Is the R3 component of the human blink reflex nociceptive in origin?

The R3 component of the blink reflex can reproducibly be evoked by noxious stimulation but can probably also be elicited by innocuous stimuli. This study was conducted to investigate the contribution of nociceptive A delta and C fibers to the generation of the electrically evoked R3 blink reflex. Electrical thresholds for detection, pain and all blink reflex components were determined and the modulatory effects of local anesthesia were investigated. The electrical R3 threshold of 4.6 +/- 0.5 mA (mean +/- SE) corresponded to 2.9 times the detection threshold and to 0.35 times the pain threshold. The R3 threshold was significantly below the pain threshold. Under local anesthesia of the supraorbital skin with a complete loss of warm and cold sensation, a loss of pinprick sensation, but a normal detection of tactile stimuli, the electrical pain threshold increased, all other thresholds remained unchanged. Under local anesthesia none of the reflex components were significantly reduced. Cutaneous A beta fibers and nociceptive A delta fibers, but not unmyelinated C fibers, contribute to the generation of the electrically evoked R3 component. According to the recruitment order in peripheral sensory nerves the electrical threshold of the R3 is mainly determined by activation of A beta fibers. Thus, it can not be assumed that the electrically evoked R3 is an adequate model to investigate nociceptive processing.