Gastric Physiology 2: Proteases, Gastric motility and emptying Flashcards
What cells produce pepsinogen?
Chief cells
What is pepsinogen when it has not been activated?
In an inactive form or precursor to a protease (zymogen)
What happens to pepsinogen to turn it into a protease?
- It is mediated by input from enteric nervous system (ACh)
- Secretion parallels HCl secretion
- Activates the lumen
What feedback loop is involved in protease activation?
- Positive feedback loop
- HCl present and helps the transformation of pepsinogen into pepsin
- Pepsin added back into loop to create more pepsin from pepsinogen, acts as a catalyse
What is the feedback loop for protease activation dependent on?
- pH dependent
- Most efficient when pH <2
- Pepsin is involved in the feedback loop
- Pepsin only active at low pH
What is the role of pepsin in protein digestion?
- Not essential (protein digestion can occur if the stomach is removed)
- Accelerates protein digestion
- Normally accounts for 20% of total protein digestion
- Breaks down collagen in meat (shreds into smaller pieces with greater surface area for digestion)
What can the volume of the stomach increase to?
- Empty stomach = 50mL
- When eating, can accommodate 1.5L with little increase in luminal pressure
- Smooth muscle in body and fundus undergoes receptive relaxation (which is mediated by the vagus nerve)
What is receptive relaxation?
- Mediated by parasympathetic nervous system acting on enteric nerve plexuses
- Co-ordination: afferent input via Vagus nerve
- Nitric oxide and serotonin released by enteric nerves mediate relaxation
Where do peristaltic waves begin?
- In gastric body (fundus), weak contraction in the body and so little mixing
What are the next steps in peristalsis?
- More powerful contraction in gastric antrum.
- Pylorus closes as peristaltic wave reaches it
- Little chyme enters duodenum
- Antral contents forced back towards body (mixing)
What cells determine the frequency of peristaltic waves?
- Pacemaker cells in muscularis propria (interstitial cells of Cajal)
- These cells undergo slow depolarisation - repolarisation cycles
- Depolarisation waves transmitted through gap junctions to adjacent smooth muscle cells
- Do not cause contraction in empty stomach
How do the strength of peristaltic contractions vary?
- Excitatory neurotransmitters and hormones further depolarise membranes
- Action potentials generated when threshold reached
How do the strength of peristaltic contractions increase?
- Gastrin
- Gastric distension (medicated by mechanoreceptors)
How do the strength of peristaltic contractions decrease?
- increase in duodenal luminal fat
- increase in duodenal osmolarity
- decrease in duodenal luminal pH
- increase in sympathetic NS action
- decrease in parasympathetic NS action
(slows down the movement of things from stomach to duodenum)
What is dumping syndrome?
- The overfilling of duodenum by a hypertonic solution causes dumping syndrome
(so a shift in fluids)
Capacity of stomach > capacity of duodenum
