Supraspinal influences on nociceptive flexion reflex and pain sensation in man.

The sensation of pain and the nociceptive flexion reflex of the biceps femoris muscle (RIII, Bi) elicited by electrical stimulation of the ipsilateral sural nerve were studied in human during 4 conditions: (1) a mental task; (2) a stress; (3) during noxious stimulation of the contralateral ulnar nerve; (4) after an intense noxious stimulation of the sural nerve itself. An inhibition of both pain sensation and RIII, Bi was observed in (1), while a facilitation of these parameters was noted in (2). In contrast, no change in RIII, Bi occurred in (3) while the pain sensation was inhibited. In (4), just after the noxious stimulation, there was a marked facilitation of RIII, Bi associated with increased pain sensation. However, 10--12 sec later, the pain sensation returned to its control values, while the RIII, Bi reflex was still facilitated. The latter recovered to its control values 28--30 sec later. These results show the possibility of a dissociation between afferent ascending nociceptive messages and nociceptive motor activity (in 3 and 4). They suggest that supraspinal descending influences can act differently on spinal dorsal horn neurons in the case of pain ascending volleys, and in the case of spinal nociceptive motor activity.