General Intestinal Histology

Activities of the Gastrointestinal Tract u Motility u Secretion u Digestion u Absorption

Intestinal Autonomic Nervous System

Parasympathetic Nervous System

Vagovagal or Long reflex

The Enteric Nervous System

Peptide Secretion

Criteria for GI Hormones u a physiologic stimulus in one part of the GI tract produces a response in another part u the response must be independent of the nervous system u the physiologic response can be duplicated with an “extract” from the stimulus site u the substance must be isolated, purified, identified chemically, and synthesized

Gastrin u Stimulates gastric acid secretion and growth of gastric oxyntic gland mucosa. u Released from the gastric antrum and perhaps the duodenum. u Release is stimulated by peptides, amino acids, gastric distention and stimulation of the vagus.

Structure of Human “little” Gastrin

Cholecystokinin (CCK) u Stimulates gallbladder contraction, pancreatic enzyme and bicarbonate secretion, and growth of the exocrine pancreas u Inhibits gastric emptying u Released from the duodenum and jejunum u Release is stimulated by peptides, amino acids and >8C fatty acids, and to a lesser extent by acid

Structure of porcine Cholecystokinin

Secretin u Stimulates pancreatic bicarbonate secretion, biliary bicarbonate secretion, growth of the exocrine pancreas, and pepsin secretion u Inhibits gastric acid secretion and the trophic effect of gastrin u Secreted by the duodenum u Secretion is stimulated by acid and to a lesser extent fat

Secretin Family of Peptides

Gastric Inhibitory Peptide (GIP) u Stimulates insulin release u Inhibits gastric acid secretion u Secreted by the duodenum and jejunum u Secretion is induced by glucose, amino acids and fatty acids

Motilin u Stimulates gastric and intestinal motility u Released by the duodenum and jejunum u Release is stimulated by nerves, fat and acid

Candidate Hormones u Pancreatic polypeptide-a 36 amino acid peptide released from the pancreas in response to all three food stuffs u Peptide YY-a 36 amino acid peptide released from the ileum and colon in response to meals, especially fat u Enteroglucagon-is present in the distal small intestine and released into the bloodstream

Paracrines u Act on cells in the immediate vicinity, not through the blood stream. –Somatostatin –Histamine

Somatostain u Inhibits gastrin release, release of other peptide hormones and gastric acid secretion. u Released from the GI mucosa and the pancreatic islets. u Release is induced by acid. u Release is inhibited by vagal stimulation.

Histamine u Stimulates gastric acid secretion. u Released from oxyntic gland mucosa and enterochromaffin-like (ECL) cells. u Released in response to gastrin.

Neurocrines u Released from neurons by an action potential, then diffuse across the synaptic cleft to the target tissue –Vasoactive intestinal peptide (VIP) –Bombesin or gastrin-releasing peptide (GRP) –Enkephalins

Vasoactive Intestinal Peptide u Stimulates intestinal and pancreatic secretion. u Relaxes sphincters and gut circular smooth muscle in general. u Released by cells in the mucosa and smooth muscle of the GI tract.

Bombesin or Gastrin-releasing peptide u Stimulates gastrin release. u Released by cells in the gastric mucosa.

Enkephalins u Stimulates smooth muscle contraction. u Inhibits intestinal secretion. u Released from cells in the GI mucosa and smooth muscle.

Smooth Muscle Cells u They are smaller than skeletal muscle cells and long, narrow and spindle-shaped. u Cells that belong to a bundle are functionally coupled by gap junctions. u There are no sacromeres, therefore no striations. u The ratio of thin to thick filaments is 15:1.

u Robert D. Specian, Ph.D. u