Gastric motility and secretion Flashcards
What are the functions of the stomach?
Food storage Barrier for harmful bacteria (Partial) digestion of macromolecules Regulation of digestion Secretion of intrinsic factor
What does gastric motility allow the stomach to do?
Reservoir for large volume of food
Fragment food and mix with gastric secretions
Empty contents into duodenum at controlled rate
How does receptive relaxation aid gastric motility?
Smooth muscle in the orad region relaxes to increase stomach volume to approx. 1.5 litres without increasing pressure
What mediates receptive relaxation?
Parasympathetic input from vagus nerve and enteric plexus
Release of NO and serotonin by enteric neurones mediates relaxation of smooth muscle cells
What is retropulsion?
Closure of the pyloric sphincter as peristaltic wave causes contraction of the antrum - this causes the contents of the antrum to be forced backwards into the body of the stomach
What is peristalsis?
Waves of alternating contractions and relaxations of smooth muscle layers that mix and squeeze contents through hollow tubes (i.e. moves bolus along GI tract)
What is segmentation?
Cycles of contractions that mix contents but do not push them in any one direction
How are contractions of smooth muscles in the GI tract achieved?
Slow waves (coordinate contractions) Action potentials
How are slow waves achieved?
Fluctuations in membrane potential spreading to adjacent sections of muscle
How is gastric emptying achieved?
Coordinated contractions of the stomach, pylorus and duodenum
What are the functions of the pyloric sphincter?
- Regulation of emptying gastric contents at optimal rate for digestion
- Prevents regurgitation of duodenal contents into stomach
How is the pyloric sphincter controlled?
Autonomic Nervous System (sympathetic and vagal)
Hormonal control
What hormones are involved in controlling the pyloric sphincter?
Gastrin
Cholecystokinin (CCK)
Gastric Inhibitory Peptide (GIP)
Secretin
[Nb. all elicit contraction of sphincter]
What factors regulate gastric emptying?
Nature of duodenal contents (e.g. high in fat, highly acidic or very hypertonic all decrease rate of gastric emptying)
What are the main components of gastric juice?
Hydrochloric acid
Mucus
Enzymes (e.g. pepsinogens, gastric lipase)
Intrinsic Factor
Where are gastric juices secreted from?
Gastric glands in mucosa
Where are parietal cells typically found?
Proximal 80% of stomach (oxyntic gland area)
Where are gastrin-producing cells typically found?
Antrum (pyloric gland area)
What are the gastric effects of Gastrin?
- Stimulation of secretion of acid, pepsinogens, mucus and bicarbonate
- Stimulation of gastric motility
- Inhibition of gastric emptying
What effects does Gastrin have on other areas of the GI tract?
- Stimulation of pancreatic enzymes and bicarbonate secretion
- Stimulation of insulin release
- Stimulation of intestinal motility
What is Histamine?
paracrine agent
Where is Histamine secreted from?
Enterochromaffin-like Cells (ECL)
What are the gastric effects of Histamine?
- Stimulation of acid secretion
2. Increased local blood flow (supports increased metabolism associated with acid secretion)
Where is somatostatin synthesised?
D cells (in antrum and body of stomach)
What are the gastric effects of Somatostatin?
- Inhibition of gastrin release
2. Inhibition of acid secretion
How are parietal cells specialised?
Truncated pyramidal shape
Invaginations of the luminal membrane form canaliculi which are lined with ion pumps
High mitochondrial content
How is acid secreted from parietal cells?
- Hydrogen ions and Bicarbonate produced from CO2 and water
- Hydrogen ions secreted into lumen via H+/K+ ATPase pump
- Bicarbonate moves across basolateral membrane via antiport with Cl-
- Cl- diffuses passively into the lumen via a Cl- channel
What physiological agents stimulate gastric acid secretion?
Gastrin (hormone)
Acetylcholine (parasympathetic input from CN X)
Histamine (paracrine agent)
What hormones inhibit gastric acid secretion?
Somatostatin
Prostaglandins (E2 and I2)
Intestinal hormones (Enteric hormones inhibiting Gastrin release)
What are prostaglandins?
Lipids derived from arachidonic acid made at sites of tissue damage or infection to control processes such as inflammation and blood flow
What enzyme catalyses prostaglandin synthesis?
Cyclooxygenase Enzyme (COX)
What is mucus made up of?
Glycoproteins
Glycopolysaccharides
Where is mucus secreted from in the stomach?
Mucus (neck) cells in the necks of gastric glands
Surface epithelial cells of the stomach
What role does mucus play in the stomach?
Forms a gastric mucosal barrier that protects the stomach against acid, proteolytic enzymes and mechanical damage
How is pepsin secreted?
Secreted by chief cells in the form of Pepsinogen
What causes pepsinogen to convert to pepsin?
Exposure of low pH in the lumen (so pepsin is only active at low pH)
What role does pepsin play in the stomach?
Accelerates protein digestion
What is intrinsic factor?
A glycoprotein secreted by parietal cells of the stomach
What is the role of intrinsic factor?
Essential for absorption of vitamin B12 in the intestines
What is Zollinger-Ellison Syndrome?
Presence of a gastrin-secreting tumour or hyperplasia of the Pancreatic Islet cells causes increased Gastrin and therefore increased H+ secretion leading to recurrent peptic ulcers
Why does Zollinger-Ellison Syndrome cause peptic ulcers?
Non-physiological gastrin secretion by a Gastrinoma is not feedback-regulated so gastrin secretion continues even when stomach contents are already very acidic.
Duodenal ulcers can occur as excess H+ load can not be buffered (movement from stomach to duodenum and neutralisation by Bicarbonate)
What investigations can be used to diagnose Zollinger-Ellison Syndrome?
Endoscopy
Pentagastrin Stimulation Test
Serum Gastrin level
Basal gastric H+ secretion
What is a Pentagastrin Stimulation Test?
Can be used to identify Zollinger-Ellison Syndrome
In healthy patients, Pentagastrin (a gastrin analogue) would stimulate increased secretion of H+ by parietal cells.
In patients with Zollinger-Ellison (or Gastrinoma), H+ secretion is already maximally stimulated so administration of a gastrin analogue does not increase H+ secretion further.
