Inhibition of the human primary motor area by painful heat stimulation of the skin

Objective: To prove whether painful cutaneous stimuli can affect specifically the motor cortex excitability. Methods: The electromyographic (EMG) responses, recorded from the first dorsal interosseous muscle after either transcranial magnetic or electric anodal stimulation of the primary motor (MI) cortex, was conditioned by both painful and non-painful CO₂ laser stimuli delivered on the hand skin. Results: Painful CO₂ laser stimuli reduced the amplitude of the EMG responses evoked by the transcranial magnetic stimulation of both the contralateral and ipsilateral MI areas. This inhibitory effect followed the arrival of the nociceptive inputs to cerebral cortex. Instead, the EMG response amplitude was not significantly modified either when it was evoked by the motor cortex anodal stimulation or when nonpainful CO₂ laser pulses were used as conditioning stimuli. Conclusions: Since the magnetic stimulation leads to transynaptic activation of pyramidal neurons, while the anodal stimulation activates directly cortico-spinal axons, the differential effect of the noxious stimuli on the EMG responses evoked by the two motor cortex stimulation techniques suggests that the observed inhibitory effect has a cortical origin. The bilateral cortical representation of pain explains why the painful CO₂ laser stimuli showed a conditioning effect on MI area of both hemispheres. Non-painful CO₂ laser pulses did not produce any effect, thus suggesting that the reduction of the MI excitability was specifically due to the activation of nociceptive afferents.