The transmission of pain. Pain messages are transmitted through a peptide transmitter known as Substance P (yellow spheres) to spinal neurone cells in the spinal cord. At centre left a spinal neurone has been opened to show the relation between the Substance P and its receptor sites (yellow c-shaped). At bottom right a spinal interneurone is just visible with a short axon terminating on an axon of the spinal neurone. Here receptor sites (red c-shaped) of opiopeptins (red spheres) are found. Opiopeptins are contained in analgesic drugs and may alleviate pain because they inhibit the release of the Substance P.