Pain and vascular reflexes in man elicited by prolonged noxious mechano-stimulation.

Interdigital webs of the hands of human volunteers were pinched for periods of 2 min duration to induce tonic pain perceptions. During stimulation, the subjects gave estimates of their pain level at 10-sec intervals on a visual analogue scale, and the stimulus induced changes of the skin blood flow in the stimulated hand was assessed by photo-plethysmography. Sustained pinching induced a tonic reflex vasoconstriction in the stimulated hand with rather slow adaptation rate and no signs of habituation between trials. Step increases of the pinching force in the course of a stimulus were reflected by a decrease in plethysmogram amplitude as well as by increased pain ratings, although the subjects were usually unaware of these steps. Lowering the stimulus strength induced opposite, albeit much smaller effects in both pain ratings and vascular reactions. It is concluded that the vasomotor responses described in this paper are mediated by nociceptor input and reflect the processing of this input mainly at a spinal level.