Excitation of the motor cortex associated with the E2 phase of cutaneous reflexes in man

We investigated excitability changes of the motor cortex associated with the E2 phase of cutaneous reflexes in the first dorsal interosseous muscle using transcranial electrical and magnetic stimulation of the motor cortex in humans. EMG responses to combined cutaneous and weak magnetic cortical stimulation, which were elicited during the E2 phase of cutaneous reflexes, were larger than those by the same magnetic cortical stimulation alone. This facilitatory effect was reduced or even inhibitory effect was seen when the intensity of cortical stimulation was increased. Responses to weak electrical cortical stimulation were less affected by the combined cutaneous stimulation. The same facilitatory effect on responses to weak magnetic cortical stimulation was also observed in single motor unit recordings, too. Dissociation between facilitatory effects on the responses evoked by weak magnetic and weak electrical cortical stimulations suggests that the motor cortical excitability is increased in association with the E2 phase. The present results are consistent with the hypothesis that the E2 phase is a kind of transcortical reflex.