Hi, After a recent bout of gallstone pain, I got to thinking that it was strange that the pain I felt should be localized to the area of the inflammation. How does the brain know that inflammation, in an internal part of the body that I was not aware of before should be signalled in a place that corresponds to the real place? For an internal pain or sensation, how does the body know that something physically placed where it is should signal pain in exactly the same place? Does that make sense? Anyway thanks for the great shows and for spreading the word on science so well. Martin Fennell
Chris Smith answered Martin's question...
Chris - Well the answer is that the way your body develops when you’re an embryo, it starts off as a flat sheet of cells and then another flat sheet of cells forms underneath that, and then the whole thing rolls up rather like a tube. So you get a tube inside a tube. If you imagine two plastic plates laying on top of each other and then the edges curving round to meet in the front, the inner tube is your gut tube, and the outer tube is you skin. And along that length the body divides itself up into segments - a bit like a millipede - and those segments are running different genetic instructions and they also have different spinal nerves coming out of the central nervous system that’s developing that supply them.
So your body has a pattern and the inputs from those nerves therefore signal where on the body surface and, to a lesser extent, where on the body insides a signal is coming from, so the body can localise pain reasonably well. It’s much better at doing it on the outside surface of the body a) because there are more nerves there, and b) because it’s more helpful to you to be able, for instance, put your hand in your pocket, feel around and found a two pence coin than being able to do that level of precision with your innards.
But it is useful to know when things are upset inside because, obviously, if you’re uncomfortable or if you have stomach ache or something like that, knowing where the pain is it can give you help as to how to make it better. So you do have the ability to do that but you don’t do it with the same level of precision.
And a good example of this is when people classically have a heart attack, or angina chest pain because of cardiac pain, they don’t just feel the pain where the heart is, they may feel the pain in their neck, they may complain of pain down their left arm. Or, if they're one of the rare individuals in the population who have their heart on the opposite side of the body, they may actually complain of pain going down their right hand side.
Caroline - So I understand that we need to feel where pain is coming from inside us and that helps but why can’t we feel any pressure at all because when we digest food we don’t feel that at all?
Chris - We do a bit when your tummy rumbles, and you get nervous and you have butterflies in your stomach. That sinking feeling, a bit like before you come on this programme, is because your ‘fight or flight’ response, your sympathetic nervous system, kicks in and it suppresses the parasympathetic nervous system which is part of your nervous system which makes you rest and digest and it strongly deactivates your intestines. And that shuts down your stomach and shuts down your guts and all the muscles relax, and that sinking feeling is everything going ‘urrgh’ as it turns off in order to prepare you to run off.
So you do have a degree of sensation but your brain is also very good at subtracting from the signals being presented to your consciousness everything which you have got used to. Which is why you put your cloths on this morning and are probably aware of pulling your jumper over your head but then pretty quickly you stop noticing “I’m wearing a jumper.” But if your bring it to mind, suddenly you’ll start noticing it again. And you think ‘ahh’ because your brain is mentally subtracting in order to prevent information overload.
Perhaps because you (or the doctor) prodded the general area, and that helped you mentally locate the actual source of the pain?