A Self-Running System

The gut has by far the largest and most independent nervous system outside the brain. Other organs have neurons too — your heart, for instance, carries its own intrinsic network that helps fine-tune its rhythm on its own — but nothing else runs a system this big or this self-sufficient.

Its main job is movement. The enteric neurons fire in waves to squeeze food along the entire length of your intestine, a steady rhythm called peristalsis that continues without a single instruction from your brain.