Key Points

The functions of the gastrointestinal tract are regulated by chemical mediators including

(1) hormones, which are delivered in the blood;

(2) paracrines, which diffuse through the interstitial fluid; and (3) neurocrines, which are released from neurons.

Neural regulation of the gastrointestinal (GI) tract is both extrinsic and intrinsic. Parasympathetic and sympathetic nerves relay information to and from the GI tract. The enteric nervous system within the gut receives and relays information to and from the extrinsic nerves and conducts signals along the gut. Two families of related peptides regulate many of the functions of the GI tract: (1) gastrin and cholecystokinin and (2) secretin, vasoactive intestinal peptide (VIP), gastric inhibitory peptide (GIF), and glucagon.

Gastrointestinal hormones are found in endocrine cells scattered over large areas of mucosa. They are released by chemicals found in food, neural activity, or physical distension and may stimulate or inhibit several processes. There are five established gastrointestinal tract hormones: gastrin, cholecystokinin, secretin, GIF, and motilin.

Somatostatin and histamine are two important paracrine agents.

Three peptides function as neurocrines: VIP, bombesin (GRP), and enkephalins.

Essential Medical Physiology, Third Edition

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