Artwork of the human brain featuring the binding sites and pathways (in red) of opiate (morphine- type) drugs. The mechanisms of pain perception & analgesia, the action of pain-killing drugs on the brain, have received intensive biochemical study. In the brainstem (rightmost two red spheres), sites involved in the transmission of pain include the nucleus raphe magnus & locus ceruleus, with other nuclei in the hypothalamus & thalamus (circles to the left). Nerve pathways (red) extend to the frontal cortex (far left) & up into the limbic system (centre-left). Sites involved in altering reactivity to pain are less well identified.