The brain's role in pain

Our brains can evoke pain even in the complete absence of bodily damage. How do we manage this process when it becomes harmful?

 
  Our brain, not our body, is responsible for the pain that we experience.   VSRao/Pixabay  (CC0)

Our brain, not our body, is responsible for the pain that we experience. VSRao/Pixabay (CC0)

 

Pain is a universal human experience. Although we often consider pain as a measure of tissue damage, correlated with the degree of injury, this is not the case at all. Instead, pain is a protective system that allows us to change our behaviour before damage occurs. All pain, no matter how it feels, sharp or dull, strong or mild, is always a construct of the brain and is uncorrelated with tissue damage.

If there is good reason to believe that protection is required, then our brain makes pain. Sometimes, however, this pain is not useful; our nervous system becomes overprotective, producing unnecessary warning signals that cause unhelpful persistent pain.

Today in Australia, one in five people suffer from chronic pain, costing the country billions of dollars every year. Too often, patients with persistent pain are prescribed medications and treatments that, in most cases, are useless or even harmful. With the latest discoveries on the role of the brain in making pain, it is time to rethink treatment approaches. Retraining the pain system is possible.

All around our body, there are sensory neurons called nociceptors. Their function is to detect actual or potential tissue-damaging events, such as thermal, mechanical or chemical stimuli, and send a “possible threat” signal to the spinal cord. Here, a second neuron takes the message and travels up the spinal cord to the brain.

The brain makes sense of this message by drawing information from current and past experience and from the state of our mind: Where are we? What are we doing? What can we see, smell, hear? Have we been here before? What happened last time? How did we solve it? Are we stressed, frightened, relaxed?

The brain assesses how dangerous the situation is and decides on a course of action. If it perceives the situation as potentially harmful, it will produce pain to draw our full attention to it. Conversely, if the brain believes there is no need for protection, it will not produce pain. That is to say, pain is not produced in the body; it is produced in the brain. A danger message coming from the body is neither sufficient nor necessary to produce pain.

 
  Whether an injury produces pain depends very much on the context in which it occurs.   rawpixel/Unsplash  (CC0)

Whether an injury produces pain depends very much on the context in which it occurs. rawpixel/Unsplash (CC0)

 

In 1995, the British Medical Journal reported the case of a 29-year-old construction worker who arrived to the emergency department with a 15cm nail stuck into his boot. Medical staff tried to alleviate his excruciating pain with opioids and anaesthesia, with little success. The nail was then pulled out from below. When the boot was removed, doctors realised that the nail had penetrated between his toes, leaving the foot entirely uninjured. Yet, the man’s brain had perceived the situation as extremely dangerous and produced enough pain to encourage him to seek medical attention.

In 2003, an 84-year-old war veteran discovered from a routine x-ray that a bullet had been embedded in his neck since June 1940. Many are the stories of war veterans who have unknowingly lived for decades with bullets in their bodies. At the time of being shot, their brain had decided that pain would not have been a helpful cue. For those soldiers, at that moment, running for their life was more useful than worrying about their injury.

So what happens in the brain when pain persists? In people with chronic pain, pain can occur in the total absence of any physical stimulus or tissue damage.

Pain emerges with the activation of a neurotag, a network of neurons in the brain. Different neurotags can wire together to share the same brain cells; hence, the activation of one neurotag can trigger the activation of another. For example, the neurotag related to low back pain may share brain cells with the neurotag involved in how we think and feel about our low back. If we believe that our back is fragile or damaged, perhaps due to an old injury or a past trauma, that will increase the activation of the back pain neurotag.

If we expect that something dangerous will happen to our low back and we tend to have a catastrophising aptitude, the pain neurotag will consequently activate and produce more pain. Any credible evidence of danger to our back — for example, suggestive language used by our health professional or viewing models of slipped discs — will turn on or turn up the pain.

When pain persists for months or years, the pain neurotag becomes more sensitive and activates with smaller stimuli. For people who have suffered from chronic low back pain for a long time, watching someone bending forward to pick up a box may be enough of a stimulus to feel back pain. And their pain is real. 

 
  Suggestive words or imagery can sometimes be enough to evoke phantom pain.   Chuck D/Flickr  (CC BY 2.0)

Suggestive words or imagery can sometimes be enough to evoke phantom pain. Chuck D/Flickr (CC BY 2.0)

 

“I think many people in pain are in a vicious cycle that is running outside of their consciousness at an information processing level,” says Lorimer Moseley, Professor of Clinical Neurosciences and Foundation Chair in Physiotherapy at University of South Australia.

This cycle consists of thinking that pain is caused by tissue damage or pathology: Damage implies the need for protection, which in turn produces more pain. “The clear access point here — that is, the thing we can change — is the erroneous but strongly-held view that pain equals damage,” says Moseley.

Moseley and his collaborators are trialling a new way to approach chronic pain. Rather than using scans, pharmaceutical treatments or surgery, their aim is to retrain the overprotective pain system.

The RESOLVE clinical trial, led by James McAuley at Neuroscience Research Australia in Sydney, grounds on the idea that chronic pain is not a symptom, but originates from the malfunctioning of the nervous system. Matthew Bagg, a PhD student from Moseley’s research group involved in the trial, says that fewer than 10% of people have specific pain that is linked to an actual tissue problem. The remaining 90% don’t appear to have any specific cause for their pain.

Because the brain is incredibly plastic, the pain system can be retrained to its normal function. Contemporary pain education is the first and fundamental step to recovery. When a chronic pain sufferer understands that his pain does not equal damage, that vicious cycle he has fallen into can be broken. This idea may seem counterintuitive and frustrating at first, as Bagg explains to an audience of elderly chronic pain sufferers at a managing pain seminar at the War Memorial Hospital in Sydney. However, when this knowledge shift happens, patients can engage again in movement, exercise, social and pleasurable activities feeling safe. This will slowly reduce the perceived need of protection and thus pain.

“A quick fix is unlikely but recovery is back on the table,” says Moseley. “It will take a journey and they will have to drive it, but there are good people around to give them the resources to do so.”

Edited by Andrew Katsis