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Block 1 > Stomach motility and secretion > Flashcards

Stomach motility and secretion Flashcards

1
Q

What are the functions of the stomach?

A

Food storage
Minimise ingestion of bacteria
Dissolve and partially digest the macromolecules in food
Regulate the rate at which stomach contents enter the small intestine
To secrete intrinsic factor

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2
Q

What are the purposes of gastric motility?

A

Allows the stomach to:
Act as a reservoir for a large volume of food
Fragment food into smaller particles and mix with gastric secretions (to make chyme)
Empty gastric contents into duodenum at controlled rate.

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3
Q

What are the divisions of the stomach?

A

Fundus
Body
Antrum

Orad/Caudad regions (in terms of motility)

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4
Q

What is receptive relaxation?

A

The empty stomach is small, mucosa folded into rugae- receptive relaxation allows accommodation of large food volumes through relaxation of gastric smooth muscle.

Mediated by the vagus nerve (parasympathetic) which coordinates with enteric plexi which release NO and serotonin to mediate relaxation of smooth muscle cells of the stomach.

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5
Q

What makes up the enteric nervous system?

A

Myenteric plexus in smooth muscle (muscularis layer)

Submucosal plexus in submucosa

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6
Q

Describe the mixing phase of gastric motility

A

Peristaltic waves begin in the body of the stomach and move towards the antrum.
Waves become more powerful towards the antrum, closing the pyloric sphincter, forcing most stomach contents back into the body of the stomach (retropulsion).

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7
Q

What brings about the action of peristalsis?

A

Patterns of electrical activity: Slow waves and action potentials

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8
Q

What causes slow waves?

A

Spontaneous fluctuation in membrane potential spreading to adjacent sections of muscle. Generated in the pacemaker zone in the wall of the body of the stomach.

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9
Q

What do slow waves do?

A

Coordinate contractions by controlling the appearance of action potentials

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10
Q

How are action potentials generated?

A

Excitatory neurotransmitters and hormones depolarise the membrane, increasing amplitude of the slow wave. If the slow wave rises above the threshold, the cell fires one or more action potentials and causes contraction of smooth muscle.

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11
Q

How is gastric emptying regulated?

A

Coordinated by the activity of the pyloric sphincter, stomach and proximal small intestine.

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12
Q

What are the functions of the pyloric sphincter?

A

Allows regulated emptying of gastric contents at a rate consistent with duodenum’s ability to process chyme.

Prevents regurgitation of duodenal contents back into the stomach

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13
Q

How is the rate of stomach emptying altered by the nature of stomach contents?

A

Materials high in fat digestion products, high acidity or hypertonicity decrease rate of gastric emptying

  • Receptors in duodenum and jejunum sense acidity, osmotic pressure, fats and amino acids and cause release of intestinal hormones.
  • Intestinal hormones inhibit antral contractions and/or elicit contraction of the pyloric sphincter, decreasing gastric emptying.
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14
Q

How is the pyloric sphincter controlled to regulate gastric emptying?

A
  • ANS: sympathetic fibres increase constriction; vagal fibres can be excitatory or inhibitory.
  • Hormones: gastrin, CCK, gastric inhibitory peptide (GIP) and secretin all elicit contraction.
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15
Q

What are the components of gastric juice? Where is it secreted from?

A

Secreted from gastric glands in the gastric mucosa.

Contains hydrochloric acid, mucus, enzymes (pepsinogen and gastric lipase), intrinsic factor

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16
Q

What cells make up gastric glands? What is their function?

A

4 types of secretory epithelial cells?

  • Surface and neck mucous cells: secrete mucous
  • Parietal cells: hydrochloric acid and intrinsic factor.
  • Chief cells: secrete pepsinogen
  • G cells: secrete gastrin across basolateral membrane into blood.
17
Q

Describe the distribution of secretory gastric cells in the stomach

A

Parietal cells located in the proximal 80% of the stomach (oxyntic gland area)

Gastrin producing cells are found in the antrum (pyloric gland area)

18
Q

What are the gastric hormones? What are they secreted by?

A

Secreted by gastric glands, control secretion of gastric juice.

  • Gastrin
  • Histamine
  • Somatostatin
19
Q

What are the actions of gastrin?

A

In the stomach:

  • Stimulates acid secretion, pepsinogen, mucus and bicarbonate.
  • Stimulates gastric motility
  • Inhibits gastric emptying

In the GI tract:

  • Increases GI motility
  • Stimulates insulin release
  • Stimulates pancreatic enzyme secretion
  • Stimulates bicarbonate release
20
Q

What are the actions of histamine in the stomach? What is it secreted by?

