The ENS is embedded in the gut wall and participates in a rich dialogue with the brain during the entire journey of food through the 30-foot-long digestive tract. The ENS cells in the lining of the gut communicate with the brain by way of the autonomic nervous system, which controls the body's vital functions. As part of that system, sympathetic nerves connect the gut to the spinal cord and then to the base of the brain. In addition, parasympathetic nerves pass to and from the base of the brain via the vagus nerve from the upper gut or the sacral nerves from the colon. The gut and brain use their shared neurotransmitters, including acetylcholine and serotonin, to transmit information back and forth by way of the sympathetic and parasympathetic nerves.
This two-way communication system between the gut and the brain explains why you stop eating when you're full (sensory neurons in your gut let your brain know that your stomach is distended), or conversely, why anxiety over this morning's exam has ruined your appetite for breakfast (the stress activated your "fight or flight" sympathetic nervous system, inhibiting gastrointestinal secretion and reducing blood flow to the gut).