Facilitation of the withdrawal reflex by repeated transcutaneous electrical stimulation: an experimental study on central integration in humans

In the present human study, we aimed to investigate the facilitation of bot h the subjective pain responses. and the withdrawal reflex to consecutive t ranscutaneous electrical stimuli as measures of temporal summation. The fre quency (0.5-20 Hz) and intensity (0.4-0.8 times the reflex threshold, xRT) of the electrical stimuli were systematically varied. When using repeated s timulation, the stimulus intensity that evoked pain was lower than that req uired by a single stimulus (temporal summation). Temporal summation leading to pain was found to depend significantly upon both frequency and intensit y (e.g. stimulation at 1 Hz caused summation at 0.8 x RT, whereas stimulati on at 20 Hz caused summation at 0.6 x RT). The strongest reflex facilitatio n, and hence the strongest pain intensity was obtained for stimulation at 1 0-20 Hz at an intensity of 0.8 x RT. In conclusion, the results of the pres ent human study demonstrate clearly that a stimulus that is perceived as a localised, repetitive tactile tap can be integrated and cause severe pain. This suggests that pathologically generated sparse nociceptive afferent act ivity causes strong pain by central integration. This might be one mechanis m to explain why clinical conditions can become excruciatingly painful desp ite the fact that the pathophysiological changes seem to be marginal (e.g. minor nerve trauma).