The gastrointestinal tract is unusual among organ systems in being governed by three autonomic divisions — parasympathetic, sympathetic, and the intrinsic enteric nervous system — rather than the customary two. Parasympathetic innervation is the "rest and digest" division, and its unifying principle is that it promotes the digestive state: it increases glandular secretion, increases motility, decreases sphincter constriction so that luminal contents can advance from one compartment to the next, and increases splanchnic blood flow. Anatomically this outflow is craniosacral — arising from brainstem cranial nerves VII, IX, and X and from sacral nerves that synapse in plexuses adjacent to the gut segments they supply — which places the concept within autonomic neurophysiology and gastrointestinal physiology as the neural-control layer of the parent discipline of human physiology.
The gastrointestinal tract is unusual among organ systems in being governed by three autonomic divisions — parasympathetic, sympathetic, and the intrinsic enteric nervous system — rather than the cus…