Regions of the brain that perceive pain also contain receptors for pain-killers, such as morphine (from poppy plants) or endorphins (produced in the brain). Teams of young men competed in pain-endurance tests by repeatedly squeezing hand-springs until reaching unbearable pain.
During pre-competition training, some teams received injections of morphine. During competition, no morphine was administered. However, some teams thought they were receiving morphine injections. Instead, they were injected with a placebo (a saline solution) or the placebo plus naloxone (an endorphin blocking drug) as shown in the following data.
