What Lies Behind Your Belly Button: The Second Brain in Your Gut

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The gut contains a vast neural network—the Enteric Nervous System—often described as the body’s “second brain”

Which organ in your body has capabilities that surpass all your other organs, and even rival your brain? It is the gut. A lot is happening behind the belly button.

What we have there is our second brain. You’ve probably used it countless times before; it’s just that when you did, you probably referred to it as gut instinct. New research is showing that this age-old phrase is surprisingly accurate.

The second brain influences our judgment, and much else besides. Known as the Enteric Nervous System (ENS)—enteric meaning “to do with the intestines”—it is an extensive network of brain-like neurons and neurotransmitters wrapped in and around our gut. It contains about 100 million neurons, more than in either the spinal cord or the peripheral nervous system.

Most of the time we are unaware of its existence, as its primary function is what one would expect: managing digestion. Yet the presence of all that brain-like complexity is no coincidence.

The ENS has the remarkable ability to work autonomously. Equipped with its own reflexes and senses, it can control gut behaviour independently of the brain. We likely evolved this system to perform digestion and excretion “on site”, rather than remotely from our brains through the middleman of the spinal cord. However, its complexity cannot be explained by the messy business of digestion and excretion alone.

For a long time the reason for this complexity remained unclear. In the 1980s researchers made a startling discovery: the ENS is awash with neurotransmitters—the biochemicals that are vital to brain activity. By the 1990s it was being referred to for the first time as the body’s second brain. Researchers worldwide are now racing to explore its implications.

The ENS is in constant communication with the brain in our skull via the body’s own information superhighway—the vagus nerve. It is now becoming clear that the signals flowing back and forth can influence our decisions, mood, and general well-being.

A clue to the important role played by the gut in our well-being comes from the fact that around 80 percent of the vagus nerve is dedicated to reporting information from the gut to the brain. Suddenly, the idea of having a gut instinct no longer seems so ridiculous.

The second brain informs our state of mind in other, more subtle ways as well. A large part of our emotional life may be influenced by the nerves in our gut. Butterflies in the stomach—signalling activity in the gut as part of our physiological stress response—is one familiar example.

Although gastrointestinal turmoil can sour one's mood, everyday emotional well-being may depend on messages travelling from the brain below to the brain above.

Cutting-edge research is now investigating how the second brain mediates the body’s immune response; after all, at least 70 percent of our immune system is aimed at the gut. Scientists are also studying how the trillions of bacteria living in the gut communicate with cells in the enteric nervous system.

These discoveries are leading researchers to think that in the coming years psychiatry may need to expand its scope—treating not only the brain in our head, but also the one within our gut.

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