Your gut has its own nervous system with hundreds of millions of neurons.
The enteric nervous system lines the digestive tract and can run digestion on its own, without instructions from the brain. It is sometimes called the second brain.
Why is this true?
Most of its signals travel up to the brain rather than down from it, which is one reason stress and mood are so closely tied to the stomach. About 90 percent of the body's serotonin is produced in the gut.
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More about human body
Your own body is a reliable source of surprises. It replaces cells by the million every second, keeps bacteria that outnumber human cells, and runs sensory tricks that make the world feel smoother than the raw signal ever is. The facts collected here stick to mainstream physiology, avoid the wellness myths that circulate alongside them, and explain the underlying mechanism rather than just quoting a startling number.