Gastric motility and secretion

Question 1

What are the functions of the stomach?

Answer 1

Food storage
Barrier for harmful bacteria
(Partial) digestion of macromolecules
Regulation of digestion 
Secretion of intrinsic factor

Question 2

What does gastric motility allow the stomach to do?

Answer 2

Reservoir for large volume of food
Fragment food and mix with gastric secretions
Empty contents into duodenum at controlled rate

Question 3

How does receptive relaxation aid gastric motility?

Answer 3

Smooth muscle in the orad region relaxes to increase stomach volume to approx. 1.5 litres without increasing pressure

Question 4

What mediates receptive relaxation?

Answer 4

Parasympathetic input from vagus nerve and enteric plexus

Release of NO and serotonin by enteric neurones mediates relaxation of smooth muscle cells

Question 5

What is retropulsion?

Answer 5

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

Question 6

What is peristalsis?

Answer 6

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)

Question 7

What is segmentation?

Answer 7

Cycles of contractions that mix contents but do not push them in any one direction

Question 8

How are contractions of smooth muscles in the GI tract achieved?

Answer 8

Slow waves (coordinate contractions) 
Action potentials

Question 9

How are slow waves achieved?

Answer 9

Fluctuations in membrane potential spreading to adjacent sections of muscle

Question 10

How is gastric emptying achieved?

Answer 10

Coordinated contractions of the stomach, pylorus and duodenum

Question 11

What are the functions of the pyloric sphincter?

Answer 11

1. Regulation of emptying gastric contents at optimal rate for digestion
2. Prevents regurgitation of duodenal contents into stomach

Question 12

How is the pyloric sphincter controlled?

Answer 12

Autonomic Nervous System (sympathetic and vagal)

Hormonal control

Question 13

What hormones are involved in controlling the pyloric sphincter?

Answer 13

Gastrin
Cholecystokinin (CCK)
Gastric Inhibitory Peptide (GIP)
Secretin

[Nb. all elicit contraction of sphincter]

Question 14

What factors regulate gastric emptying?

Answer 14

Nature of duodenal contents (e.g. high in fat, highly acidic or very hypertonic all decrease rate of gastric emptying)

Question 15

What are the main components of gastric juice?

Answer 15

Hydrochloric acid
Mucus
Enzymes (e.g. pepsinogens, gastric lipase)
Intrinsic Factor

Question 16

Where are gastric juices secreted from?

Answer 16

Gastric glands in mucosa

Question 17

Where are parietal cells typically found?

Answer 17

Proximal 80% of stomach (oxyntic gland area)

Question 18

Where are gastrin-producing cells typically found?

Answer 18

Antrum (pyloric gland area)

Question 19

What are the gastric effects of Gastrin?

Answer 19

1. Stimulation of secretion of acid, pepsinogens, mucus and bicarbonate
2. Stimulation of gastric motility
3. Inhibition of gastric emptying

Question 20

What effects does Gastrin have on other areas of the GI tract?

Answer 20

1. Stimulation of pancreatic enzymes and bicarbonate secretion
2. Stimulation of insulin release
3. Stimulation of intestinal motility

Question 21

What is Histamine?

Answer 21

paracrine agent

Question 22

Where is Histamine secreted from?

Answer 22

Enterochromaffin-like Cells (ECL)

Question 23

What are the gastric effects of Histamine?

Answer 23

1. Stimulation of acid secretion

2. Increased local blood flow (supports increased metabolism associated with acid secretion)

Question 24

Where is somatostatin synthesised?

Answer 24

D cells (in antrum and body of stomach)

Question 25

What are the gastric effects of Somatostatin?

Answer 25

1. Inhibition of gastrin release | 2. Inhibition of acid secretion

Question 26

How are parietal cells specialised?

Answer 26

Truncated pyramidal shape Invaginations of the luminal membrane form canaliculi which are lined with ion pumps High mitochondrial content

Question 27

How is acid secreted from parietal cells?

Answer 27

1. Hydrogen ions and Bicarbonate produced from CO2 and water 2. Hydrogen ions secreted into lumen via H+/K+ ATPase pump 3. Bicarbonate moves across basolateral membrane via antiport with Cl- 4. Cl- diffuses passively into the lumen via a Cl- channel

Question 28

What physiological agents stimulate gastric acid secretion?

Answer 28

Gastrin (hormone) Acetylcholine (parasympathetic input from CN X) Histamine (paracrine agent)

Question 29

What hormones inhibit gastric acid secretion?

Answer 29

Somatostatin Prostaglandins (E2 and I2) Intestinal hormones (Enteric hormones inhibiting Gastrin release)

Question 30

What are prostaglandins?

Answer 30

Lipids derived from arachidonic acid made at sites of tissue damage or infection to control processes such as inflammation and blood flow

Question 31

What enzyme catalyses prostaglandin synthesis?

Answer 31

Cyclooxygenase Enzyme (COX)

Question 32

What is mucus made up of?

Answer 32

Glycoproteins | Glycopolysaccharides

Question 33

Where is mucus secreted from in the stomach?

Answer 33

Mucus (neck) cells in the necks of gastric glands | Surface epithelial cells of the stomach

Question 34

What role does mucus play in the stomach?

Answer 34

Forms a gastric mucosal barrier that protects the stomach against acid, proteolytic enzymes and mechanical damage

Question 35

How is pepsin secreted?

Answer 35

Secreted by chief cells in the form of Pepsinogen

Question 36

What causes pepsinogen to convert to pepsin?

Answer 36

Exposure of low pH in the lumen (so pepsin is only active at low pH)

Question 37

What role does pepsin play in the stomach?

Answer 37

Accelerates protein digestion

Question 38

What is intrinsic factor?

Answer 38

A glycoprotein secreted by parietal cells of the stomach

Question 39

What is the role of intrinsic factor?

Answer 39

Essential for absorption of vitamin B12 in the intestines

Question 40

What is Zollinger-Ellison Syndrome?

Answer 40

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

Question 41

Why does Zollinger-Ellison Syndrome cause peptic ulcers?

Answer 41

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)

Question 42

What investigations can be used to diagnose Zollinger-Ellison Syndrome?

Answer 42

Endoscopy Pentagastrin Stimulation Test Serum Gastrin level Basal gastric H+ secretion

Question 43

What is a Pentagastrin Stimulation Test?

Answer 43

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.