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Neural and Hormonal Integration

Endocrine and paracrine hormones and neurotransmitters control GI motility, secretion, perfusion and growth. Reflexes proceed within the mesenteric and submucosal plexus (enteric nervous system, ENS), and external innervation modulates ENS activity.

Local reflexes are triggered by stretch sensors in the walls of the esophagus, stomach and gut or by chemosensors in the mucosal epithelium and trigger the contraction or relaxation of neighboring smooth muscle fibers. Peristaltic reflexes extend further towards the oral (ca. 2 mm) and anal regions (20-30 mm). They are mediated in part by interneurons and help to propel the contents of the lumen through the GI tract (peristalsis).

External innervation of the GI tract (cf. p. 78ff.) comes from the parasympathetic nervous system (from lower esophagus to ascending colon) and sympathetic nervous system. Innervation is also provided by visceral afferent fibers (in sympathetic or parasympa-thetic nerves) through which the afferent impulses for supraregional reflexes flow.

ENS function is largely independent of external innervation, but external innervation has some advantages (a) rapid transfer of signals between relatively distant parts of the GI tract via the abdominal ganglia (short visceral afferents) or CNS (long visceral afferents); (b) GI tract function can be ranked subordinate to overall body function (c) GI tract activity can be processed by the brain so the body can become aware of them (e.g., stomach ache).

Neurotransmitters. Norepinephrine (NE) is released by the adrenergic postganglionic neurons, and acetylcholine (ACh) is released by pre- and postganglionic (enteric) fibers (^ p. 78ff.). VIP (vasoactive intestinal peptide) mediates the relaxation of circular and vascular muscles of the GI tract. Met- and leu-enkephalin intensify contraction of the pyloric, ileocecal and lower esophageal sphincters by binding to opioid receptors. GRP (gastrin-re-leasing peptide) mediates the release of gastrin. CGRP (calcitonin gene-related pep-tide) stimulates the release of somatostatin (SIH).

All endocrine hormones effective in the GI tract are peptides produced in endocrine cells of the mucosa. (a) Gastrin and cholecystokinin (CCK) and (b) secretin and GIP are structurally similar; so are glucagon (^ p. 282ff.) and VIP. High concentrations of hormones from the same family therefore have very similar effects.

Gastrin occurs in short (G17 with 17 amino acids, AA) and long forms (G34 with 34 AA). G17 comprises 90% of all antral gastrin. Gastrin is secreted in the antrum and duodenum. Its release (^ A1) via gastrin-releasing peptide (GRP) is subject to neuronal control; gastrin is also released in response to stomach wall stretching and protein fragments in the stomach. Its secretion is inhibited when the pH of the gastric/duodenal lumen falls below 3.5 (^ A1). The main effects of gastrin are acid secretion and gastric mucosal growth (^ A2).

Cholecystokinin, CCK (33 AA) is produced throughout small intestinal mucosa. Long-chain fatty acids, AA and oligopeptides in the lumen stimulate the release of CCK (^ A1). It causes the gallbladder to contract and inhibits emptying of the stomach. In the pancreas, it stimulates growth, production of enzymes and secretion of HCO3- (via secretin, see below) (^ A2).

Secretin (27 AA) is mainly produced in the duodenum. Its release is stimulated by acidic chyme (^ A1). Secretin inhibits acid secretion and gastric mucosal growth and stimulates HCO3- secretion (potentiated by CCK), pancreatic growth and hepatic bile flow (^ A2).

GIP (glucose-dependent insulinotropic pep-tide, 42 AA; formerly called gastric inhibitory polypeptide = enterogastrone) is produced in the duodenum and jejunum and released via protein, fat and carbohydrate fragments (e.g., glucose) (^ A1). GIP inhibits acid secretion (^ A2) and stimulates insulin release (this is why oral glucose releases more insulin than intravenous glucose).

Motilin (22 AA) is released by neurons in the small intestine and regulates interdigestive motility (^ A1, 2).

Paracrine transmitters. Histamine, soma-tostatin and prostaglandin are the main para-crine transmitters in the GI tract.

A. Gastrointestinal hormones

Stretch Peptides, amino acids

Fatty acids

Stretch Peptides, amino acids

Fatty acids

Prostaglandins Tract

2 Main effects of gastrointestinal hormones

(Partly after L.R. Johnson)

2 Main effects of gastrointestinal hormones

(Partly after L.R. Johnson)

tu CT

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