Pain and its mechanisms are very complex events. It's not fully understood by the most knowledgeable researchers.
We do know that tissue damage causes the release of many inflammatory cells. The result is to get receptors going that pick up painful messages in the area of injury. The more receptors that get fired up, the quicker pain is felt.
Not only that, but the tissues are actually sensitive to psychosocial factors. Feelings of helplessness and loss of control are linked with increased pain. Injury and pain in a local area actually results in a global response. In other words, the person feels pain in other areas of the body far away from the spine with much less pressure or provocation compared to someone with no injury.
Many scientists are studying the process of pain. So far we seem to have more questions than answers in this area.