Figure 3. Response of area 3a neurons to thermoneutral vs. thermonoxious skin contact (from Whitsel et al., 2009).

A: Spike train data obtained from a neuron in area 3a using indent-and-hold stimulation – on each trial the skin was indented 0.5mm for 7s. For this neuron, 51°C (trials 7-12) evoked a significant response, but not 38°C (trials 1-6 and 13-18). B: Spike trains obtained from a neuron in area 3a, using a “wind-up” protocol which evokes slow/2^nd/burning pain in humans. Brief (0.8 sec duration) skin indentations were delivered repetitively (1 stimulus every 2 sec). In contrast to this neuron's insensitivity to 38°C contact, a significant elevation of spike firing occurred during exposures to 55°C stimulation, and this elevation of spike firing persisted following the stimulus. Panels at top: raster-type displays of spike trains recorded during and following a series of successive contacts delivered to the skin by a probe maintained at a thermoneutral (38°C) or thermonoxious temperatures (51°C – neuron in A; 55°C – neuron in B). Vertical bar indicates time of probe retraction. Panels in second row: superimposed PST histograms showing mean firing rates (MFR) in response to 38°C (dark shading) vs. 51 ° C or 55°C (light shading) skin contact; horizontal arrow along the ordinate indicates spontaneous (no-stimulus) firing rate. Panels in third row: “difference PSTs” showing the difference between the mean firing rates (ΔMFR) recorded in noxious test trials vs. non-noxious control trials. Bottom panels: difference PSTs showing the difference between the mean firing rate recorded in non-noxious recovery trials (13-18) vs. non-noxious control trials (1-6).