To explain what the gut can feel in a literal sense, Katie Haylor met up with consultant gastroenterologist Jeremy Woodward from Cambridge University Hospitals...
Jeremy - We talk about the gastrointestinal tract as being from the mouth to the anus. There's a lot of confusing terminology with this. People talk about the tummy when they mean the abdomen, short for stomach and the stomach is a key part of the gastrointestinal tract. But when you follow it down from the mouth you have a muscular tube, the gullet or oesophagus, which then leads into the stomach and the stomach then leads into the small intestine and then into the colon and then out into the toilet.
Katie - I've heard that the gut actually has its own brain.
Jeremy - Well it's not a brain like our head brain, if you like. We call it the gut brain because there are so many nerves present in the gut, about half a billion nerve cells which is the same as you'd find in the spinal cord. But the other reason which we talk about having a gut-brain, is actually the fact that it can function almost autonomously. We know that because if we transplant intestine, which we do here in this hospital, they actually function perfectly well when we move the gut into another person.
Katie - Provocative question - what makes the gut so special that it deserves its own brain?
Jeremy - Well one could turn that question around and say why do we have a head brain to be honest! The gut is the oldest organ in the body and the nervous system evolved around the gut in order to provide ways of helping it work, but also helping to fill it, in other words to get food into it. So when you look through evolution, all of the things that are developed on the outside of animals are really ways of actually getting food into the gut. And to a certain extent the head, where sensory organs are concentrated. The head brain if you like, for thinking and planning, to a large extent, when it comes down to the bottom line, it's about how we get food and how we fill the gut. In many ways, if you look at the evolutionary story, the gut brain came first and the head brain second as a result of basically the evolution of nerves in the gut.
Katie - Not that you're biased or anything.
Jeremy - Of course not.
Katie - Can you tell us a bit about how it's displayed in the gut?
Jeremy - Okay so it doesn't look anything like a brain. What it is, is just a network of nerve cells, long processes spreading out from the nerve cell body which are like the electric wiring if you like, that are able to transmit messages from one point to another. And in the gut that they're thinly spread between the layers of the muscle and also under the surface lining of the gut, the mucosa.
Katie - Can you tell us a little bit about how food actually progresses down the gut?
Jeremy - The gut works by a process called peristalsis and most people have heard of this. It’s a very clever mechanism where food goes in one direction usually - the gut senses a lump, it relaxes below it, and contracts above it and that way we get a moving wave of contraction moving down the gut. It's extraordinary to watch - it's the most the most beautiful thing, and there are even videos of people standing on their heads and you can see the peristalsis moving against gravity.
Katie - I was going to actually ask you what would happen if I stood on my head and ate my breakfast? I imagine you would not be endorsing that as someone who has to look after guts! But is this perhaps an example of what you were saying about the gut brain or enteric nervous system being able to work by itself?
Jeremy - So the gut actually is able to control its own movement entirely through the enteric nervous system. It's got a network of nerves responsible for contracting and relaxing below, and moving this wave down the gut and that's entirely independent of any nerves coming from outside and you wouldn't be aware of it. Interestingly, it is the oesophagus, the gullet, where the brain has actually in humans taken over some of the control of the muscle contraction. We don't see that in other animals. It seems to be specific to more recently evolved mammals, but further down in the stomach and the intestine this is all totally controlled within the nerves of the gut itself.
Katie - Can you tell me a bit about what the gut can feel and by that I mean sense or detect?
Jeremy - Well you name it, effectively. There's no end to what the gut can do. It's an extraordinary organ. I've already mentioned that it can sense the presence of a lump or a bolus of food, but the lining of the gut can actually even taste specific chemicals. So if there's a lot of fat in the meal it will affect the way the gut works into more of a churning motion. Fat is also detected by cells that secrete hormones that make us release bile to help us to absorb it. And this is all regulated within the gut itself.
But the gut can also sense noxious things. The feeling of nausea and sickness when we've eaten something we shouldn't. The Yew tree berry for instance which looks very attractive but is highly toxic. The gut will be able to sense that and it will automatically make us vomit. The gut can recognise the difference between particles and fluid, that's part of the way that peristalsis works. It can sense the presence of bacteria and even specific bacteria. It's capable of responding to stretch and even probably to be able to tell apart the consistencies of different foods going through it.
Katie - I'm glad I can respond to stretch, because isn't that how I managed to eat my Christmas dinner or a couple of Christmas dinners, right?
Jeremy - Well effectively, yes.
Katie - I'm curious. As a gut expert, what do you think looking forward are the more interesting questions to ask around how the gut is controlled or innervated?
Jeremy - For me as a clinician, understanding a lot more about the afferent nerves, these sensory nerves from the gut to come back to the brain, and what fires them. How the gut is sensitive to things - I have to deal a lot with people who have a lot of sensations from the gut, pain and nausea. These sensations can be extremely unpleasant and we as yet are very limited in our ability to understand the mechanisms for how they are occurring in these patients but also how to make them better.