A

Secreted by enterochromaffin-like (ECL) cells in the gastric mucosa (paracrine agent)

Stimulates acid secretion
Increases local blood flow to support the increased metabolism from acid secretion.

21
Q

What are the actions of somatostatin? What secretes it?

A

D cells in gastric glands.

Inhibits gastrin release
Inhibits acid secretion from parietal cells.

22
Q

Describe the structure of parietal cells

A

Pyramidal shape
High surface area from extensive invaginations of luminal membrane forming canaliculi.
Canaliculi lined with ion pumps facing lumen of stomach.
High mitochondrial content

23
Q

Describe the mechanism of acid secretion by parietal cells

A

H+ and HCO3- formed from CO2 and H2O.

H+ secreted into the lumen by H+/K+ ATPase pump.

HCO3- moves out of the cell across the basolateral membrane via antiport and Cl- moves in.

Cl- diffuses passively into the lumen via a Cl- channel.

24
Q

How is gastric acid regulated?

A

Stimulated by: gastrin, acetylcholine, histamine

Inhibited by: somatostatin, prostaglandins I2 and E2, intestinal hormones

25
Q

How does acetylcholine stimulate gastric acid secretion?

A

Released from cholinergic parasympathetic nerve terminals near parietal cells.

Binds to muscarinic receptors on parietal cells to directly stimulate acid secretion.
Also binds to muscarinic receptors on ECL cells to increase histamine secretion which further acts on H2 receptors of parietal cells to increase acid secretion.

26
Q

How does gastrin stimulate acid secretion?

A

Secreted by G cells into the blood, travels to parietal cells and binds to gastrin receptors to stimulate secretion of acid.

Also act gastrin receptors on on ECL cell to release histamine which further increases acid secretion by binding to H2 receptors on parietal cells.

27
Q

How does histamine increase secretion of stomach acid?

A

Secreted from ECL cells, binds to H2 receptors on parietal cells to stimulate increased acid secretion.

28
Q

What is mucus made up of?

A

Glycoproteins and glycopolysaccharides

29
Q

What is the gastric mucosal barrier? What are its functions?

A

Formed from mucus and the alkaline entrapments within it (bicarbonate).
Protects the stomach from acid, proteolytic enzymes and mechanical damage.

Viscosity prevents it from rapidly mixing with stomach contents.

30
Q

What is mucus secreted by?

A

Surface and neck mucous cells in the necks of gastric glands

31
Q

What does the gastric mucosal barrier require?

A

Mucus and bicarbonate

32
Q

How is the gastric mucosal barrier renewed?

A

Local irritation to the mucosa stimulates the production of prostaglandins which increase production of mucus and HCO3- and inhibit acid secretion.

Secretion of HCO3- and mucus is also stimulated by acid secretion itself.

33
Q

How are prostaglandins synthesised? What are their roles?

A

Made at sites of tissue damage or infection, synthesised from arachidonic acid by cyclo-oxygenase enzyme. Inhibits gastric acid secretion and stimulates production of mucus and bicarb

Involved in the healing process.

34
Q

What are the actions of pepsin? How is it secreted?

A

Secreted by chief cells in its inactive precursor pepsinogen.
Exposure to low pH in the gastric lumen causes conversion of pepsinogen to pepsin.

Only active at low pH
Accelerates protein digestion.

Pepsin secretion parallels acid secretion

35
Q

What is intrinsic factor? What are its actions?

A

Glycoprotein secreted by parietal cells.

Essential for the absorption of B12 in the small intestine- binds to it and travels with it to the ileum.

36
Q

Why do NSAIDs have an inflammatory effect on gastric mucosa?

A

Inhibit the production of prostaglandins by inhibiting cyclo-oxygenase enzyme.
PGs E2 and I2 are required to inhibit gastric acid secretion and stimulate bicarbonate and mucus, therefore renew gastric mucosal barrier

37
Q

How does somatostatin decrease stomach acid secretion?

A

Binds to somatostatin receptors on ECL cells, preventing the secretion of histamine which prevents the stimulation of parietal cells to secrete stomach acid.

Also acts directly on the parietal cell by binding to its somatostatin receptors and inhibiting acid secretion.

Inhibits gastrin

38
Q

How does PGE2 inhibit acid secretion?

A

PGE2 formed from arachidonic acid binds to PGE2 receptors on parietal cells, preventing acid secretion

Block 1 (31 decks)

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