Effect of noninvasive transspinal direct current electrical stimulation of cervical enlargement on the excitability of different hand muscles

<i>Objective.</i>Transspinal electrical direct current (DC) stimulation is a noninvasive method of modulating the activity of spinal cord neurons. However, the effects and their mechanisms remain unclear. A pivotal question pertains to the localization of the effects of transspinal stimulation. In this study, the effects of anodal DC stimulation (1.5 mA for 11 min) of cervical enlargement on the excitability of upper limb motoneuron pools and other networks are identified.<i>Approach.</i>This is achieved using the spinal motor evoked potentials (sMEP) evoked by impulse transspinal spinal cord electrical stimulation. This study assessed the proximal and distal muscles on both sides of the body.<i>Main results.</i>The present study identified lateralization of stimulation effects, with the most significant effects observed in the proximal shoulder muscles, i.e. the biceps brachii and triceps brachii (TB). The most significant changes in motor responses were observed in the TB 15 min after stimulation, with a strong lateralization of responses and a decrease in sMEP amplitude. Moreover, a general tendency for the sMEP to decrease for approximately one hour after the cessation of stimulation was also observed.<i>Significance.</i>Thus, we observe a unique and selective effect of stimulation on the spinal cord. Perhaps understanding the placement of electrodes and stimulation protocols may lead to improved effects.