Pain is a sensation unique to an individual. Its perception depends on the injury involved and the situation or context. A bruise obtained in a football game may not be appreciated at the time of the injury, yet in other circumstances the pain from a minor injury, such as stubbing a toe, may be overwhelming. The extent of the injury does not predict the amount of pain experienced; it is this wide variability that makes the treatment of pain difficult.

Within the brain, there are two systems that can appreciate the sensation of pain. One deals with the objective component and tells the exact location of the injury and what type of injury it is. The other is more diffuse and comprises the "hurt." Many people have experienced both types of pain. Touching a hot object or stubbing a toe is quickly...