Analgesia

The first line of defense against pain in the body is the endorphins. These chemicals are peptides (compounds of amino acids) found in the brain and parts of the central nervous system. Like opiates, they produce a sense of well being (euphoria) and relieve pain. Endorphins are released from a transmitting nerve cell and then bind to the receptor sites of a receiving cell. After an endorphin sends the message to block pain signals to the receptor site, it is annihilated, thus allowing other endorphins to be produced.

Morphine molecules flow through the spaces (synapses) between these sending and receiving cells and position themselves on the receptor sites, locking out endorphins. The morphine molecule sends the same message as the endorphin to block the pain signal. With long-term use of morphine, the endorphins decline in number and so do the receptor sites, leading to the twin problems of addiction and drug tolerance.