Comparative study of perceived pain and nociceptive flexion reflex in man

The purpose of this study was to compare the amplitude of the flexion reflex of the biceps femoris muscle (BF) 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.It was observed that: (1) Stimulation of the sural nerve (either on or through the skin) elicits two different reflex responses in the BF: the first (RII) is of short latency, low threshold and corresponds to a tactile reflex. The second (RIII) is of longer latency and higher threshold, and corresponds to a nociceptive reflex. The threshold of RIII was found to be the threshold of a pain sensation. (2) Stimulation of the skin elicits only a late nociceptive (RIII) response in the BF. The threshold of this response was also found to be that of pain. (3) The thresholds of both pain and RIII were found to be higher for sural nerve stimulation (10 mA) than for cutaneous stimulation (5 mA).It was suggested that the large diameter cutaneous fibers could have an inhibitory effect on both pain and the nociceptive reflex. This was supported by the results obtained during 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 are discussed.