Propriospinal cutaneous-induced EMG suppression is unaltered by anodal tDCS of healthy motor cortex

Cervical propriospinal premotoneurons (PN) relay descending motor commands and integrate peripheral afferent feedback. Effects of anodal transcranial direct current stimulation (a-tDCS) on propriospinal excitability in the upper limbs are unknown.

Healthy right-handed adults received a-tDCS or sham tDCS over primary motor cortex (M1) at 1mA (Experiment 1, n=18) or 2mA current intensity (Experiment 2, n=15). Propriospinal excitability was assessed by suppression of background electromyography (EMG) in extensor carpi radialis (ECR) from electrical stimulation of the superficial radial nerve during bilateral (Experiment 1 and 2) or unilateral (Experiment 2 only) activation of the left and/or right ECR. EMG suppression could be attributed to an early propriospinal component and late cortical component. Motor evoked potentials (MEP) were obtained as a manipulation check.

Before tDCS, propriospinal-mediated cutaneous-induced suppression was present in each arm for early and late components. ECR MEP amplitude increased after 1mA, but not 2mA, a-tDCS. Neither 1mA nor 2mA a-tDCS modulated either component of ipsilateral or contralateral propriospinal excitability during bilateral or unilateral tasks.

Propriospinal-mediated cutaneous-induced suppression was not modulated by a-tDCS in healthy adults.

Reporting non-significant findings is paramount for the development of clinically-relevant tDCS protocols.