Pain is usually generated in some peripheral part of the body (muscles, joints, organs skin etc) and travels to the brain via the spinal cord or via the cranial nerves. The pain signals can be modified (increased or decreased) at various different points along the pathway to the brain. This communication goes both ways- there are pathways from the brain to the peripheries which are meant to tone down the "volume" of pain if the brain assesses the pain as non-threatening or less important than something else one is concentrating on. For example, if you hurt yourself while trying to score a touchdown in the middle of a football match you may not notice the pain as much as if you felt the same pain while feeling anxious about something.
There are a number of chronically-painful conditions which are frustrating for patients and often difficult for doctors to treat. Pain is perceived in the brain. In some of these it is as though the "pain dial" has been turned to maximum.
The BC Women's Hosptial has a useful article on a number of those overlapping painful conditions in which the central nervous system has become excessively sensitized to pain which can be found here.
David is a fan of books and no doubt will be sharing some good reads here.