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First published online on September 14, 2006.
Copyright © 2006 by The Physiological Society
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Received June 21, 2006
Revised July 13, 2006
Accepted after revision September 4, 2006

Similar nociceptive afferents mediate psychophysical and electrophysiological responses to heat stimulation of glabrous and hairy skin in humans

G D Iannetti1*, L Zambreanu1, and I Tracey1

1 University of Oxford

* To whom correspondence should be addressed. E-mail: giandomenico.iannetti{at}anat.ox.ac.uk.

The ability to perceive and withdraw rapidly from noxious environmental stimuli is crucial for survival. When heat stimuli are applied to primate hairy skin, first pain sensation is mediated by type-II A-fibre nociceptors (II-AMHs). In contrast, the reported absence of first pain and II-AMH microneurographical responses when heat stimuli are applied to the hand palm has led to the notion that II-AMHs are lacking in this primate glabrous skin. Aim of this study was to assess the effect of hairy and glabrous skin stimulation on neural transmission of nociceptive inputs elicited by different kinds of thermal heating. We recorded psychophysical and EEG brain responses to radiant and contact heat stimuli (laser- and contact heat-evoked potentials, LEPs and CHEPs) delivered to the dorsum and the palm of the hand in normal volunteers. Brain responses were analysed at a single-trial level, using an automated approach based on multiple linear regression. Laser stimulation of hairy and glabrous skin at the same energy elicited remarkably similar psychophysical ratings and LEPs. This finding provides strong evidence that first pain to heat does exist in glabrous skin, and suggests that similar nociceptive afferents, with the physiological properties of II-AMHs, mediate first pain to heat stimulation of glabrous and hairy skin in humans. In contrast, when contact heat stimuli were employed, a significant higher nominal temperature had to be applied to glabrous skin in order to achieve psychophysical ratings similar to those obtained following hairy skin stimulation, and CHEPs following glabrous skin stimulation had significantly longer latencies (N2 wave: +25%, P2 wave: +24%) and smaller amplitudes (N2 wave: -40%, P2 wave: -44%) than CHEPs following hairy skin stimulation. Irrespectively of the stimulated territory, CHEPs always had significantly longer latencies (hairy skin N2 wave: +75%, P2 wave: +56%) and smaller amplitudes (hairy skin N2 wave: -42%, P2 wave: -19%) than LEPs. These findings are consistent with the thickness-dependent delay and attenuation of the temperature waveform at nociceptor depth when conductive heating is applied, and suggest that the previously reported lack of first pain and microneurographical II-AMH responses following glabrous skin stimulation could have been the result of a search bias consequent to the use of long-wavelength radiant heating (i.e. CO2 laser) as the stimulation procedure.


Key words: Human • Nociceptor • Pain




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G. D. Iannetti, N. P. Hughes, M. C. Lee, and A. Mouraux
Determinants of Laser-Evoked EEG Responses: Pain Perception or Stimulus Saliency?
J Neurophysiol, August 1, 2008; 100(2): 815 - 828.
[Abstract] [Full Text] [PDF]




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