Site of action of morphine. Increased painful nerve impulses from the cutaneous, muscular, articular or visceral tissues from the nociceptors, in yellow to the spinal cord, then red to the cerebral cortex via the medulla oblongata and the mesencephalon of the brainstem seen in horizontal cuts. Inhibition by descending pathways in pink. zoom on the enkephalin neuron of the spinal cord in blue and the opiate receptors on the yellow neuron