Undamaged nerve fibres - not those that are injured - may cause long-term chronic pain, research suggests.
Millions suffer from chronic pain
Ongoing pain affects one-in-five adults across Europe, and costs an estimated £23 billion a year in lost work days.
Inflammation caused by damaged nerve fibres triggered nearby undamaged ones to send signals to the brain, the University of Bristol researchers said.
In the journal Neuroscience, they say their finding may aid the development of more effective painkillers.
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.
It is particularly difficult to live with because it is often impossible to treat with currently available painkillers.
Previous research into ongoing pain has tended to focus on the damaged nerve fibres after injury or disease and overlooked the intact fibres.
Key nerve cells
Lead researcher Professor Sally 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."
The Bristol team found that the key was nerve cells called nociceptors, 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 fibres fire, the stronger the pain becomes.
The Bristol team found that firing seems to be triggered in undamaged fibres by inflammation produced by neighbouring dying or degenerating fibres which have been damaged by injury within the same nerve.
The researchers said further work was needed to establish how this mechanism might contribute to the ongoing pain associated with a wide variety of diseases.
Dr Beverly Collett, president of the British Pain Society, said: "This is important because it throws more light on to what happens when people suffer neuropathic pain from trauma, surgery, or conditions like diabetes and shingles.
"Any research like this that helps us to understand why people suffer from pain should ultimately help us to develop new treatments."