Topographical distribution of pinprick and warmth thresholds to CO2 laser stimulation on the human skin

We studied the topographical distribution of laser sensory thresholds on th e human hairy skin, using a small laser beam for pinprick and a large beam for warmth sensations. The threshold for pinprick sensation correlated posi tively with the distance from the brain, suggesting that A delta nociceptor s, the fibers which convey pinprick sensation, are more dense at proximal t han at distal body sites. This finding adds information to skin biopsy stud ies of epidermal free nerve endings which showed a similar gradient, but co uld not differentiate small myelinated from unmyelinated fiber afferents. P ossibly because of a diffuse low density of warmth receptors, laser warmth thresholds showed no trend. (C) 2000 Elsevier Science Ireland Ltd. All righ ts reserved.