Acute pain could be seen as a biological protection so that animals can sense danger or if they are injured they can move away from the danger. And obviously what you need is some sort of receptor out on the periphery that can pick up the danger and in some ways relay that to the centre through some sort of wiring system which is the nervous system.

So what we have is pain receptors out on the periphery and nerves which relay this to your spinal cord, which is like a relay station except it’s more active than that, and then it goes up to your brain and the brain sort of sorts out what’s going on here and makes you pay attention to it and tells you what’s wrong and makes you take action.

So acute pain acts as a sort of warning system and tells the animal either to avoid something or that there’s been damage so to remove itself and if you can think in terms of designing some sort of robot or something so that it wouldn’t injure itself any further.

What you’d want to do would be to have this acute warning system to say something’s gone wrong, get out of the way, and if you’ve injured a part, stop using it. And you’d want to stop using that part, say you’d broken your wrist you’d want to stop using your arm and your hand until the wrist had healed and then once it had all healed you’d want to be able to use it again.

So what happens is the human nervous system changes during the time of that injury and during the first part of healing so that pain signals are amplified and made much worse and that makes the animal reluctant to use that so it will keep it still and it won’t hurt.

Then once the healing process has happened the nervous system goes back to normal and hopefully the person won’t suffer pain anymore. The problem is what happens if it doesn’t go back to normal after healing. And we think that this wind up, sensitisation or whatever you like to call it in the nervous system in some people doesn’t go back to normal after healing so some patients are left with a sensitised nervous system so the slightest thing causes pain and it doesn’t resolve after the healing process which it should have done.

In the old days we used to think there was just one pain pathway where it ran, say I injured my finger, it would run from the tip of my finger, up a nerve in my arm, into my neck, into the spinal cord and then up to the brain. We now know that it’s a lot more complicated than that and maybe we could say there are two main systems.

The first system the nerve delivers it up to the spinal cord and then one pathway goes up into the sensory part of the brain through an area called the thalamus which acts as a sort of relay station in the brain and that system tells you you’ve injured your finger, it feels like a pinprick or a burn or whatever and it tells you in other words the discriminatory aspects. Where has been hurt and what sort of injury it is. But it doesn’t really make you do anything about it.

The other pathway goes from the spinal cord through a different route in the brain up into the emotional centres in the brain around the limbic system and other areas and that grabs your attention, you cannot ignore this.

And this makes pain different from other things like say vision or sound. These pain messages go straight into the attention mechanisms so you can’t just say ‘Oh that’s interesting. I’ve got my hand in a fire. It makes you do something about. It grabs your attention and says ‘You must remove your hand. It also tells you there’s something very unpleasant going on.

So the combination of the two, you’ve got one system saying ‘Pay attention. Something really nasty’s going on’ and then you’ve got another bit of your brain which says ‘Oh, it’s your finger and you’ve stuck it in a fire’ and the two go well together and enables us to take avoiding action and to then get out of that situation that’s causing the damage.

One good way of explaining to people the sort of mechanism we are talking about with sensitisation in the nervous system would be the analogy of sunburn. You imagine you go off to Spain on your holiday and you’re sitting on the balcony of your hotel, reading a really good book and you totally forget that you’ve gone from Britain to Spain and it’s really hot and the sun’s beating down and your right shoulder is in the sun. Your left shoulder is in the shade. Your right shoulder gets badly sunburned.

The next day you take a shower. Well what’s going to happen? Your left shoulder which is normal it’s just going to be fine. It’s just going to be warm water. But your right shoulder, the one that was sunburnt, it’s going to be agony. Now why is that when it’s the same stimulus on the two shoulders? Now what’s happening here is that the nerves that go to this shoulder have been changed by nasty chemicals released by the damage to the skin. It isn’t the damage to the skin itself that causes the sensitisation and if you think, that’s logical.

Damaging things doesn’t make them work better, it’s a result of that damage. When you damage the skin, nasty chemicals called inflammatory mediators, one of them is prostaglandin gets out from all these tissues onto the nerves and that sensitises the nerves and for example brings down the threshold at which that fibre would fire for pain. Normally this would be about 45 degrees. So on this shoulder you would have normal skin with normal nerve endings which would tell you it’s painful at 45 degrees. On this shoulder the threshold might have gone down to 38 degrees.

So you go on a shower at 40 degrees, this shoulder is 5 degrees below the pain threshold, that’s fine. But this shoulder at 40 degrees that water is 2 degrees above the pain threshold so it says ‘That’s very painful. The lesson here is the stimulus is the same. It’s the nervous system that’s changed and whether or not someone feels pain doesn’t just depend on the stimulus. It depends on the state of your nervous system as well.

