Facilitation of the human nociceptive reflex by stimulation of A beta-fibres in a secondary hyperalgesic area sustained by nociceptive input from the primary hyperalgesic area.

Hyperalgesia was induced in healthy volunteers by topical capsaicin applied on the dorsum of the foot within the receptive field of the sural nerve. Under presence of hyperalgesia different normally non-noxious conditioning stimuli were applied to the hyperalgesic area and the polysynaptic nociceptive spinal reflex and pain ratings were used to assess central excitability. The nociceptive reflex was measured in the knee extensor and flexor muscles evoked by electrical stimulation of the sural nerve trunk at an intensity of 1.5 times the initial reflex threshold (an intensity above the pain threshold). Thermal stimulation of the primary hyperalgesic area (re)established both on-going spontaneous pain and secondary hyperalgesia. Thus, increased nociceptive reflexes were recorded and increased pain intensity reported when A beta-fibres in the secondary hyperalgesic area were activated concurrently with the reflex testing after a non-noxious thermal stimulation of the primary hyperalgesic area. The A beta-fibre activation was achieved by continuous low-intensity electrical stimulation (40 Hz) that was initiated after on-going pain produced by the thermal stimulation had waned. The same measurement without prior thermal conditioning stimulation of the primary area resulted in no reflex facilitation, indicating rapid changes in the central excitability with existence of on-going nociceptive activity. This indicates that the development and maintenance of secondary hyperalgesia are dependent on sustained peripheral nociceptive activity. The study also shows that a central summation of nociceptive and non-nociceptive afferent activity can occur once secondary hyperalgesia is present.