Inhibition of biceps brachii muscle motor area by painful heat stimulation of the skin

This study aimed to evaluate whether painful cutaneous stimuli can affect specifically the excitability of the arm proximal muscle motor area. The motor evoked potentials (MEPs), recorded from the right biceps brachii muscle after either transcranial magnetic or electrical anodal stimulation of the left primary motor (MI) cortex, were conditioned by painful CO₂ laser stimuli delivered either on the right hand dorsum or on the lateral surface of the right arm. Painful CO₂ laser stimuli delivered on the hand skin reduced significantly the amplitude of MEPs evoked by the transcranial magnetic stimulation of the contralateral MI area, while the MEP amplitude was not significantly modified by CO₂ laser pulses delivered on the arm skin. The inhibitory effect followed the arrival of the nociceptive inputs to the cerebral cortex. The amplitude of MEPs evoked by anodal electrical stimulation of the motor cortex was not decreased by conditioning painful stimuli delivered on the hand dorsum. Since the magnetic stimulation led to transynaptic activation of pyramidal neurons, while the anodal stimulation activated directly corticospinal axons, our findings suggest that CO₂ laser pulses delivered on the hand are able to inhibit the arm proximal muscle motor area.