Pain appears to be a simple thing to understand, you stub your toe - it hurts, you're obviously feeling the damage to the body's tissues whether that be the bone, tendon, muscle etc.
But it's not quite that simple, say, for example, instead of stubbing your toe you get a splinter in your foot, if you've ever had a splinter you'll know how horrible this can feel. But when you look at your foot to try and get the splinter out, wow you notice a big bruise on your leg, how did that get there? You didn't feel it happen and yet it plainly has done more damage to your body's tissues than the splinter did. In this example the pain you feel has no correlation to the amount of tissue damage.
Let's go back again to the example of stubbing your toe.
Say you're getting yourself ready for a fantastic night out, you're in a great mode, you've been looking forward to it for ages and bang you stub your toe on the side of the wardrobe, it stings, maybe makes you hop for a moment or two, you check it out, nothings appears broken, you can still wiggle it, a little sore maybe but quickly you're back to what you were doing and by the time you head out the door you've forgotten all about it.
The question is, would it feel just the same if you had been huffing round the room in a bad mood really cross at something or someone? I suspect you'd feel it little more intensely and for considerably longer even though the actual incident was the same. What changed here was your mood and the surrounding circumstances, not the tissue damage.
What these simple examples show us is there are many more factors involved in how we experience pain not just a nerve signal about tissue damage, in fact sometimes, as in the case of chronic or persistant pain, you don't need any nerve signal at all.
This way of looking at pain is called "The Neuromatrix Theory of Pain", the theory basically suggests that pain is an output or experience constructed in the brain from many different inputs a bit like an orchestra playing music, the different instruments are all inputs which when played together produce an output which is the music.
A nerve signal to the brain about a particular event that's happened in your body may be one of those inputs in to the neuromatrix but depending on the other inputs may or may not be output as a sensation of pain.
There isn't really any such thing as a pain receptor that sends pain signals to your brain. We have an array of different types of sensory receptors in our body, Proprioceptors which provide information about our balance and position in space and between body parts, Mechanoreceptors which detect changes such as pressure and temperature and Nociceptors which detect noxious stimuli or things that could be a threat to the system.
It is Nociceptors which are often confusingly described as pain receptors, but as we have seen already, pain is far more than just a signal from a specific type of nerve or sensory receptor.
Other inputs to the neuromatrix could include:
- Other sensory inputs from mechanoreceptors and proprioceptors
- Memories of past events and experiences
- Anxieties, fears, beliefs, consequences
- Emotions, mood, stress levels, hormones
This list is not exhaustive but gives an idea of some things your brain may use to work out if a particular event or experience should be output as pain.
Does this mean pain is all in your mind? Not in the sense that most people use that phrase, which is to suggest that the pain you're experiencing is in some way made up and you could if you so wished stop feeling it. Pain however is very real and often very debilitating, it is a part of being being human and being alive and on an evolutionary level is there to protect us, but sometimes it stops us doing things we want or need to do and sometimes the whole process can become problematic.
The Neuromatrix theory of pain, put forward by Ronald Melzack Ph.D. in "Pain and the Neuromatrix in the Brain" is the most current and influential theory on how we experience pain.
"Reconceptualising Pain According to Modern Pain Science" Lorimer Moseley