Transcranial direct current stimulation effects on the excitability of corticospinal axons of the human cerebral cortex.

BACKGROUND: Transcranial direct current stimulation (tDCS) of the human cerebral cortex modulates cortical excitability non-invasively in a polarity-specific manner: anodal tDCS leads to lasting facilitation of motor cortex excitability.

OBJECTIVE: To further elucidate the underlying physiological mechanisms of tDCS.

METHODS: We recorded corticospinal volleys evoked by single-pulse transcranial magnetic stimulation of the primary motor cortex before and after a 20 min period of anodal tDCS in a conscious patient who had electrode implanted in the cervical epidural space for the control of pain. We performed magnetic stimulation of the motor cortex using a direction of the induced current in the brain capable of activating both corticospinal axons, evoking D-wave activity, and cortico-cortical axons projecting upon corticospinal cells, evoking I-wave activity.

RESULTS: Anodal tDCS increased the excitability of cortical circuits generating both D and I-wave activity, with a more prolonged effect on D-wave activity. The changes in motor evoked potential recorded from hand muscles produced by tDCS were in agreement with the effects produced on intracortical circuitry.

CONCLUSIONS: Epidural recordings of corticospinal activity in our patient indicate that anodal tDCS develops its facilitatory effects by an increase in the excitability of corticospinal axons and by an increase of activity in cortico-cortical projections onto pyramidal tract neurones, modulating motor cortex excitability with both synaptic (I waves) and non-synaptic (D waves) mechanisms.