Figure 2.

This figure shows schematically the descending inhibitory pathways. Areas shaded grey display a high expression of opioid receptors and their endogenous ligands. MOP agonists produce analgesia either by indirectly increasing neuronal traffic through the descending pathway at the NRPG and PAG, or by directly inhibiting nociceptive afferents in the periphery. MOP agonists act at the NRM to indirectly inhibit spinal pain transmission and, in addition, reduce spinal nociception. PAG = periaqueductal grey; NRPG = nucleus reticularis paragigantocellularis; NRM = nucleus raphe magnus; LC = locus correolus.