Nociceptive flexion reflex

The nociceptive flexion reflex, or nociceptor flexion response (NFR) or RIII reflex is a muscle withdrawal reflex involving numerous spinal cord synapses in response to activation of A-delta nociceptors (fast pain signalling nerve fibers). The NFR is measured by using an electromyogram to monitor the muscle activity in the upper leg (biceps femoris) while applying increasing electrical stimulation to the lower leg (sural nerve) on the same side of the body. The stimulus intensity at which the NFR is evoked is often the intensity at which the subject reports the onset of pain, and the strength of the NFR is correlated with the strength of the pain experienced.