Experimental Physiology (1992), 77, 765-768 Printed in Great Britain

An Investigation into the Extent to which Flare in Human Skin crosses the Mid-Line

G. Z. Mentis and B. Lynn

Department of Physiology, University College London, Gower Street, London WCIE 6BT

SUMMARY

We investigated whether flare crossed the mid-line in various positions in humans using laser Doppler flowmetry to confirm visual observations. On the back and forehead, flare crossed the midline. However, on the posterior part of the neck, flare did not cross the mid-line, an observation that may be related to the elongated shape of flare in this region.

INTRODUCTION

The phenomenon of flare, the vasodilator response around skin injuries, is believed to be due to an axon reflex mechanism initiated in cutaneous nociceptor terminals (Lewis, 1927). Activity in nociceptive neurones is transmitted via the main axon to the spinal cord and also back along peripheral branches that terminate around the stimulated area. Release of mediators from nociceptor terminals results in vasodilatation and flare appears (Lisney & Bharali, 1989). Flare is usually more or less radially symmetrical around the stimulus. One exception reported was the forehead, when stimuli close to the mid-line produced asymmetric flare that did not cross the mid-line (Helme & McKernan, 1984). However, on the back, it has been claimed that flare does cross the mid-line (LaMotte, Shain, Simone & Tsai, 1991). In this paper, we report a study of the shape of flare for noxious stimuli applied close to the posterior mid-line at levels from the middle of the back to the neck (Fig. I C) and close to the anterior mid-line on the forehead. To standardize our methods, most flare has been produced using controlled skin heating and symmetry of responses has been checked using laser Doppler flowmetry.

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