Responses of Cutaneous A-Fiber Nociceptors to Noxious Cold

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Abstract
Simone, Donald A. and Keith C. Kajander. Responses of cutaneous A fiber nociceptors to noxious cold. J. Neurophysiol. 77: 2049–2060, 1997. Responses of cutaneous nociceptors to natural stimuli, particularly mechanical and heat stimuli, have been well documented. Although nociceptors are excited by noxious cold stimuli, there have been few studies of their stimulus-response functions for cold stimuli over a wide range of stimulus temperatures. Furthermore, the proportion of nociceptors excited by noxious cold is not clear. In the present study, we examined responses of mechanosensitive Aδ-nociceptors and low-threshold mechanoreceptors to a wide range of cold stimuli that included stimulus temperatures 2 contact thermode from a base temperature of 32°C. Stimuli were applied in descending order of 2°C decrements. Stimulus ramp rate was 5°C/s, and stimulus temperatures were applied for a duration of 10 s. A total of 90 A fibers was studied, of which 61 were nociceptors and had conduction velocity in the Aδ-range (2–30 m/s). Nociceptors were classed initially as mechanical, mechanoheat, and mechanocold nociceptors. The remaining 29 fibers were low-threshold mechanoreceptors with conduction velocity in the Aδ- or Aβ-range (>30 m/s). These were subgrouped according to their adaptive properties as slowly or rapidly adapting, and according to whether they were excited by hair movement (hair follicle afferent fibers). All nociceptors were excited by noxious cold. Only 30% of nociceptors were considered sensitive to cold on initial classification with the use of a cold stimulus of 0°C. However, all nociceptors were excited by stimulus intensities