Circuit of pain of the muscles, joints, viscera in the brain. Rise of painful nerve impulses of the cutaneous, muscular, artic or visceral tissues from the nociceptors, in yellow to the spinal cord, then in red to the cortex brain through the medulla oblongata and the mesencephalon of the brain stem seen in horizontal sections. Inibition by descending routes in pink. zoom on the enkephalin neuron of the spinal cord in blue and the opiate receptors on the yellow neuron