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. 2008 Mar;135(1-2):196-208.
doi: 10.1016/j.pain.2007.11.013. Epub 2008 Jan 14.

Nociceptive sensations evoked from 'spots' in the skin by mild cooling and heating

Barry G Green  1 , Carolyn Roman, Kate Schoen, Hannah Collins
Affiliations

Nociceptive sensations evoked from 'spots' in the skin by mild cooling and heating

Barry G Green et al. Pain. 2008 Mar.

Abstract

It was recently found that nociceptive sensations (stinging, pricking, or burning) can be evoked by cooling or heating the skin to innocuous temperatures (e.g., 29 and 37 degrees C). Here, we show that this low-threshold thermal nociception (LTN) can be traced to sensitive 'spots' in the skin equivalent to classically defined warm spots and cold spots. Because earlier work had shown that LTN is inhibited by simply touching a thermode to the skin, a spatial search procedure was devised that minimized tactile stimulation by sliding small thermodes (16 and 1mm(2)) set to 28 or 36 degrees C slowly across the lubricated skin of the forearm. The procedure uncovered three types of temperature-sensitive sites (thermal, bimodal, and nociceptive) that contained one or more thermal, nociceptive, or (rarely) bimodal spots. Repeated testing indicated that bimodal and nociceptive sites were less stable over time than thermal sites, and that mechanical contact differentially inhibited nociceptive sensations. Intensity ratings collected over a range of temperatures showed that LTN increased monotonically on heat-sensitive sites but not on cold-sensitive sites. These results provide psychophysical evidence that stimulation from primary afferent fibers with thresholds in the range of warm fibers and cold fibers is relayed to the pain pathway. However, the labile nature of LTN implies that these low-threshold nociceptive inputs are subject to inhibitory controls. The implications of these findings for the roles of putative temperature receptors and nociceptors in innocuous thermoreception and thermal pain are discussed.

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Figures

Figure 1
Figure 1
Shown are the percentages of temperature-sensitive sites classified as thermal, bimodal and nociceptive found using the 0.16 cm2 Peltier thermode in the sliding search procedure of Exp. 1. The graphs on the left and right side of the figure contain the results obtained with the 28° and 36°C stimuli, respectively. The number of each type of site that was found is indicated in the parentheses above each bar.
Figure 2
Figure 2
The percentage of temperature-sensitive sites in which multiple (gray bars) or single (hatched bars) temperature-sensitive spots were found during sliding searches with the 1.0 mm2 thermode adjusted to 28° (top) or 36°C (bottom). The number of each type of site that was studied is indicated in the parentheses above each pair of bars.
Figure 3
Figure 3
The percentages of thermal (Th), bimodal (Bi) and nociceptive (Noci) spots found within cold-sensitive (a,b,c) and heat-sensitive (d,e,f) thermal, bimodal and nociceptive sites. The spots were located using the 1.0 mm2 thermode and the sliding search procedure. The N indicated in each graph refers to the total number of spots studied for each category of site.
Figure 4
Figure 4
The stability of cold- and heat-sensitive sites is expressed as the percentage of each type of site that was found when the originally identified sites were retested to obtain ratings of perceived intensity during sliding contact. Complete stability would be indicated by 100% of the sites remaining in the original category (indicated by the arrows). Th = thermal sites; Bi = bimodal sites; Noci = nociceptive sites; Ins = Insensitive sites. Note that bimodal and nociceptive sites, which were more unstable than thermal sites, more often became thermal or bimodal sites than becoming insensitive.
Figure 5
Figure 5
Shown are the log means of perceived intensity ratings given in response to the 28° and 36° stimuli on cold- and heat-sensitive sites under conditions of sliding (open bars) and normal (hatched bars) contact. Vertical bars indicate standard errors of the means. Letters on the right y-axis of each graph represent descriptors on the psychophysical scale of perceived intensity (gLMS) that was used: NS = no sensation; BD = barely detectable; W = weak; M = moderate; S = Strong. The dashed lines highlight the finding that during normal contact, nociceptive sensations were on average rated below ‘barely detectable’.
Figure 6
Figure 6
The qualities of sensation reported in the sliding (open bars) and normal contact (hatched bars) conditions of experiment 1 for cold- and heat-sensitive sites expressed as percentages of the total number of trials on which thermal stimulation was delivered. The data for bimodal and nociceptive sites have been combined.
Figure 7
Figure 7
Log mean ratings of perceived intensity of thermal (empty circles) and nociceptive (filled circles) sensations are shown for cold- and heat-sensitive sites as a function of stimulus temperature. Vertical bars indicate standard errors of the means. Letters on the right y-axis of each graph represent descriptors on the psychophysical scale of perceived intensity (gLMS) that was used: NS = no sensation; BD = barely detectable; W = weak; M = moderate; S = Strong. The dashed line provides a reference to indicate stimuli that were rated above ‘barely detectable’.
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