Computer illustration of three of the major subtypes of opioid receptor. At left is the mu-opioid receptor, binding morphine. At centre the delta-opioid receptor binding an endorphin, and at right the kappa-opioid receptor binding a dynorphin. Opioid receptors are found on nerve cells. When pain is detected by the body, endorphins are released and attach to the receptors, starting a reaction that modulate pain perception. Opioid drugs such as morphine mimic endorphins.