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Key points

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    Transcranial constant current polarization of the human brain is on the increase in neurological practice because it improves several motor and cognitive functions of the human nervous system and because it is non-invasive and technically simple.
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    Here we show that transcranial brain polarization in anaesthetized animals not only affects cortical neurons, as is often assumed, but also facilitates activation of neurons in all investigated subcortical motor systems.
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    In addition, the subcortical facilitation greatly outlasts (by at least hours) the period of transcranial polarization. These findings provide new evidence of plasticity at subcortical levels, the mechanisms for which remain to be investigated.
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    In clinical practice, the subcortical effects of transcranial polarization may thus make an essential contribution to the beneficial effects of the treatment of motor impairments.

Abstract  The main aim of the study was to examine the effects of transcranial polarization on neurons in two descending motor systems, rubro- and reticulospinal. Anodal DC current was applied through an electrode in contact with the skull over the contralateral sensori-motor cortex, against an electrode placed between the skull and the ipsilateral temporal muscles in deeply anaesthetized cats. Its effects were estimated from changes in descending volleys evoked by electrical stimuli applied in the red nucleus (RN), medial longitudinal fascicle (MLF; to reticulospinal fibres) and the pyramidal tract (PT; to corticospinal or corticoreticular fibres). The descending volleys were recorded from the surface of the spinal cord at a cervical level. Rubrospinal neurones were activated either directly or indirectly, via interpositorubral fibres. Reticulospinal neurons were likewise activated directly and indirectly, via other reticulospinal or corticospinal fibres. Transcranial polarization facilitated transsynaptic activation of both rubrospinal and reticulospinal neurons, shortening the latency of the indirect descending volleys and/or increasing them, Direct activation of descending axons was much less affected. The facilitation of all subcortical neurons examined was potentiated by repeated applications of transcranial direct current stimulation (tDCS) and outlasted the polarization by at least 1–2 h, replicating tDCS effects on indirect activation of cortical neurons. The results indicate that the beneficial effects of tDCS on motor performance in humans may be due to more efficient activation of not only cortical but also subcortical neuronal systems. Combined actions of tDCS on cortical and subcortical neurones might thus further improve recovery of motor functions during rehabilitation after central injuries. 249/250