Stomach motility and secretion Flashcards
What are the functions of the stomach?
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
What are the purposes of gastric motility?
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.
What are the divisions of the stomach?
Fundus
Body
Antrum
Orad/Caudad regions (in terms of motility)
What is receptive relaxation?
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.
What makes up the enteric nervous system?
Myenteric plexus in smooth muscle (muscularis layer)
Submucosal plexus in submucosa
Describe the mixing phase of gastric motility
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).
What brings about the action of peristalsis?
Patterns of electrical activity: Slow waves and action potentials
What causes slow waves?
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.
What do slow waves do?
Coordinate contractions by controlling the appearance of action potentials
How are action potentials generated?
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.
How is gastric emptying regulated?
Coordinated by the activity of the pyloric sphincter, stomach and proximal small intestine.
What are the functions of the pyloric sphincter?
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
How is the rate of stomach emptying altered by the nature of stomach contents?
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.
How is the pyloric sphincter controlled to regulate gastric emptying?
- ANS: sympathetic fibres increase constriction; vagal fibres can be excitatory or inhibitory.
- Hormones: gastrin, CCK, gastric inhibitory peptide (GIP) and secretin all elicit contraction.
What are the components of gastric juice? Where is it secreted from?
Secreted from gastric glands in the gastric mucosa.
Contains hydrochloric acid, mucus, enzymes (pepsinogen and gastric lipase), intrinsic factor
What cells make up gastric glands? What is their function?
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.
Describe the distribution of secretory gastric cells in the stomach
Parietal cells located in the proximal 80% of the stomach (oxyntic gland area)
Gastrin producing cells are found in the antrum (pyloric gland area)
What are the gastric hormones? What are they secreted by?
Secreted by gastric glands, control secretion of gastric juice.
- Gastrin
- Histamine
- Somatostatin
What are the actions of gastrin?
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
What are the actions of histamine in the stomach? What is it secreted by?
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.
What are the actions of somatostatin? What secretes it?
D cells in gastric glands.
Inhibits gastrin release
Inhibits acid secretion from parietal cells.
Describe the structure of parietal cells
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
Describe the mechanism of acid secretion by parietal cells
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.
How is gastric acid regulated?
Stimulated by: gastrin, acetylcholine, histamine
Inhibited by: somatostatin, prostaglandins I2 and E2, intestinal hormones