If we pursue the sunburn analogy a little further you imagine the day after you’ve had the sunburn things are at their worst. You’ve damaged the skin, everything is all wound up. Over the next few days things gradually heal. The skin will heal and gradually come back to normal, and the nervous system will also re-adjust itself gradually back to normal, so that after a few days that change in threshold which you’ll remember came down from about 45 to maybe 38 degrees, re-adjusts back up again until it’s at 45 degrees.

The problem is that we think in chronic pain some people don’t re-adjust after the injury so their nervous system is left in that sensitised state. Their thresholds are permanently lowered and they also get amplification, like turning up a volume control within the nervous system, so the whole thing is made worse by this sensitisation and amplification of the problem within the nervous system.

And we believe that in many people who suffer chronic pain after injuries and maybe after other sorts of illnesses, like viral illnesses, it may be that the nervous system has changed and everything’s been stirred up and sensitised but instead of re-adjusting to normal after a few days after healing takes place, they are left with this chronically sensitised nervous system that gives them a chronic pain syndrome.

There are several different sorts of pain, in the old days we used to classify pain really according to the timescale and we’d talk about acute pain, meaning pain of a fairly sudden onset that didn’t last long. For example, you fall over and bang your knee, it hurts for a while and then it all gets better or you have an operation, say, you have your appendix out and you have that acute pain of the inflammation of your appendix, the acute pain of having had the operation and then the whole thing settles down and goes back to normal.

And we talked about more chronic pains like back pain or the pain, say, of post-herpetic neuralgia, the neuralgia that people get after shingles that can go on for months and years. Nowadays we tend to think of pain more in terms of what was the mechanism that caused it and one of the most important differences is where you have something like a tissue injury, for example the damaged knee when you fall over and bang your knee, you injure your knee, your knee hurts and the nerves pick that up and relay it back to the nervous system.

The pain I mentioned from the shingles and the post-herpetic neuralgia, the nervous system itself is actually damaged there. Shingles is caused by the chicken pox virus and the virus actually attacks the nerves around the spine and damages those nerves, so these are abnormal nerves transmitting unpleasant signals even though in fact nothing may be happening at the far end of that nerve.

The problem lies within the nervous system itself and we call that a neuropathic pain. And there are several other examples of this trigeminal neuralgia in the face, sciatica where people get pain down their leg from a disc, all these are pains where the nerve itself is the origin of the pain and they need different treatments from these pains where for example, if you damaged your knee and it’s all inflamed you might need an inflammatory, an anti-inflammatory painkiller or an ordinary sort of painkiller like paracetamol or something a bit stronger like morphine but these drugs aren’t so good for neuropathic pains and in fact we use completely different sorts of drugs that work more on the nervous system such as the tricyclic anti-depressants like amitriptyline and the anti-convulsants like gabapentin.

Inflammatory pain for example, rheumatoid arthritis may be different again and we know that in many painful arthritic conditions the nerves in the joint actually change and nerves that normally are silent can wake up so-called silent nociceptors and these nerve endings become extremely sensitive and they have a very low threshold. That means they respond to the slightest thing by giving you pain and it may be this change within the nerves in the joint that actually make conditions like rheumatoid arthritis so painful rather than the actual joint damage itself.

So there are many different sorts of pain and they may need different sorts of treatment and I think that as we learn more about pain and further research is done, we’ll probably find that there are other sorts of pain as well and we also know clinically that people often have mixed pain syndromes and that their pain may have some elements of the normal, what we call nociceptive type of pain and other elements may be neuropathic. So it’s a complicated business.

Pain is a very complex thing and as Pat Wall, the greatest of all the British pain scientists and doctors of the past century once said ‘Pain isn’t a thing, it’s a process’ and all sorts of different things go on in our nervous system and our brain when we suffer pain. One aspect of pain is the simple sensory stuff of where does it hurt, what sort of pain is it, how long is it going on for, all that sort of thing.

But we have to realise that even at the sort of hardware level of the nervous system there are other aspects of pain. There are different pathways that go to other parts of the brain and there are pathways that go directly from your spinal cord up into the emotional centres of the brain and one of the recent bits of research from Tony Dickinson in London has shown that you actually have a pathway that goes down from the emotional centres to within the spinal cord and actually amplifies the pain at the spinal cord level.

This is not the way I would have assumed that it would have worked so that the emotional centres as it were go across the brain from the emotional side to the more sense, purely sensory side. They actually go right down to the spinal cord and turns things up at that level and then that fires up to the sensory side so the interplay of the sensory amount of pain that you feel and your emotions and all the other factors going on in your life are very complex but very real.

And we know that what is going on in your life in terms of emotions, feelings, all these sorts of things have a direct action within the nervous system which can turn up your pain and perhaps if we are lucky can also turn down people’s pain. And that is why it is so useful to have Clinical psychologists and others working with us in the field of chronic pain to try and help with these problems.