Scientists Find a Second 'Brain' in the Gut

blackboard drawing of gut with bacteria
blackboard drawing of gut with bacteria. (Photo: T.L. Furrer/Shutterstock)

Humans can eat an astounding variety of foods, allowing humans to survive — and thrive — in radically different landscapes with varied diets, from the mostly-meat-and-fat meals of the Inuit people of Canada's far north, to plant-munching vegans.

Ultimately, the credit for that flexibility goes to our digestive system. The complex human gut can process the milk and meat of other animals, cooked and raw combinations of foods, and a wide variety of plants — be them fermented, roasted, dried, salted or something else. The system that manages the physical and chemical breakdown of foods, the extraction of nutrients from them, and the efficient ejection of waste is so complex that it turns out it even has its own brain. And as Giulia Enders, a German writer and scientist, explains in her TEDTalk above, there's a lot more to know. But we are making progress on that front.

How our second brain works

The Enteric Nervous System (ENS) is a collection of over 500 million neurons — the most found outside the big, main brain that lives in your skull. It probably evolved before our other main brain did; it's ancient hardware. Where is the ENS? Well, it's in every part of your gut, lining your esophagus, down into your stomach, throughout your intestines large and small, and all the way to the (ahem), end. Instead of a singular lump of an organ, it's more like a mesh that's integrated into the whole system. The ENS has all sorts of neurotransmitters zipping around, sending chemical signals from one part of the gut to another. (In fact, the same neurotransmitters as are found in the brains in our skulls.) It's not conscious like our main brain is, but without it, we couldn't survive. However, the ENS will still function even if we are otherwise brain dead or damaged.

Scientists have known about the ENS, but didn't really know how it worked until recently. According to a new study published in JNeurosci, they've figured it out. Using mice, the scientists were able to observe a "novel pattern of rhythmic coordinated neuronal firing" — basically the neurons in the ENS coordinated with each other to move muscles around the large intestine that in turn physically moved pellets through the mice guts. Without the mouse having to think "I want to digest this," the pellets were digested.

Yes, our bodies have plenty of automatic functions, but this is different. The gut isn't governed by the central nervous system (CNS), though it does communicate with it via the vagus nerve. The ENS operates independently. It can exchange information with the the CNS, but it doesn't need it to digest food. This is part of the reason we don't have to eat at the same time each day, or continuously. The system is always "on," but it isn't actively working until we eat. This "smart system" in our gut allows digestive movement to occur only when it's needed — and now we know how that happens.

"This is the first direct observation of rhythmic firing in the ENS," the researchers wrote in their paper.

New answers raise new questions

Interestingly, the scientists also observed that in between digestions, there was still "ongoing activity" in the ENS. What was it doing? We have no idea. There's obviously plenty more to learn about the ENS, but knowing that it causes digestion in the way it does advances our understanding of both digestion and the capacities of this second "brain."

Holistic health advocates have long suggested a brain-gut connection. We already know that 90 percent of the serotonin our body makes (often called the "happy chemical") is produced in special cells in the GI tract. Half of the dopamine we make is located there, too. Both serotonin and dopamine have a physiological function and also influence mood when they make their way out of the gut and into the blood stream. Connecting the dots between the known function of those neurotransmitters, it's possible the ENS could act in a more direct way to influence our emotions and state of mind, though this has yet to be understood fully by scientists.

All of this is not only food for thought, it reminds us that "going with your gut" when it comes to decisions might not be such a brainless way to make choices after all.