Gastric Motility And Secretions Flashcards
Gastric accommodation
The ability of the stomach to relax when in the presence of high amounts of food
- vagus nerve controls this
different from receptive relaxation in that this is NOT automatic when initiating swallowing
Mechanical actions of the stomach
1) propulsion
- peristaltic wave moves food bolus towards a pyloris
2) grinding
- antrum cells stimulated by gastrin increase contractions of stomach and churn the chyme better
3) retropulsion
- minimal amounts of the chyme in the stomach gets through the pyloric sphincter, whereas the rest is sent back into the stomach to repeat 1-2 steps.
How does the body regulate motility of the stomach?
1) Parasympathetics and hormones gastrin and motilin
- all three of these work to increase the frequency of action potentials and force of contractions
2) Sympathetics and hormones secretin and GIP
- all three work to decrease the frequency of action potentials and force of contractions
Migrating myoelectric complexes
Motilin mediated gastric contractions that occur in 90-minute intervals
- function to clear stomach of any remaining residues left over
What two factors are major in slowing/inhabiting gastric emptying?
Fat and H+ ions
- both stimulate CCK release
Also H+ ions stimulate ENS reflexes which slows gastric smooth muscle down, permitting time for pancreatic bicarbonate to come in and decrease acid before duodenum
Regulation of pepsinogen
Zymogen that is released in the presence of peptides
- strongest stimulant of pepsinogen secretion is ACh
Is spontaneously lyzed into pepsin at pH less than 5
- however pepsin only functions at pH lower than 3 (hence why the stomach is usually less than 2 pH where as the duodenum is >4 pH
Vitamin B12 (cobalamin) absorption
Mediated by intrinsic factor (IF)
Cobalamin is cleaved by pepsin from dietary proteins and released into the stomach
Gastric glands secrete haptocorrin (R-protein) => binds to cobalamin
Gastric parietal cells secretes IF and the pancreas secretes bicarbonate and proteases
Haptocorrin complex, bicarbonate and proteases all go into the small intestine
- complex is released due to degradation of haptocorrin
Cobalamin binds to IF and forms a complex which allows it to be absorbed by small intestine by Ileal enterocytes
Gastric acid secretion steps
- requires parietal cells*
1) carbon dioxide and water combine to form carbonic acid in parietal cells
2) carbonic acid spontaneously loses H+ ions in parietal cells and becomes bicarbonate
3) H+/K+ ATPase pumps H+ ions into the lumen and K+ ions into the parietal cells
4) bicarbonate leaves parietal cells via HCO/Cl- channels
- Cl comes in, HCO goes out
5) chloride diffuses through the membrane of parietal cells into the stomach lumen and combines with H+ ions to form HCL
Regulation of gastric acid secretion
ACh, histamine, gastrin somatostatin, and PG2 all help regulate this
ACh = muscarinic nerves on M3 receptors (increases IP3/DAG)
- Gq channels
Gastrin = G-cells and bind to CCK2 PLC receptors (increase IP3/DAG)
- **can also bind to ECL cells and release histamine to help its effect
- Gq channels
Histamine = ECL cells and binds to H2 AC receptors (increases cAMP)
- Gs channels
Somatostatin and PG1 = binds to their own receptors but works to inhibit Adenylate Cyclase in parietal cells
- GI channels
What is the cephalad phase of gastric acid secretion?
Is what occurs in the stomach once swallowing has initiated
Vagus nerve stimulates parietal cells via ACh and G cells via GRP
- G cells = makes gastrin to further stimulate parietal cells
Gastric phase of gastric acid secretion
Vagovagal reflexes based on distention of stomach and presence of amino acids and peptides
Stimulates the same things as seen in the cephalic phase, but requires afferents to get sent to vagus first and then the vagus goes.
Effects of eating on acid secretion
1) within 1 hr of digesting food
- stomach is full and buffers the H+ ions in the stomach
- total levels of H+ ions tanks (<100mL)
- H+ secretion rate starts to increase
2) within 1-2 hrs
- stomach begins to empty
- total levels of H+ ions very slightly increases
- H+ secretion rates are at max magnitude (very high)
3) >3hrs
- stomach is empty
- total levels of H+ ions are very high z(back to resting level = 800mL)
- H+ secretion rates tank (super low)
Why are total numbers of H+ ions and rate of H+ secretion inversely related?
Because gastrin is the prime mover in H+ secretion and it is inhabited in low pH (high H+ concentrations)
Also low pH = increases somatostatin release
