Each of these receptors is involved in different brain functions. For example, mu is responsible for the pain-relieving effects of the drug morphine. The limbic system controls emotions. Opiates change the limbic system to produce increased feelings of pleasure, relaxation and contentment. The brainstem controls things your body does automatically, like breathing or coughing.
Opiates can act on the brainstem to stop coughing or slow breathing.
The spinal cord transmits pain signals from the body. By acting here, opiates block pain messages and allow people to bear even serious injuries. They're powerful pain killers, and doctors sometimes prescribe them to control severe diarrhea. If you look on a cough medicine label, you might find that codeine is one of the ingredients. When used properly for medical purposes, opiates don't produce an intense feeling of pleasure, and patients have very little chance of becoming addicted.