Comparative study of perceived pain and nociceptive flexion reflex in man

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Abstract
The amplitude of the flexion reflex of the biceps femoris muscle (BF) was compared with the intensity of the painful sensation elicited by a nociceptive stimulation resulting from application of a constant-current either on the sural nerve or on the skin in its distal receptive field. Experiments were carried out on 15 normal volunteers. Stimulation of the sural nerve (either on or through the skin) elicited 2 different reflex responses in the BF: the 1st (RII) was of short latency, low threshold and corresponds to a tactile reflex. The 2nd (RIII) was of longer latency and higher threshold, and corresponds to a nociceptive reflex. The threshold of RIII was the threshold of a pain sensation. Stimulation of the skin elicited only a late nociceptive (RIII) response in the BF. The threshold of this response was also that of pain. The thresholds of both pain and RIII were higher for sural nerve stimulation (10 mA) than for cutaneous stimulation (5 mA). The large diameter cutaneous fibers could have an inhibitory effect on both pain and the nociceptive reflex. This was supported by a selective ischemic block of the largest diameter fibers in the sural nerve, when a 10 mA stimulation was applied to the nerve. In this case, a decrease of the RII reflex was observed in BF, together with an increase of both RIII and pain sensation. Functional implications of these results were discussed.