Gastric Motility and Pancreatic Function

Question 1

Explain the mechanisms involved in the neutralisation of gastric acid in the duodenum, including control of bicarbonate secretion in the duodenum.

Answer 1

• Bicarbonate (HCO3) secretion from Brunner’s gland dust cells (submucosal glands).

Acid in duodenum triggers;

• Long (vagal) and short (ENS) reflexes –> HCO3 secretion.
• Release of secretin from S cells –> HCO3 secretion (from pancreas and liver) –> acid neutralisation –> inhibits secretin release (negative feedback control).

Question 2

Explain the role of gastric motility in digestion.

Answer 2

• Peristaltic waves: body –> antrum.
  Body;
• Thin muscle = weak contraction, no mixing.
  Antrum;
• Thick muscle = powerful contraction, mixing.
• Contraction of pyloric sphincter means small quantity of gastric content (chyme) enters duodenum at a time and further mixing occurs as antral contents are forced back towards body.

Question 3

Describe the mechanisms controlling gastric emptying.

Answer 3

• Gastrin –> increases contraction.
• Distension of stomach wall –> long/short reflexes –> increased contraction.
• Fat/acid/amino acid/hypertonicity in duodenum –> inhibition of motility.

Question 4

Describe the actions of secretin and cholecystokinin (CCK) on pancreatic and bile secretion and the stimuli which will cause their release.

Answer 4

• Acid in duodenum –> secretin –> bicarbonate secretion.
• Fat/amino acids in duodenum –> cholecystokinin (CCK) –> zymogen secretion.
  Both also under neural control (vagal/local reflexes) triggered by arrival of chyme in duodenum.

Question 5

Describe the anatomy and histology of the pancreas.

Answer 5

• Head (located within curvature of duodenum, body, tail (extends to spleen).
• Endocrine portion –> pancreatic islets (islets of Langerhans): produce insulin, glucagon and somatostatin (control secretion or prior two hormones).
• Exocrine portion –> acinar cells (salivary glands) –> lobules –> connected by intercalated ducts –> intralobular ducts –> interlobular ducts –> main pancreatic duct –> common bile duct –> hepatopancreatic ampulla (Sphincter of Oddi) –> duodenum.
• Secretion of bicarbonate by duct cells.
• Secretion of digestive enzymes by acinar cells.

Question 6

Describe the structure of the exocrine pancreas.

Answer 6

• Responsible for digestive function of pancreas.
• Acini –> ducts –> pancreatic duct.
• Secretion of bicarbonate by duct cells.
• Secretion of digestive enzymes by acinar cells.

Question 7

Describe the production of peristaltic waves.

Answer 7

• Peristaltic rhythm (3/min) generated by pacemaker cells (longitudinal muscle layer, connected by gap junctions)
• Slow waves: spontaneous de- and repolarisation.
• Slow wave rhythm = basic electrical rhythm (BER)
• Slow wave depolarisation sub-threshold: requires further depolarisation, slow waves alone cannot cause contraction.
• Number of APs/wave determines strength of contraction.

Question 8

Describe zymogens.

Answer 8

• Acinar cells contain digestive enzymes stored as inactive zymogen granules.
• Prevents autodigestion of pancreas.
• Enterokinase (bound to brush border of duodenal enterocytes) converts trypsinogen to trypsin.
• Trypsin converts all other zymogens to active forms.

Question 9

Describe the categories of pancreatic enzymes.

Answer 9

• Proteases: cleave peptide bonds.
• Nucleases: hydrolyse DNA/RNA.
• Elastases: collagen digestion.
• Phospholipases: phospholipids –> fatty acids.
• Lipases: triglycerides –> glycerol + fatty acids.
• Alpha-amylase: Starch –> glucose + maltose.