The Science of Pain
Chris - Now tell us first of all, why do we have to have pain. What role does it serve?
Irene - Well it's a very important role because pain is obviously something that alerts you to the fact that you're going to damage your tissue so it's a self-preserving phenomenon and therefore a very important one. The body has a pretty complicated set of systems geared up for alerting you that something is painful and you'd better do something about it.
Chris - Let's start outside and work our way in then. What sorts of things do we interpret as painful? I don't mean pinches and punches. What's actually going on inside the body to alert nerve fibres that there's something painful happening?
Irene - We do have a set of nerve fibres that specifically detect pain or tissue-damaging types of signals and we generally divide painful stimuli into three broad categories. One would be a thermal type of stimulus, so noxious or unpleasant heat; one would be mechanical like a pin prick, knife wound or a mechanical crushing type pain; and the other type which is less common is chemical pain. That would be something like an acidic pain or if you've ever chopped chilli peppers and then rubbed your eye, you'll realise that it really really hurts afterwards. We have in our peripheral nervous system underneath the skin we've got these specialised fibres and receptors that pick up those three broad categories of pain inducing stimuli. Basically what they do is start the whole process that we call nociception and that is detecting those stimuli. They then send those signals up to the brain and the brain will then unravel all that and tell you that it hurts.
Chris - How does the body discriminate between a tickle or a rub and a painful stimulus?
Irene - Well in terms of them being sensory stimuli, they're all sensory stimuli, so we're aware of where they came from. Whether it's painful and thus if you should withdraw your hand or need to rub it, that's where the brain kicks in and where the type of object causing the pain in the first instance is very important. We're just really starting to understand the difference between the painful phenomena and non-painful phenomena because we've had the ability to look inside the human brain for the first time with these brain imaging tools. A lot of the work we do in Oxford and in other groups around the world is to take normal healthy people, put them in our scanners and image the brain in action as they're detecting those pain stimuli. So we'll take people and give them painful heat, and pressure pain and see how the different parts of the brain respond to that pain inducing stimulus as opposed to something that's not painful, such as just a rub. And what we're finding is that there's a whole network of brain regions that get activated when the situation's painful versus when it's just something normal sensory. That's what we're basically trying to unravel at them moment.
Chris - What about phantom pain? For instance when someone has a part of their body amputated for various reasons they will sometimes say that they can still feel the missing part of the body and that it's painful.
Irene - That's right and that's a very serious condition. There's a couple of different theories describing what's going on there. The most simple one to explain is that where you've lost the limb, you've obviously got raw nerves that have been cut. Those nerves are sending signals into the brain signalling that a very traumatic event occurred and they've just switched on permanently. What can happen after months and years is that brain areas that respond to these painful signals and tell you that this was painful get hard wired and switched on permanently. That can be devastating for the patient because in effect that pain is now being generated by the brain and is as real as if it was happening from the outside and switched on permanently. This is why we need to understand what's going on in the brain so we can then target the therapies.
Chris - One person has suggested that in the same way as that you get that phantom pain from a missing part of the body, that tinnitus could be caused by a bit of your cochlea that's been damaged. This converts sound waves into nerve signals. The missing cochlea is a bit like the missing bit of limb. So your tinnitus is phantom pain in an auditory sense.
Irene - Yes I think that that's exactly right. It's something that is unpleasant. You can broaden out the concept of pain to a very unpleasant smell or taste. This idea of pain being some sensory phenomenon can be broadened out to all the senses, where it's just got to the level where it's very unpleasant and discomforting. People with tinnitus try many methods to get their brains to ignore the unpleasant stimulus.
Chris - Given that you're able to pin point the parts of the brain that are becoming active in these syndromes and phenomena, are we any closer to understanding exactly what's driving these things and how to get rid of them?
Irene - We've done very well over the past ten to fifteen years in terms of understanding that complicated network of structures that have to activate to give you a conscious perception of pain. Now what we're doing is trying to target them selectively with drugs and surgical therapies using things like cognitive behavioural therapies. Why is it that when you listen to a piece of music, it can take your mind off the pain and make the pain less? Why is it that when you get into the fight or flight situation of a sport event that you support quite a traumatic injury but you don't notice it at the time? Then when the event is over and the situation has calmed down they realise that their leg is cut. So we're starting to understand very well actually what the brain is doing in all those different scenarios. So that where it's important for you not to perceive pain because you need to do something immediately then, you can switch the pain signals off. And in other situations where you need to be alert to the fact that it is painful, you can multiply them and make the pain experience much worse. Those amplification and attenuation processes are starting to be understood at a much better level. This bodes very well for the development of drugs and the development of target areas for surgery and rehabilitation.