title: Transcranial direct current stimulation effects on the excitability of corticospinal axons of the human cerebral cortex.
abstract: 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.
authors.affiliation: Institute of Neurology, Campus Biomedico University, Via Alvaro del Portillo 200, 00128 Rome, Italy. v.dilazzaro@unicampus.it
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Di Lazzaro
authors.forename: Federico
authors.initials: F
authors.lastname: Ranieri
authors.forename: Paolo
authors.initials: P
authors.lastname: Profice
authors.forename: Fabio
authors.initials: F
authors.lastname: Pilato
authors.forename: Paolo
authors.initials: P
authors.lastname: Mazzone
authors.forename: Fioravante
authors.initials: F
authors.lastname: Capone
authors.forename: Angelo
authors.initials: A
authors.lastname: Insola
authors.forename: Antonio
authors.initials: A
authors.lastname: Oliviero
mesh_terms: D000328:Adult; D001369:Axons / Q000502:physiology*; D004558:Electric Stimulation; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D009044:Motor Cortex / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D011712:Pyramidal Tracts / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation
keywords: Corticospinal axons; Corticospinal volleys; D-wave; I-waves; Transcranial direct current stimulation