Further studies on the role of afferent input from relatively large diameter fibers in transmission of nociceptive messages in humans.

The effects of a sural nerve conditioning stimulation (S2) were studied on nociceptive flexion reflexes (RIII) from the biceps femoris muscle elicited by a test stimulation (S1) of the same sural nerve. The experimental procedure was designed so as to allow an indirect measurement of the conduction velocity (c.v.) of the conditioning inputs. When the S2 parameters produced an activation of afferent fibers belonging only to the A alpha beta group, it produced a facilitation of the RIII reflex which was maximal when 40 m/s conducting fibers were recruited. In contrast, when the S2 parameters activated also the A delta and C groups, the facilitatory effects were maximal when 20 m/s and 1 m/s conducting fibers were activated.