Sunday, 7 September 2014

How we feel and the causes of pain – Part 1 ‘Nociception’

This blog is part of a series of blogs I’m writing explaining how we feel pain and the causes of pain. In this first part I will give an explanation of what is known as ‘nociceptive’ pain. This is the type of pain you feel if you stub your toe, cut yourself or anything else that causes an initial quick sharp pain. It’s your brain and body’s initial alarm system, to protect you from damage, potential damage or the perception of damage. That is right if you get pain it doesn’t always mean you’ve damaged yourself, I’ll explain why.

It is now commonly accepted that all pain is produced by the brain. It was previously thought that we have ‘pain receptors’ in the body’s tissue that would send pain signals to the brain to let it know something had been damaged. We now know that we don’t have ‘pain receptors’ and what we do have is receptors for mechanical load, chemical stimulation and temperature.   What usually happens when we feel pain is that there is sufficient input into these receptors, to trigger a fast acting signal from the periphery (Muscles, joints, ligaments etc), into the central nervous system, up to the brain to let it know there is potential danger in that area of the body. The input needs to be strong enough to stimulate receptors beyond there threshold to activate a signal into the central nervous system, for instance if you put your hand on a hot pan or drop a heavy weight on your toe. This is known as a ‘nociceptive signal’. These receptors won’t be activated unless there is sufficient input i.e. high mechanical load (banging your thumb with a hammer), high or low temperatures or chemical irritation.

When the brain receives this information it then has to decide whether to cause pain. Most of the time if the brain receives nociceptive information it will cause pain. This is where it becomes interesting, because there are times when it may decide that it is not advantageous to cause pain at that moment, even if there is actual tissue damage. There are also times when there may not be any damage, but the brain perceives things to be worse than they really are and causes pain anyway. I will go on to explain situations where this can happen in later blogs. So, this strong ‘nociceptive’ information is received by a certain part of the brain, what happens next is that this area of the brain then communicates very quickly with other parts of the brain. It will communicate with the visual cortex to see if it has noticed the danger; it may communicate with the amygdala or insula, where we store our fear memories to see if they think this situation is dangerous or the auditory cortex to see if it heard anything dangerous. Your brain very quickly collates all this information and then decides whether to cause pain or not.

Why would the brain decide not to cause pain? Well it maybe something simple, like it was distracted at the time an injury occurred and not enough danger information was received. Have you ever cut yourself or had a bruise that you didn’t notice, but only became painful once you did see it? Your brain may have been distracted by something more important at the time the injury occurred and didn’t register the injury, so it didn’t see the need to cause pain. It’s only when it gets the visual input that it realises it needs to protect this area, so that is when it causes pain.

There are also times when even though the brain recognises there is damage it still decides not to cause pain, for instance when you are playing sport or if you are a soldier in battle. If you are trying to win a match and you get injured your brain may decide it does not want to cause pain, as the pain
will prevent you from playing well and winning the match. A lot of elite athletes often compete with injuries, but the motivation of winning overrides the pain. Likewise, if you are a soldier in a war zone and have just been shot, you may still be in danger and there may still be a risk of further injury or fatality. Again the brain may decide not to cause pain at this time as there is still a greater risk and you may lose your life, so pain at that time may distract you from survival and would be disadvantageous.

In the same way that the brain can decide not to cause pain even when there is severe tissue damage, pain can be triggered when there is no tissue damage at all. Over the next series of blogs I’ll explain how and why this occurs and how the brain and nervous system can adapt to continue causing pain long term, even without tissue damage and how a build-up of stress and negative thinking can cause pain and lead to these adaptive changes in the nervous system being maintained.

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