These unexpected findings, by Dr Laiche Djouhri, Professor Sally Lawson and colleagues from the University of Bristol, UK, are reported in the Journal of Neuroscience today [25 January, 2006].
Previous research into ongoing chronic pain has tended to focus on the damaged nerve fibres after injury or disease and overlooked the intact fibres. This new understanding may help pharmaceutical companies formulate novel pain killers.
Professor Lawson said: "The cause of this ongoing pain and why it arises spontaneously was not understood before. Now that we know the type of nerve fibres involved, and especially that it is the undamaged fibres that cause this pain, we can examine them to find out what causes them to continually send impulses to the brain. This should help in the search for new analgesics that are effective for controlling ongoing pain."
Ongoing pain is a burning or sharp stabbing/shooting pain that can occur spontaneously after nerve injury. Unlike 'evoked' pain caused, for example, by hitting your thumb with a hammer, ongoing pain is particularly difficult to live with because it is often impossible to treat with currently available pain killers.
Djouhri and Lawson show that the nerve cells responsible are 'nociceptors' or damage detectors. There are thousands of these nerves cells, each of which has a very long, fine nerve fibre emerging from it. These fibres run within nerves and connect the skin or other tissues to the spinal cord.
When activated through damage or disease, these nerve fibres fire electrical impulses that travel along the fibre from the site of injury to the spinal cord, from where information is sent to the brain. The faster the undamaged fine fibres fire, the stronger the ongoing pain becomes.
Dr Djouhri added: "The cause of this firing appears to be inflammation within the nerves or tissues, caused by dying or degeneration of the injured nerve fibres within the same nerve."
The mechanism described by Djouhri and Lawson occurs following nerve injury and in nerve and tissue inflammation. Further research is now needed to establish how generally this mechanism may contribute to ongoing pain associated with a wide variety of diseases such as back pain or shingles.