Pain has been considered as part of a defensive strategy whose specific role is to signal an immediate active danger to the organism. This definition fits well for acute pain. It does not work well, however, for chronic pain that is maintained even in absence of an ongoing, active threat. Currently, acute and chronic pain are considered to be separate conditions.
What follows is a review of the different theories about pain and its history. Different hypotheses regarding pain mechanisms are illustrated. New data emerging from scientific research on chronic pain (migraine in particular) involving innovative imaging techniques are reported and discussed.