MEPs: whereas anodal tsDCS increased resting motor threshold [(mean ± SE) 107.33 ± 3.3% increase immediately after tsDCS and 108.37 ± 3.2% increase 30 min after tsDCS compared with baseline] and had no effects on MEP area and latency, cathodal tsDCS increased MEP area (139.71 ± 12.9% increase immediately after tsDCS and 132.74 ± 22.0% increase 30 min after tsDCS compared with baseline) without affecting resting motor threshold and MEP latency. Our results show that tsDCS induces polarity-specific changes in corticospinal excitability that last for >30 min after tsDCS offset and selectively affect responses in lower limb muscles innervated by lumbar and sacral motor neurons.

Keywords: corticospinal system; direct current stimulation; motor potentials; spinal cord; spinal cord stimulation; transcranial direct current stimulation; transcranial magnetic stimulation; transcutaneous spinal direct current stimulation.

Fig. 1.

A and B: lower limb motor evoked potentials (MEPs) before (baseline), immediately after (T0), and at 30 min after (T30) anodal (A) or cathodal (B) transcutaneous spinal direct current stimulation (tsDCS) in a representative subject. Each trace is the superimposition of 5 sweeps. Table on right reports average values of resting motor threshold (RMT), MEP area, and MEP latency for the represented subject. Note that whereas anodal tsDCS decreased the MEP area at T0 and T30, cathodal tsDCS increased the MEP area. Vertical arrows represent the stimuli. C and D: upper limb MEPs before (baseline), immediately after (T0), and at 30 min after (T30) anodal (C) or cathodal (D) tsDCS in a representative subject. Each trace is the superimposition of 5 sweeps. Table on right reports average values of RMT, MEP area, and MEP latency for the represented subject. Note that neither anodal nor cathodal tsDCS affected MEPs. Vertical arrows represent the stimuli.

Fig. 2.

A and B: effects of tsDCS on RMT (A) and MEP area (B) when responses were recorded from abductor hallucis muscle (AH; data are expressed as % of baseline). Group data are presented as mean ± SE changes induced by anodal or cathodal tsDCS immediately after current offset (T0) and 30 min later (T30) (error lines are SE). Note that anodal and cathodal tsDCS induced significantly different, opposite changes in RMT and MEP area. C and D: effects induced by tsDCS on RMT (C) and MEP area (D) when responses were recorded from abductor digiti minimi (ADM) muscles. Group data are presented as mean ± SE changes induced by anodal or cathodal tsDCS immediately after current offset (T0) and 30 min later (T30) (error lines are SE). Neither anodal nor cathodal tsDCS significantly changed RMT or MEP area. **P < 0.05. N.S., not significant.