Dr. Vania Apkarian, a professor at the Northwestern University Feinberg School of Medicine, published a study last year that revealed how chronic back pain actually registers in a different part of the brain (the prefrontal cortex) than acute pain (the thalamus). Dr. Apkarian also found that the longer a person has been suffering from chronic pain, the more activity there was in the prefrontal cortex. He was able to accurately determine the number of years the person had experienced back pain just by reading the MRI scan - the scans showed a cumulative tape of how much pain the brain had registered.
Basically, Dr. Apkarian has identified that with chronic pain there appears to be an old memory that essentially gets stuck in a rut in the prefrontal cortex - the part of the brain that governs emotions and learning. Even after the injury/original problem that caused the pain is healed, the brain seems to keep remembering the problem and can't forget it.
More recently, in a follow-up study using laboratory rats, Dr. Apkarian has found that the drug D-Cycloserine seems to help erase or at least dampen that memory. The rats in the study who had chronic pain from an injured limb seemed to be pain-free after 30 days of treatment with the drug.
If this drug works in humans, it would be a very exciting development for many of those coping with chronic back pain.