Neural control of the gut wall, showing (1) the myenteric and submucosal plexuses (black fibers); (2) extrinsic control of these plexuses by the sympathetic and parasympathetic nervous systems (red fibers); and (3) sensory fibers passing from the luminal epithelium and gut wall to the enteric plexuses, then to the prevertebral ganglia of the spinal cord and directly to the spinal cord and brain stem (green f ibers).