Your Gut Contains More Neurons Than Your Spinal Cord

The enteric nervous system — sometimes called the "second brain" — lines the gastrointestinal tract and contains roughly 500 million neurons, more than the spinal cord. It operates largely independently of the brain and communicates bidirectionally via the vagus nerve. Gut bacteria directly produce neurotransmitters including serotonin (90% of the body's total supply is made in the gut), influencing mood and cognition.

Source: The Good Gut — Justin Sonnenburg; Nature Reviews Neuroscience