title: Thermal detection thresholds of Aδ- and C-fibre afferents activated by brief CO2 laser pulses applied onto the human hairy skin.
abstract: Brief high-power laser pulses applied onto the hairy skin of the distal end of a limb generate a double sensation related to the activation of Aδ- and C-fibres, referred to as first and second pain. However, neurophysiological and behavioural responses related to the activation of C-fibres can be studied reliably only if the concomitant activation of Aδ-fibres is avoided. Here, using a novel CO(2) laser stimulator able to deliver constant-temperature heat pulses through a feedback regulation of laser power by an online measurement of skin temperature at target site, combined with an adaptive staircase algorithm using reaction-time to distinguish between responses triggered by Aδ- and C-fibre input, we show that it is possible to estimate robustly and independently the thermal detection thresholds of Aδ-fibres (46.9±1.7°C) and C-fibres (39.8±1.7°C). Furthermore, we show that both thresholds are dependent on the skin temperature preceding and/or surrounding the test stimulus, indicating that the Aδ- and C-fibre afferents triggering the behavioural responses to brief laser pulses behave, at least partially, as detectors of a change in skin temperature rather than as pure level detectors. Most importantly, our results show that the difference in threshold between Aδ- and C-fibre afferents activated by brief laser pulses can be exploited to activate C-fibres selectively and reliably, provided that the rise in skin temperature generated by the laser stimulator is well-controlled. Our approach could constitute a tool to explore, in humans, the physiological and pathophysiological mechanisms involved in processing C- and Aδ-fibre input, respectively.
authors.affiliation: Institute of Neuroscience (IONS), Université catholique de Louvain, Brussels, Belgium.
authors.forename: Maxim
authors.initials: M
authors.lastname: Churyukanov
authors.forename: Léon
authors.initials: L
authors.lastname: Plaghki
authors.forename: Valéry
authors.initials: V
authors.lastname: Legrain
authors.forename: André
authors.initials: A
authors.lastname: Mouraux
mesh_terms: D000328:Adult; D005073:Evoked Potentials, Somatosensory / Q000502:physiology / Q000528:radiation effects; D005260:Female; D006358:Hot Temperature; D006801:Humans; D054020:Lasers, Gas; D008297:Male; D009413:Nerve Fibers, Myelinated / Q000502:physiology* / Q000528:radiation effects; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology* / Q000528:radiation effects; D009619:Nociceptors / Q000502:physiology* / Q000528:radiation effects; D017288:Pain Threshold / Q000502:physiology* / Q000528:radiation effects; D011930:Reaction Time / Q000502:physiology / Q000528:radiation effects; D012867:Skin / Q000528:radiation effects*; D012881:Skin Temperature / Q000528:radiation effects