The ability to sense pain is vitally important. When a painful sensation occurs, a signal is transmitted by the nerves from the site of the pain to the spinal cord and brain. On its journey to the brain, the pain signal will travel along many nerves, being passed from one nerve to another by chemical messengers.Naturally occurring opioids in the body - such as endorphins - prevent the pain signal from being passed between the nerves. The three active ingredients of opium - morphine, codeine and thebaine - mimic the action of the endorphins, preventing the transmission of the pain sensation.