Download 3. Regulation of Digestive Function

AHS 2013 Digestion and Metabolism 3
Sarah Harney
3. Regulation of Digestive Function
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How does the nervous system influence digestion
•
What hormones regulate digestive function?
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Regulation of motility
•
Regulation of secretion
•
Reading: Stanfield pp 592-594
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Peripheral Nervous System
Afferent
Efferent
Information from periphery
to CNS
Information from CNS to periphery
Sensory neurons
Somatic Nervous System
Autonomic Nervous System
Skeletal muscle
Smooth muscle – gut, blood vessels,
bladder
Sympathetic Nervous
System
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Parasympathetic Nervous
System
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Autonomic Control of GI Activity
Autonomic Nervous System
Involuntary activity
Smooth muscle – gut, blood vessels, bladder
Glands
Parasympathetic Nervous
System
Sympathetic Nervous
System
‘Fight or Flight’
‘Rest and Digest’
Inhibition of GI tract contraction
and secretion
Increases GI motility and secretion of
digestive enzymes and hormones
(via vagus nerve)
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Signalling Pathways
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Regulation
Phases
Mechanisms
• Cephalic
• Enteric Nervous System
• Gastric
• Central Nervous System
• Intestinal
• Gastric hormones
• Intestinal hormones
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Cephalic phase
•
‘Anticipatory’
occurs before food reaches stomach
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Sight, smell, hunger, thirst
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Stimulation of saliva secretion
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Increases gastric secretion
– Activity of vagus nerve increases secretion
of HCl and pepsinogen by secretory cells
– Vagal stimulation of G cells increases gastrin
secretion - gastrin also increases HCl and
pepsinogen secretion
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Famously
described in
Pavlov’s dog
experiments
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Gastric phase
• Begins when food reaches the stomach
• Stimuli in stomach act to increase gastric secretion
– Protein, distension, caffeine, alcohol
• Mechanism (as for cephalic)
– Activation of vagal nerve and intrinsic nerves of enteric
nervous system
→ Increased gastrin secretion
→ increased HCl and pepsinogen secretion
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Intestinal phase
• Occurs when chyme enters small intestine
• Acts to shut off gastric secretion as stomach empties – therefore, is
an inhibitory phase
- Reduced protein in stomach removes stimulus for gastrin secretion so
gastric secretion is reduced
-Lower pH in stomach with less food also reduces secretion
• Stimuli - Fat, acid or distension of duodenum inhibits gastric
emptying
• Mechanism
Cholecystokinin (CCK) release (from small intestine mucosa)
inhibits gastric emptying and secretion
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Example of Phases
Gastric Motility
Cephalic
Gastric
• Stimulated by
- Anticipation, smell
- Anger, aggression
Intestinal
• Stimulated by
• Stimulated by
- Protein, distension
- Protein, fat, H+,
of stomach
distension in
duodenum
Mediated by
• Mediated by
gastrin release
CCK release
Mediated by
gastrin release
Increased secretion
• Inhibited by
Increased force of
contraction
-Stomach emptying,
low pH
• Inhibited by
- Fear, depression, pain
-CCK, GIP release
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Reduced Gastric
motility
Reduced gastric
secretion
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Gut Hormones
• 4 principal gut hormones:
–
–
–
–
Gastrin
Cholecystokinin (CCK)
Secretin
Glucose-dependent
insulinotropic peptide (GIP)
(formerly known as gastric inhibitory peptide)
• Released by endocrine cells in
gastric/duodenal mucosa
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Gut hormones — 1
GASTRIN
Source: G cells in gastric glands of stomach
Release: protein in stomach
Functions
• Stimulates gastric secretion
• Increases gastric and intestinal motility
• Induces colonic mass movements
• Trophic to gastric and small intestinal mucosa
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Gut hormones — 2
SECRETIN
Source: duodenal mucosa
Release: acid in duodenum
Functions
• Inhibits gastric emptying and secretion
• Increases NaHCO3 secretion by pancreas/in bile
• Trophic to exocrine pancreas
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Gut hormones — 3
CCK (CHOLECYSTOKININ)
Source: duodenal mucosa
Release: duodenal nutrients (esp. fat)
Sherwood 16-13
Functions
• Inhibits gastric emptying and
secretion
• Increases enzyme secretion by
pancreas
• Promotes bile release
• Trophic to exocrine pancreas
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Gut hormones — 4
Glucose-dependent insulinotrophic peptide (GIP)
(formerly known as gastric inhibitory peptide)
Source: duodenal mucosa
Release: duodenal fat, acid, glucose, distension
Functions
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Increases insulin secretion by endocrine pancreas
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Inhibits gastric emptying and secretion
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Regulation of Gastric Function —
Outline
Stomach
Protein + products
Distension
Parasympathetic stim.
Gastrin
+ Gastric Motility
and Secretion
-
Small Intestine
Secretin
H+
GIP
Glucose
Fats
H+
Distension
CCK
Fats
Protein +
products
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Regulation of gastric function
• Stimulus: Protein /Amino
acids, distension in stomach
• Signal: gastrin, neural input
• Effect: increased HCl and
pepsin production
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Regulation of bile function
• More complex…
• Two hormones
– Secretin — synthesis
– CCK — release
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Regulation of Pancreatic Function
• Endocrine
(secretion into the blood)
– Islet cells ,
secretion of
insulin,glucagon
• Exocrine
(secretion into external
environment)
– Acini – enzymes
i.e. amylase,
lipases, proteases
– Ducts bicarbonate
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Regulation of Pancreatic Function
• Pancreatic secretions are increased by CCK
and secretin
CCK
Enzyme secretion
Secretin
Bicarbonate secretion
(acinar cells)
(duct cells)
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Regulation of Pancreatic Function
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AHS 2013 Digestion and Metabolism 3
Sarah Harney
Regulation of motility
Gastric motility
Increased by:
Reduced by:
• Gastrin
• GIP
•Secretin
•CCK
• Cephalic phase
stimuli: anger,
aggression
• Cephalic phase
stimuli : pain, fear,
depression, anxiety
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