enteric nervous system
The enteric nervous system is a vast web of nerve cells woven into the wall of your gut, running from the lower part of the food pipe all the way through the stomach and intestines. It holds an astonishing number of neurons, on the order of hundreds of millions, more than the spinal cord, all packed into the lining of a single organ. Because it is so large and so capable of running on its own, scientists often nickname it the second brain.
Its day job is to manage digestion. It senses what is in the gut, stretches and chemistry alike, then coordinates the muscle squeezes that push food along, the release of digestive juices, and the flow of blood to the gut wall. The remarkable part is that it can do most of this without checking in with the brain in your head. Cut the main nerve connecting the two, and the gut keeps digesting, which is why it counts as semi-autonomous rather than fully independent.
It is not truly a separate brain, though. It is a specialized branch of the body's wider control wiring, and it stays in constant two-way conversation with the head brain through nerves and chemical messengers. This gut-brain dialogue is why nervousness can churn your stomach, and why what happens in the gut can quietly nudge your mood and appetite.
Before a big exam your stomach knots and your gut feels uneasy, even though no food has changed, because the head brain's stress signals are reaching the enteric nervous system through the gut-brain link.
Butterflies in the stomach are a small, everyday demo of the gut and brain talking to each other.
It is usually counted as a third division of the autonomic nervous system, alongside the sympathetic and parasympathetic branches.