Transcutaneous spinal direct current stimulation (tsDCS) can modulate neuronal excitability within the human spinal cord; however few studies have used tsDCS at a cervical level. This study aimed to further characterise cervical tsDCS by observing its acute effects on motor responses to transcranial magnetic stimulation (TMS) and cervicomedullary stimulation. In both Study 1 and 2, participants (Study 1: n = 8, 4F; Study 2: n = 8, 3F) received two periods of 10 min, 3 mA cervical tsDCS on the same day through electrodes placed in an anterior-posterior configuration over the neck; one period with the cathode posterior (c-tsDCS) and the other with the anode posterior (a-tsDCS). In Study 1, electrically-elicited cervicomedullary motor evoked potentials (eCMEPs) and TMS-elicited motor evoked potentials (MEPs) were measured in biceps brachii and flexor carpi radialis (FCR) before, during and after each tsDCS period. In Study 2, eCMEPs and magnetically-elicited CMEPs (mCMEPs) were measured before, during and after each tsDCS period. For Study 3, computational modelling was used to observe possible interactions of cervical tsDCS and electrical cervicomedullary stimulation. Study 1 and 2 revealed that eCMEPs were larger during c-tsDCS and smaller during a-tsDCS compared to those elicited when tsDCS was off (P < 0.05), with no changes in MEPs or mCMEPs. Modelling revealed that eCMEP changes may result from modifications of electrical field direction and magnitude when combined with cervical tsDCS. Bidirectional eCMEP changes are likely caused by an interaction between cervical tsDCS and electrical cervicomedullary stimulation, thus care should be taken when combining such electrical stimuli in close proximity.
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