"Warmth-insensitive fields": evidence of sparse and irregular innervation of human skin by the warmth sense

Although more acute in some areas of the body than in others, temperature s ensitivity is assumed to be present throughout the skin. Only when very sma ll stimuli have been used (e.g., approximate to 1 mm(2)) has sensitivity to warming or cooling appeared discontinuous. Here we report the discovery of patches of skin several square centimeters in area within which heating ca nnot: be detected until skin temperature exceeds the thresholds of C heat-s ensitive nociceptors (>41 degrees C). These warmth-insensitive fields (grea ter than or equal to 5 cm(2)), which appear to lack low-threshold warm fibe rs, were also found to have reduced responsiveness to non-painful heating a nd significantly higher heat pain thresholds compared to surrounding areas of skin. The existence of such sites corroborates reports that warm fibers are rare in human cutaneous nerves and confirms the classical theory that c utaneous innervation by the warmth sense is punctate and sparse. The insens itive areas also provide unique opportunities for assessing the contributio n of the low-threshold warmth system to perception of heat and heat pain, a nd their existence in healthy young adults contraindicates use of warmth se nsitivity in neurological assessments of C-fiber function.