Although cathodal transcranial direct current stimulation (tDCS) decreases cortical excitability, the mechanisms underlying DC-induced changes remain largely unclear. In this study we investigated the effect of cathodal DC stimulation on spontaneous neural activity and on motor responses evoked by stimulation of the central and peripheral nervous system. We studied 17 healthy volunteers. Transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES) of the motor area were used to study the effects of cathodal tDCS (1.5 mA, 10 min) on resting motor threshold and motor evoked potentials (MEPs) recorded from the contralateral first dorsal interosseous muscle (FDI). The electroencephalographic (EEG) activity in response to cathodal tDCS was analysed by power spectral density (PSD). Motor axonal excitability changes in response to transcutaneous DC stimulation of the ulnar nerve (0.3 mA, 10 min) were assessed by testing changes in the size of the compound muscle action potential (CMAP) elicited by submaximal nerve stimulation. Cathodal tDCS over the motor area for 10 min increased the motor threshold and decreased the size of MEPs evoked by TMS for at least 60 min after current offset (t0 71.7 ± 5%, t20 50.8 ± 11%, t40 47.7 ± 7.7%, and t60 39.7 ± 6.4%, P <0.01). The tDCS also significantly decreased the size of MEPs elicited by TES (t0 64 ± 16.4%, P = 0.09; t20 67.6 ± 10.8%, P = 0.06; and t40 58.3 ± 9.9%, P <0.05). At the same time in the EEG the power of delta (2-4 Hz) and theta (4-7 Hz) rhythms increased (delta 181.1 ± 40.2, P <0.05; and theta 138.7 ± 27.6, P = 0.07). At the peripheral level cathodal DC stimulation increased the size of the ulnar nerve CMAP (175 ± 34.3%; P <0.05). Our findings demonstrate that the after-effects of tDCS have a non-synaptic mechanism of action based upon changes in neural membrane function. These changes apart from reflecting local changes in ionic concentrations, could arise from alterations in transmembrane proteins and from electrolysis-related changes in [H+] induced by exposure to constant electric field.
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