control of gastric secretions
TRANSCRIPT
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Control of Gastric secretions
Dr Raghuveer Choudhary
Associate ProfessorDept. of Physiology
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Gastric secretion
Gastric juice- hydrochloric acid
- conversion of pepsinogen to pepsine- bacteriostatic effect
- pepsin- protein digestion- replaceable by pancreatic enzymes
- mucus- protective coating, lubricant- part of gastric mucosal barrier
- intrinsic factor- binds B
12vitamin, absorption in the ileum
- the only indispensable substance in gastric juice
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Functions of HCl:
Turn pepsinogen into pepsin & provide a
medium of low pH favoring pepsinaction;
Kill many swallowed virulent organisms;
Promote the secretion of pacreas & bile;
Promote the absorption of calcium &ferrus
Pepsinogen
Secreted bychief cells & activate by HClFunction: digest proteins into proteoses,
peptones,
polypeptides, with little free amino acids
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Parietal cell - secretingTubulovesicles fuse with
canaliculus, increased surface area
and numbers of H+K+ATP ase
increases acid secretion into lumen
of gut.
H+
Acid secretion is against a 3
million fold concentration
gradient
H+ inside = 4x 10-8
MH+ outside = 0.1 M
NEEDS ENERGY
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MECHANISM OF GASTRIC ACIDSECRETION
The oxyntic cells produce andsecrete acid,described as follows
H+ is produced through the action of carbonic anhydrase,which produces carbonic acid from CO2 and H2O.
The H+/ K+-ATPase is used to pump H+ from the cytoplasminto the stomach lumen in exchange for K+. K+ used in thisexchange process is available from food or saliva, but it is alsosecreted via a luminal membrane K+ channel.
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Cl must be secreted to yield HCl. Cl uptakeinto oxyntic cells from the extracellular fluidoccurs via the Cl/HCO3exchange at thebasolateral cell membrane.
HCO3 exits the cell in such a large quantitythat the gastric venous blood becomes
alkaline; this is known as thepostprandialalkaline tide.
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Cl is secreted into the lumen via a Cl
channel in the luminal membrane, whichresults in the generation of a large lumen-negative transepithelial potential differenceacrossthe stomach mucosa.
H+ is transported against a largeelectrochemical gradient, which is reduced bya lumen-negative
voltage.
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Regulation of Gastric Function
Gastric secretion and motility is divided into threestages:
1) Cephalic
2) Gastric
3) Intestinal phases
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- is stimulated by food in the stomach
- accounts for two-thirds of gastric
secretion.
2) The Gastric Phase
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a) the enterogastric reflex.b) local hormones
Secretin
Cholecystokinin
gastric inhibitory peptide
3) The Intestinal Phase
After entering small intestines,
chyme inhibit gastric secretion and mobilityvia:
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1. The taste or smell of food, tactile
sensations of food in the mouth, oreven thoughts of food stimulate themedulla oblongata (green arrow).
2. Parasympathetic action potentialsare carried by the vagus nerves tothe stomach (pink arrow).
3. Preganglionic parasympatheticvagus nerve fibers stimulatepostganglionic neurons in the entericplexus of the stomach.
4. Postganglionic neurons stimulatesecretion by parietal and chief cellsand stimulate gastrin secretion byendocrine cells.
5. Gastrin is carried through thecirculation back to the stomach(purple arrow), where it stimulatessecretion by parietal and chief cells.
Cephalic Phase
GastrinCirculation
Secretionsstimulated
Taste or smell of foodTactile sensation in mouth
Medulla oblongata
Vagus nerves
Stomach
1
2 3
5
4
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The taste or smell of food, tactile sensations of food in the mouth, oreven thoughts of food stimulate the medulla oblongata (green arrow).
Taste or smell of food
Tactile sensation in mouth
Medulla oblongata
Cephalic Phase
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Parasympathetic action potentials are carried by the vagusnerves to the stomach (pink arrow).
Vagus nerves
Cephalic Phase
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Preganglionic parasympathetic vagus nerve fibers stimulatepostganglionic neurons in the enteric plexus of the stomach.
Vagus nerves
Cephalic Phase
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Cephalic Phase
Postganglionic neurons stimulate secretion by parietal and chiefcells and stimulate gastrin secretion by endocrine cells.
Gastrin
Stomach
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Cephalic Phase
Gastrin is carried through the circulation back to the stomach (purple
arrow), where it stimulates secretion by parietal and chief cells.
Gastrin
Circulation
Secretionsstimulated
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Stimulation of acid secretion
cephalic phase
blocked byvagotomy
sham feeding
hypoglycemia
role of GRP(bombesin)
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Stomach
Local reflexesstimulated bystomachdistention
Distention
Secretionsstimulated
Vagus nervesMedullaoblongata
1.Distention of the stomach activates aparasympathetic reflex. Action potentialsare carried by the vagus nerves to themedulla oblongata (green arrow).
2.The medulla oblongata stimulates stomachsecretions (pink arrow).
3.Distention of the stomach also activateslocal reflexes that increase stomachsecretions (purple arrow).
Gastric Phase1
2
3
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Local reflexes
stimulated bystomach distention
Distention
Vagus nervesMedullaoblongata
Distention of the stomach activates a parasympathetic reflex.
Action potentials are carried by the vagus nerves to the medulla
oblongata (green arrow).
Gastric Phase
Stomach
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Gastric Phase
Secretions
stimulated
The medulla oblongata stimulates stomach secretions (pink arrow).
Vagus nerves
Stomach
Decreasedgastricsecretions
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Gastric Phase
Local reflexesstimulated bystomach distention
Distention of the stomach also activates local reflexes that
increase stomach secretions (purple arrow).
Stomach
Distention
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Gastric phase
60 % ofacid response
distension
peptides
calciumalcoholcaffeine
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Experiment of Sham feeding byPavlov
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1.Chyme in the duodenum witha pH less than 2 or containingfat digestion products (lipids)inhibits gastric secretions bythree mechanisms.
2.Sensory vagal action potentials
to the medulla oblongata(green arrow) inhibit motoraction potentials from themedulla oblongata (pink arrow).
3.Local reflexes inhibit gastricsecretion (orange arrows).
4.Secretin, gastric inhibitorypolypeptide, and cholecystokininproduced by the duodenum(brown arrows) inhibit gastricsecretions in the stomach.
Intestinal Phase
Secretin, gastric inhibitorypeptide, cholecystokinin
Circulation
pH
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Intestinal Phase
Sensory vagal action potentials to the medulla oblongata (greenarrow) inhibit motor action potentials from the medulla oblongata
(pink arrow).
pH
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Intestinal Phase
Local reflexes inhibit gastric secretion (orange arrows).
pH
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Secretin, gastric inhibitory polypeptide, and cholecystokininproduced by the duodenum (brown arrows) inhibit gastric
secretions in the stomach.
Secretin, gastric inhibitory
peptide, cholecystokinin
pH
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Stimulation of gastric acid secretion
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l f l
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Regulation of gastrin release
Gastrin CellSomatostatin
Cell
Acetylcholine
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