In my last blog I explained the process of ‘nociception’ and how we feel pain. In this blog I’m going to explain how the nervous system adapts in response to pain and a process known as central sensitisation. It’s important to remember here that you don’t need tissue damage to cause pain. You only need a signal that is strong enough to be deemed as dangerous by the brain to trigger the protective mechanism of pain.
To recap when the brain receives information or a signal from peripheral tissues like muscles, joints, nerves and ligaments it makes a decision on what to do with that information. If it decides the information is too strong or dangerous it is likely to produce pain to protect the area. Once it has received this information it then has the capability to enhance the signal from the periphery, pay more attention to it and increase pain or inhibit the signal, pay less attention to it and decrease the pain. If there is extensive tissue damage it will probably pay more attention to it and increase the pain response. This makes you more protective of the area and reduces the chance of further injury, a very good survival mechanism.
Overtime, if these signals continue to be received from the periphery, adaptions start to occur in the central nervous system and in the brain. The brain begins to pay more attention to the signals and wants to know what’s going on in that area. In essence it starts allowing more signals to come through. The pathway from the painful area to the brain opens up. Before the area was painful, the pathway may have been a quiet country lane, but as signals continue and the brain pays more attention to it more lanes open up until the pathway becomes a motorway.
As the brain pays more attention to the affected area as well as opening up the pathway, the brain may also change the signals coming from the periphery. Signals that were weaker non-dangerous signals can become enhanced to strong dangerous signals or ‘nociceptive’ signals. This process is known as central sensitisation and occurs in an area of the spinal cord called the dorsal horn. The dorsal horn acts like a switchboard for information going from the periphery up to the brain.
An example of central sensitisation is feeling pain when an injury has healed or there is no injury in the first place. Let’s say for instance that you have injured your arm and signals have been sent in from the periphery to let the brain know about the injury. The brain has also been enhancing the signals and now the cells in the dorsal horn (the switchboard) are receiving lots of information and have become overstimulated. The injury in the arm heals up, but these cells remain overstimulated and continue to be protective of the arm by perceiving information in the wrong way. So that if someone lightly brushes against your arm it feels painful, even though the healing has taken place and the light brush hasn’t caused any damage.
Overtime the pain system can get stuck, with too many strong signals coming from the periphery and the brain enhancing these signals. If this carries on it will lead to chronic pain. There is evidence to show that central sensitisation is involved with lots of chronic pain conditions including chronic lower back and neck pain, temporomandibular joint disorder, Irritable bowel syndrome and fibromyalgia (Kindler et al., 2011). It is often why people have more than one painful condition at the same time and if the whole system is sensitised and hyperactive you are more likely to feel more pain generally.
So the big question, I think, is why does this happen? What is the point of making things more painful than they should be? And what is the benefit to someone of this enhanced painful response? In my next blog I will explain why I think this happens, how psychological stress and negative thinking can lead to central sensitisation and an overactive pain system and why it can be of benefit to have this response. The good news is that this is reversible and by understanding how and why the adaptations occur and changing how we think about pain we can switch this response off.