Gastric Physiology 2 Flashcards
Which cells produce pepsinogen?
Chief cells
What is pepsinogen?
The inactive (zymogen) form of pepsin.
What stimulates pepsinogen secretion?
- gastrin
- vagus nerve
What is the relationship between pepsinogen secretion and HCl secretion?
They occur in parallel (both stimulated by gastrin and the vagus nerve).
How is pepsinogen activated?
Converted into pepsin when it mixes with HCl in the stomach lumen.
Most efficient at pH < 2
How does pepsin activation result in a positive feedback loop?
Pepsin catalyses the reaction between pepsinogen and HCl that produces pepsin.
How is pepsin inactivated?
HCO3- is secreted from Brunner glands in the submucosa of the small intestine. This irreversibly inactivates pepsin, as pepsin is only active at low pHs.
What is the role of pepsin in protein digestion?
- accelerates digestion
- accounts for around 20% of total protein digestion
- breaks down collagen in meat which provides a greater surface area for digestion
What is the volume of the empty stomach?
Around 50ml
How great a volume of the stomach can be reached without much increase in luminal pressure?
Around 1.5l
What is receptive relaxation?
The relaxation of smooth muscle in the body and the fundus of the stomach upon the bolus entering the stomach via the oesophagus and pharynx.
How is receptive relaxation mediated?
- afferent input via vagus nerve
- parasympathetic nervous system acts on enteric nerve plexuses
- enteric nerves release nitric oxide and serotonin to mediate relaxation
How does peristalsis of the stomach work?
- peristaltic waves begin in the gastric body (weak contraction = little mixing of chyme)
- contraction grows more powerful in the antrum
- pylorus closes as the peristaltic wave reaches it, so the chyme is forced back into the body where it mixes
How is the frequency of the peristaltic waves determined?
Pacemaker cells in the muscularis propria undergo slow depolarisation-repolarisation cycles.
Waves of depolarisation are transmitted 3 times per minute through the gap junctions to the adjacent smooth muscle cells, causing contraction when the stomach contains chyme.
Does the empty stomach undergo peristaltic contraction?
No, the pacemaker cells continue to undergo their depolarisation-repolarisation cycles, but the waves of depolarisation so not cause significant contraction of smooth muscle cells when the stomach is empty.
