Enteric nervous system

The gut does not wait for orders from the brain in order to work. It has nerves of its own, embedded in its wall, and that whole set is called the enteric nervous system. It is not a cable running down from above: it is a mesh of neurons that lives inside the digestive tract, end to end.

That mesh is organized into two networks, called plexuses. One is the submucosal plexus, also known as Meissner's plexus. It takes in information from two kinds of sensors: mechanoreceptors, which pick up stretch, and chemoreceptors, which pick up the chemical makeup of whatever is passing through. With that information it handles two things, how much gets secreted and how much blood arrives.

The other is the myenteric plexus, and it forms a continuous network running from the upper esophagus to the internal anal sphincter. That one is in charge of the smooth muscle in the wall, that is, of motility: the movement that pushes the contents along.

Even so, it does not work alone. The parasympathetic nervous system, the side that takes over at rest, usually excites it, and the sympathetic, the alert side, usually holds it back. The vagus nerve, which runs down from the skull, activates its motor neurons.