THE EFFECTS OF STIMULUS RATES UPON MEDIAN, ULNAR AND RADIAL NERVE SOMATOSENSORY-EVOKED POTENTIALS

We examined the effect of stimulus rates on the somatosensory evoked p otential (SEP) amplitude following stimulation of the median nerve (MN ) and the ulnar nerve (UN) at the elbow or wrist, and the radial nerve (RN) at the wrist in 12 normal subjects. We measured the amplitude of frontal (P14-N18-P22-N30) and parietal peaks (P14-N20-P26-N34) at a s timulus rate of 1.1, 3.5 and 5.7 Hz. The amplitude attenuation was fou nd at frontal P22 and N30 and to a lesser degree al parietal N20 and P 26 peaks with an increasing stimulus rate from 1.1 to 5.7 Hz. The ampl itude attenuation was greatest at the elbow when compared to the wrist stimulation for both MN and UN. The attenuation was least for wrist s timulation for the RN. The UN block by local anesthesia just distal to the stimulus electrode at the elbow abolished the amplitude attenuati on caused by the fast stimulus rate. The observed amplitude attenuatio n with the faster stimulus rate is probably due, in part, to interfere nce from the ''secondary'' afferent inputs. The secondary afferent inp uts arise from peripheral receptor stimulation (muscle, joint and/or c utaneous) as a subsequent effect of efferent volleys initiated from th e point of stimulation. The greater number of peripheral receptors bei ng activated as more proximal sites of stimulation in a mixed nerve wo uld result in greater attenuation of the SEP recorded from scalp elect rodes. We postulate that the attenuation of frontal peaks by the fast stimulus rate is due to the frontal projection of interfering ''second ary'' afferent inputs.