Gastric Motility and Secretion Flashcards
Outline the functions of the stomach
- Store Food
- Minimise ingestion of bacteria
- Dissolve + partially digest macromolecules
- Regulate emptying
- Secrete intrinsic factor
What are the layers of the GI tract? (From Lower Oesophagus to anal canal)
- Mucosa
- Submucosa
- Muscularis
Inner - circular
Outer - longitudinal - Serosa
Inner oblique in muscularis of stomach
Outline receptive relaxation of the stomach
When food is swallowed, the smooth muscle in the wall of the orad region of the stomach relaxes. Stomach increases volume from 50ml to 1.5L.
How is receptive relaxation of the stomach mediated?
Vagus nerve -> Enteric plexus
- Enteric neurons release NO and serotonin to relax muscles.
- Coordinate with swallowing centre
Outline mixing in the stomach
1) Contractions start in body and move toward antrum (Peristalsis)
2) When wave reaches antrum pyloric sphincter closes
3) Food is forced backwards into body (Retropulsion)
Define peristalsis
Waves of alternating contractions and relaxations of smooth muscle layers that mix and squeeze the contents through hollow tubes.
Move a bolus along the length of the GI tract.
How are GI peristaltic waves generated?
Coordinated contractions of smooth muscle: Slow Waves and Action Potentials
What are Slow waves?
Pacemaker zone in stomach generate membrane potentials which spreads across smooth muscle cells. 3 slow waves/min.
Slow waves don’t elicit contractions, they bring the threshold potential high enough for an action potential to initiate.
What is the pyloric sphincter?
Ring of smooth muscle and connective tissue between the gastric antrum and the duodenum.
What are the functions of the pyloric sphincter?
1) Regulated emptying of gastric contents
2) Prevents regurgitation of duodenal content.
What decreases the rate of gastric emptying?
- High fat food
- Highly acidic food
- Very hypertonic food
What are the components of gastric secretions?
- enzymes: pepsinogens, gastric lipase
- intrinsic factor
- HCl
- mucus
In which parts of the stomach are parietal cells and G cells mainly found?
Proximal 80 % of the stomach (“oxyntic gland area”)
G-cells are found typically in the antrum (“pyloric gland area”).
Where is Gastrin produced and what does it do?
G cells Increases: - Acid, mucus and pepsinogen - Gastric motility (but Inhibits gastric emptying) - Insulin and HCO3- release - Intestinal motility
Where is Histamine produced and what does it do?
Enterochromaffin-like (ECL) cells
Stimulates acid secretion and blood flow to stomach
Where is Somatostatin produced and what does it do?
D cells of antrum
Inhibit acid secretion and gastrin release.
What do chief cells secrete?
Pepsinogen (converted to pepsin in low pH)
What do parietal cells secrete?
HCl
Intrinsic factor - Binds VitB12 for absorption
How is the parietal cell specialised?
- Pyramidal shape
- Canaliculi lined with H+/K+ATPases
- High mitochondria
What stimulates and inhibits gastric acid secretion (Give 3 for each).
Stimulated by:
gastrin
acetylcholine
histamine
Inhibited by:
somatostatin
prostaglandins E2 and I2
intestinal hormones
What are prostaglandins?
A group of lipids derived from arachidonic acid.
Involved in the healing;
Control inflammation and blood flow.
Synthesis is catalysed by a cyclooxygenase enzyme.
Where is mucus produced in the stomach? And what does it do?
Surface epithelial and neck cells. Forms gel that protects stomach lining against acid (Mucosal Barrier)
How is the stomach protected from autodigestion?
Prostaglandins increase production of mucus and HCO3- and inhibit acid secretion.
Surface epithelial cells secrete HCO3-
Stomach has high cell turnover rate
What are the consequences of failure of gastric mucus protection?
Gastritis
Gastric Ulcers
Overuse of which drug can cause gastric ulcer formation?
NSAIDs
