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Schematic drawing of the nociceptive system with ascending and descending fibers. RF: reticular formation; PAG: periaqueductal gray; NRM: nucleus raphe magnus; NRGC: nucleus reticularis gigantocellularis; NRPG: nucleus reticularis paragigantocellularis; NRL: nucleus reticularis lateralis; 5-HT: 5-hydroxytryptamine; ENK: enkephalin; NA: noradrenaline.
