[Neurophysiological mechanisms of pain perception]

Information on noxious stimuli is perceived in the periphery by free nerve endings and conveyed to the spinal cord by A delta and C-fibres. They mostly terminate within the substantia gelatinosa (SG) of the dorsal horn. Within this structure, transmission of pain information is modified by serotoninergic and enkephalinergic mechanisms. Neurones from the deeper layers of the dorsal horn relay pain information via the ventrolateral tract to the brain stem reticular formation and thalamic nuclei. From there, pathways to the somatosensory cortex, basal ganglia, and limbic structures were described, which probably mediate conscious perception, subconscious motor reactions and emotional colouring of pain perception, respectively. For the efferent control of pain transmission, the periaqueductal gray matter (PGM) is the central structure. Electrical stimulation of PGM can relieve patients at least temporarily from states of chronic pain. PGM is connected to the midbrain raphe nuclei, from where serotoninergic fibres descend to the spinal cord to modify pain transmission in neurones within SG. Enkephalin is present in the terminals of SG interneurones and reduces pain transmission by acting on opiate receptors on the terminals of nociceptive primary afferents and probably also postsynaptic sites. Analgesia produced by natural or electrical stimulation of other cutaneous afferents and by acupuncture is believed to be mediated by these enkephalin-containing interneurones within SG of the dorsal horn.