General Intestinal Histology

Activities of the Gastrointestinal Tract

Motility Secretion Digestion Absorption

Intestinal Autonomic Nervous System

Parasympathetic Nervous System

Vagovagal or Long reflex

The Enteric Nervous System

Peptide Secretion

Criteria for GI Hormones

a physiologic stimulus in one part of the GI tract produces a response in another part

the response must be independent of the nervous system

the physiologic response can be duplicated with an “extract” from the stimulus site

the substance must be isolated, purified, identified chemically, and synthesized

Gastrin

Stimulates gastric acid secretion and growth of gastric oxyntic gland mucosa.

Released from the gastric antrum and perhaps the duodenum.

Release is stimulated by peptides, amino acids, gastric distention and stimulation of the vagus.

Structure of Human “little” Gastrin

Cholecystokinin (CCK)

Stimulates gallbladder contraction, pancreatic enzyme and bicarbonate secretion, and growth of the exocrine pancreas

Inhibits gastric emptying Released from the duodenum and jejunum Release is stimulated by peptides, amino acids

and >8C fatty acids, and to a lesser extent by acid

Structure of porcine Cholecystokinin

Secretin

Stimulates pancreatic bicarbonate secretion, biliary bicarbonate secretion, growth of the exocrine pancreas, and pepsin secretion

Inhibits gastric acid secretion and the trophic effect of gastrin

Secreted by the duodenum Secretion is stimulated by acid and to a

lesser extent fat

Secretin Family of Peptides

Gastric Inhibitory Peptide (GIP)

Stimulates insulin release Inhibits gastric acid secretion Secreted by the duodenum and jejunum Secretion is induced by glucose, amino

acids and fatty acids

Motilin

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

acid

Candidate Hormones

Pancreatic polypeptide-a 36 amino acid peptide released from the pancreas in response to all three food stuffs

Peptide YY-a 36 amino acid peptide released from the ileum and colon in response to meals, especially fat

Enteroglucagon-is present in the distal small intestine and released into the bloodstream

Paracrines

Act on cells in the immediate vicinity, not through the blood stream.

– Somatostatin– Histamine

Somatostain

Inhibits gastrin release, release of other peptide hormones and gastric acid secretion.

Released from the GI mucosa and the pancreatic islets.

Release is induced by acid. Release is inhibited by vagal stimulation.

Histamine

Stimulates gastric acid secretion. Released from oxyntic gland mucosa and

enterochromaffin-like (ECL) cells. Released in response to gastrin.

Neurocrines

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

Stimulates intestinal and pancreatic secretion.

Relaxes sphincters and gut circular smooth muscle in general.

Released by cells in the mucosa and smooth muscle of the GI tract.

Bombesin or Gastrin-releasing peptide

Stimulates gastrin release. Released by cells in the gastric mucosa.

Enkephalins

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

smooth muscle.

Smooth Muscle Cells

They are smaller than skeletal muscle cells and long, narrow and spindle-shaped.

Cells that belong to a bundle are functionally coupled by gap junctions.

There are no sacromeres, therefore no striations.

The ratio of thin to thick filaments is 15:1.

Robert D. Specian, Ph.D. rspeci@lsuhsc.edu