Gastric Function

Question 1

(Recap) What controls the motor function of the stomach?

Answer 1

1. Extrinsic (ANS):
   - SNS: NA, inhibits GI smooth muscle. SNS efferent = decreases motility and contracts sphincters
   - PNS: ACh, excitatory (motor, secretions). PNS efferent = ^ motility, secretion, relaxes sphincters.
2. Intrinsic (Enteric NS):
   - Have GI specific nerve fibres. Allow self-regulation.
   - Have submucosal and myenteric plexuses
   - Sensory: afferent neurons respond to stretch,
   tension, chemical nature of contents.
   - Motor: efferent neurons control GI contractile &
   secretory activity.
   - Excitatory = ACh, inhibitory = NO, VIP motor nerves.

Question 2

What are the three main motor functions of the stomach?

Answer 2

1. Storage
   - In proximal stomach (gastric accommodation reflex cause relaxation).
   - Via Receptive and Adaptive relaxation.
2. Mixing
   - In distal stomach
   - Spontaneous or via basal electrical rhythm (BER) (slow waves) by pacemaker cells.
   - Force ^ contractions reach pyloric sphincter.
   - Aid mixing and homogenisation of the chyme.
3. Emptying
   - In distal stomach
   - Pyloric sphincter muscles contract due to peristalsis.
   - Only a small amount of chyme is expelled into the duodenum with each wave.

Question 3

Explain the role that the stomach plays in digestion.

Answer 3

• Protein/peptide digestion by pepsin (low pH)
• Gastric lipase begins lipid digestion (small amount, pancreas is primary source of secretion)

Question 4

Name important substances absorbed by the stomach.

Answer 4

• Small amounts of lipid soluble compounds (alcohol, ASA, NSAIDs)

Question 5

What is the physiological function of Mucus cells in the stomach?

Answer 5

• Secrete protective mucus and HCO3-
• For protection

Question 6

What is the physiological function of parietal (oxyntic) cells in the stomach?

Answer 6

• Secrete HCl and intrinsic factor (allows absorption of B12)

Question 7

What is the physiological function of chief cells in the stomach?

Answer 7

• Secrete pepsinogen (inactive form of pepsin)

Question 8

How is pepsinogen activated?

Answer 8

• HCl cleaves a small fragment of pepsinogen molecules to create pepsin.
• Pepsin digests proteins into smaller fragments.

Question 9

What are the stimulatory pathways within oxyntic cells that result in gastric acid secretion?

Answer 9

• Cephalic (CNS directed): when you see, smell, taste, or think of food, increases gastric juice production.
• Gastric: arrival of food in stomach, distension, increase in gastric content pH, presence of undigested materials in stomach, especially proteins and peptides.
• Intestinal phase: when chyme first enters the SI, controls the rate of gastric emptying to ensure the SI can carry out its functions effectively.

Question 10

What are the mediators of gastric acid secretion?

Answer 10

• Ach (neural), gastrin (hormonal), histamine/ somatostatin (paracrine).
• All increase insertion of H+/K+ ATPase into the plasma membrane to increase secretion except .

Question 11

What are the main inhibitors of gastric acid secretion?

Answer 11

• Somatostatin and prostaglandin, by decreasing cAMP production and thereby Ca++.

Question 12

List the main causes of gastritis.

Answer 12

• NSAIDs
• EtOH
• Stress
• H. pylori
• Autoimmunity
• Inflammation of the gastric mucosa

Question 13

List the main causes of peptic ulcer disease.

Answer 13

• H. pylori
• NSAIDs, sore as a result of inflammation