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General Intestinal Histology
Activities of the Gastrointestinal Tract
Motility
 Secretion
 Digestion
 Absorption
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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.
 [email protected]
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