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 both the subjective pain responses. and the withdrawal reflex to consecutive transcutaneous electrical stimuli as measures of temporal summation. The frequency (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 stimulation, the stimulus intensity that evoked pain was lower than that required by a single stimulus (temporal summation). Temporal summation leading to pain was found to depend significantly upon both frequency and intensity (e.g. stimulation at 1 Hz caused summation at 0.8 x RT, whereas stimulation at 20 Hz caused summation at 0.6 x RT). The strongest reflex facilitation, and hence the strongest pain intensity was obtained for stimulation at 10-20 Hz at an intensity of 0.8 x RT. In conclusion, the results of the present 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 activity causes strong pain by central integration. This might be one mechanism to explain why clinical conditions can become excruciatingly painful despite the fact that the pathophysiological changes seem to be marginal (e.g. minor nerve trauma).