Interactions between nociceptive and non-nociceptive afferent projections to cerebral cortex in humans.

We investigated the effect of a tonic discharge of muscle nociceptive afferents on somatosensory evoked potentials (SEPs) in humans in response to stimulation of non-nociceptive afferents arising from the same muscle. Conditioning nociceptive muscle stimulation was achieved by local injection of 50 mg levo-ascorbic acid (in a volume of 0.3 ml) in the body of the extensor digitorum brevis muscle (EDB). The test stimulus for SEPs was an electrical pulse applied to the EDB nerve at an intensity below the motor threshold. The main finding was that tonic muscle nociceptive stimulation strongly depressed the middle-latency P60-N75 complex without modifying the size of the early P40-N50 complex of SEPs. Depression of the P60-N75 complex was correlated with the pain-induced loss of proprioception of the foot, making it plausible that this cortical complex reflects neuronal processes leading to perception.