Sex differences in nociceptive withdrawal reflex and pain perception.

Experimentally induced pain often reveals sex differences, with higher pain sensitivity in females. The degree of differences has been shown to depend on the stimulation and assessment methods. Since sex differences in pain develop anywhere along the physiological and psychological components of the nociceptive system, we intended to compare the nociceptive flexion reflex (NFR) as a more physiological (spinal) aspect of pain procession to the verbal pain report of intensity and unpleasantness as the more psychological (cortical) aspect. Twenty female and twenty male healthy university students were investigated by use of nociceptive flexion reflex threshold (staircase method) after electrical stimulation of the N. suralis. Furthermore, we assessed supra-threshold reflex responses (latency, amplitude and area) by applying 10 stimuli 5 mA above reflex threshold. Following each stimulation, the subjects provided pain ratings of intensity and unpleasantness on a visual analogue scale. Females exhibited marked lower nociceptive flexion reflex thresholds than males, while the supra-threshold reflex response tailored to the individual reflex threshold did not show any significant differences. The verbal pain ratings, corrected for NFR threshold, were not found to differ significantly. The large sex differences in nociception that were present in NFR threshold but not in the pain ratings corroborate the hypothesis that spinal processes contribute substantially to sex differences in pain procession.