Motor evoked potentials(MEP) were elicited in the thenar muscles of 32 normal subjects via transcranial magnetic stimuli to obtain normal parameters of MEPs as well as a standard methodology, and to evaluate the significance of excitability threshold in neurophysiologic and clinical investigation. The threshold intensity was determined at relaxed state and MEPs were recorded with suprathreshold stimuli during voluntary contraction. Central motor conduction time(CMCT) was 8.16±1.02msec in non-facilitation and this value was shorten to 5.99±0.81msec with facilitation and not affected with stimulus intensity. The intensity of excitability threshold was 62.9±11.4%/65.9±9.7% (right/left), CMCT was 6.04±0.81/5.94±0.82msec (right/left) in MEPs recorded from abductor pollicis brevis muscle. Increased threshold was found in aged group(>40 years), but CMCT was not correlated with age. The threshold intensity was correlated with visual and auditary simple reaction time, but CMCT was not correlated. We conclude that it is desirable to investigate MEPs by 120% stimulus intensity of excitability threshold in minimal voluntary contraction (facilitation) as the standard method in clinical investigation. The excitability threshold may be involved in the excitability of central neural network and the high cortical motor function as well as the excitability of pyramidal tract and spinal motor neuron pool.
