Trigeminal sensory input elicited by electric or magnetic stimulation interferes with the central motor drive to the intrinsic hand muscles

In 6 normal subjects, unilateral supraorbital magnetic or electric stimulation resulted in a consistent symmetrical inhibition of the motor evoked potentials (MEPs) of the relaxed and preactivated first dorsal interosseus (FDI) muscle. A supraorbital stimulus caused a significant reduction in amplitude when the trigeminal CS was given 30 to 65 ms before transcranial magnetic stimulation (TMS). In addition, supraorbital magnetic stimulation induced a bilateral EMG suppression of the isometrically contracting FDI muscles, starting about 40 to 50 ms after the magnetic stimulus. In 4 subjects, MEPs evoked by transcranial electric stimulation or by TMS during slight muscle contraction showed a comparable trigeminomotor inhibition. These findings demonstrate that electromagnetic stimulation of trigeminal afferents interferes with the motor output to the intrinsic hand muscles inducing a bilateral inhibition which is probably mediated by a multisynaptic subcortical network. In all 6 subjects, TMS over the motor hand area or the cerebellum elicited a reproducible blink reflex. Since the blink reflex is a sensitive indicator of trigeminal excitation, one has to assume that TMS is associated with a significant excitation of trigeminal afferents. Therefore, trigeminomotor inhibition has to be considered in all TMS studies that use a conditioning-test design.