How do gastric contents move through the stomach?
By peristaltic waves
In what section of the stomach does most mixing occur and why?
The antrum as it has a thick powerful muscle
The muscle around the body of the stomach is much weaker and so does little more than move the contents along
How does the pyloric sphincter control movement of gastric contents?
Once a small amount of gastric contents hits the duodenum the pyloric sphincter closes forces most of the contents back up towards the body to be mixed again
How fast is the gastric peristaltic rhythm?
~3 waves/min
What regulates the peristaltic rhythm?
Spontaneous depolarisation in pacemaker cells in the longitudinal muscle layer
What do we call the basic electrical rhythm of the gastric muscle?
Slow Wave Rhythm
What parts of the GI tract are slow waves conducted to and how?
The stomach, and intestines.
The longitudinal muscle is connected by gap junctions all the way down
The slow wave depolarisation happens constantly, why isnt your stomach contracting even when empty?
The slow wave depolarisation is sub threshold and requires further depolarisation from another source to trigger an action potential
What other sources of depolarisation add to the slow wave depolarisation?
Gastrin -> Increases Contraction
Distension of stomach wall -> Vagal and ENS reflexes (long & Short) -> Increased Contraction
What factors reduce gastric motility?
Acid in duodenum -> More Secretin from S cells -> Reduce gastric motility
Fat/amino acids/hypertonicity in duodenum -> Cholecystokinin -> reduce gastric motility
What chemical neutralises stomach acid upon reaching the duodenum? and where is it produced?
Bicarbonate (HCO3-)
Made in ductal cells (pancreatic duct) and brunners glands (duodenal submucosa)
How is HCO3- secretion regulateD?
Acid is detected in the duodenum:
1) Long vagal & short ENS reflexes
2) Secretin released from S cells
- triggers HCO3- secretion in ductal cells
- acid neutralisation then inhibits secretin release
Describe the endocrine portions of the pancreas?
Islets of langerhans
They secrete:
- insulin & glucagon.
- Somatostatin (controls insulin/glucagon release)
Describe the exocrine portion of the pancreas and the passage to the duodenum
Acinar cells forms lobules
- > Lobules connected by intercalated ducts
- > Join into intralobular ducts
- > Join into interlobular ducts
- > Combine into pancreatic duct
- > Joins common bile duct at hepatopancreatic ampulla
- > Passes through sphincter of Oddi
- > Exits into duodenum at the major duodenal papilla
How do we compensate for a blocked pancreatic duct?
WE have a smaller accessory pancreatic duct that can open if needed
What type of epithelium lines the pancreatic duct and what do they produce?
Cuboidal epithelium makes up the ductal cells.
They produce bicarbonate
What does the exocrine portion of the pancreas secrete?
Acinar cells -> Digestive enzymes
Ductal cells -> Bicarbonate
Why do acinar cels store their secretions as zymogens?
So they dont self digest
How are the zymogens of digestive enzymes activated upon hitting the duodenum?
Enterokinase is bound to the brush border of duodenal enterocytes
- > It activates trypsinogen to trypsin
- > Trypsin then converts the rest of the zymogens to active forms
Name the 6 types of pancreatic enzymes and what they do:
Proteases - cleave peptide bonds
Nucleases - Hydrolyse DNA/RNA
Elastases - Digest collagen
Phospholipidases - break down phospholipids to fatty acids
Lipases - break TAG to fatty acids + glycerol
Alpha-amylase - Breaks starch to maltose & glucose
Summarize the control of pancreatic secretions
Bicarbonate:
- Secretin controlled
- Released in response to acid in the duodenum
Zymogens:
- Controlled by Cholecystokinin (CCK)
- Released in response to fats & amino acids in the duodenum
Neural control through long vagal and short ENS reflexes also trigger bicarbonate and zymogen release in response to organic nutrients in the duodenum.
