EFFECTS OF VOLUNTARY CONTRACTION ON TIBIAL NERVE SOMATOSENSORY-EVOKEDPOTENTIALS - GATING OF SPECIFIC CORTICAL RESPONSES

We evaluated vertex-parietal P37, N50, and contralateral N37 somatosen sory evoked potentials (SEPs) to posterior tibial nerve stimulation du ring weak (20 to 30%) and strong (80 to 90%) ipsilateral gastrocnemius -soleus contraction. The results were compared with data obtained duri ng full relaxation. P37 and N50 were attenuated significantly during w eak contraction and then abolished during strong contraction, whereas the contralateral N37 was not. The N37 potential spreads over the vert ex and over the ipsilateral parietal region during strong contraction. The Cz'-F3 montage was not appropriate for detecting these SEP patter ns. These findings suggest that thalamic or cortical gating mechanisms affect specific cortical responses. P37 and N50 could reflect the arr ival of the afferent volley into the motor areas from thalamic and cor tical (subareas 1 and 2 of S1) projections. N37 could be generated in subarea 3b. Differential analysis of N37 and P37 is required in clinic al practice, mainly in those conditions that involve the motor system and in those conditions in which tonic muscular activity is increased.