Biology/Chapter 1: Digestive System of Man
1.4 Role of the Nervous System and Gastrin Hormone On the Secretion of Gastric Juice
Gastric Juice:
Explanation level
The secretion of gastric juice is regulated by both nervous and hormonal mechanisms. The nervous system responds to the sight, smell, taste, and presence of food, while the hormone gastrin is released when food enters the stomach. Together, they ensure that gastric juice is secreted at the right time and in the right amount.
Definition
Gastric Juice: Gastric juice is the digestive fluid secreted by the gastric glands of the stomach. It contains hydrochloric acid (HCl), pepsinogen, mucus, intrinsic factor, and water. Approximately 2–3 litres of gastric juice are produced each day.
Gastrin: Gastrin is a hormone secreted by endocrine cells (G cells) in the gastric glands of the pyloric region. It is released into the blood and travels to the parietal cells, where it stimulates additional gastric juice secretion.
Main Idea
- Nervous mechanisms — The vagus nerve stimulates secretion in response to the sight, smell, taste, and thought of food, as well as the presence of food in the stomach.
- Hormonal mechanisms — Gastrin is released when food enters the stomach and stimulates parietal cells to secrete more gastric juice.
The greatest volume of gastric secretions is initiated by the presence of food in the stomach.
Core Concept
- Cephalic phase — Nervous stimulation before food enters the stomach (sight, smell, taste, thought of food).
- Gastric phase — Both nervous and hormonal stimulation when food is in the stomach.
- Intestinal phase — Regulation when chyme enters the small intestine.
- Gastrin — Hormone that stimulates gastric juice secretion.
- Acetylcholine — Neurotransmitter released by vagus nerve that stimulates secretion.
Detailed Explanation
Nervous Regulation of Gastric Juice Secretion
- Taste of food
- Smell of food
- Stimulation of tactile receptors during chewing and swallowing
- Pleasant thoughts of food
- These stimuli stimulate centres within the medulla that influence gastric secretion.
- Neuronal stimulation of the stomach mucosa results in the secretion of acetylcholine.
- Acetylcholine stimulates the secretory activity of both the parietal cells and chief cells.
- Acetylcholine also stimulates the secretion of gastrin from endocrine cells.
- The vagus nerve (cranial nerve X) is the main nerve responsible for stimulating gastric secretion.
- It carries signals from the brain to the stomach.
- Stimulation of the vagus nerve increases gastric juice secretion.
Hormonal Regulation of Gastric Juice Secretion
- Gastrin is released by endocrine cells (G cells) in the gastric glands of the pyloric region.
- It is released into the circulation and travels to the parietal cells.
- It stimulates additional gastric juice secretion.
- Distention of the stomach
- Presence of amino acids and peptides in the stomach
- Acetylcholine from the vagus nerve
- Secretin — Inhibits gastric secretion; stimulates pancreatic bicarbonate secretion.
- Gastric inhibitory polypeptide (GIP) — Inhibits gastric secretion.
- Cholecystokinin (CCK) — Inhibits gastric secretion; stimulates pancreatic enzyme secretion and gallbladder contraction.
Phases of Gastric Secretion
| Phase | Stimulus | Mechanism | Effect |
|---|---|---|---|
| Cephalic phase | Sight, smell, taste, thought of food | Vagus nerve → acetylcholine | Stimulates parietal and chief cells; stimulates gastrin release |
| Gastric phase | Food in stomach (distention, amino acids, peptides) | Vagus nerve → acetylcholine; gastrin release | Greatest volume of gastric secretion |
| Intestinal phase | Chyme in small intestine | Secretin, GIP, CCK | Inhibits gastric secretion |
Gastric Motility
- Gentle, rhythmic contractions of the stomach wall.
- Mix food with gastric juice.
- Form chyme.
- Stronger, more powerful contractions.
- Occur less frequently than mixing waves.
- Force chyme near the periphery of the stomach toward the pyloric sphincter.
- Each peristaltic contraction is sufficiently strong to force a small amount of chyme through the pyloric opening into the duodenum.
- Usually remains partially closed because of mild tonic contraction.
- Regulates the passage of chyme into the duodenum.
Intermediate Level
Nervous vs Hormonal Regulation
| Feature | Nervous Regulation | Hormonal Regulation |
|---|---|---|
| Stimulus | Sight, smell, taste, thought of food; stomach distension | Food in stomach (distention, amino acids, peptides) |
| Pathway | Vagus nerve → acetylcholine | G cells → gastrin → blood → parietal cells |
| Target | Parietal cells, chief cells, endocrine cells | Parietal cells |
| Effect | Stimulates secretion | Stimulates secretion |
| Speed | Rapid | Slower but longer-lasting |
Role of Acetylcholine
- Stimulates parietal cells to secrete HCl.
- Stimulates chief cells to secrete pepsinogen.
- Stimulates endocrine cells to secrete gastrin.
Role of Gastrin
- Travels through the blood to parietal cells.
- Stimulates parietal cells to secrete additional HCl.
- Increases gastric motility.
- Stimulates chief cells to secrete pepsinogen.
Regulation of Gastric Secretion
| Factor | Effect on Gastric Secretion |
|---|---|
| Vagus nerve stimulation | Increases |
| Acetylcholine | Increases |
| Gastrin | Increases |
| Histamine | Increases |
| Secretin | Decreases |
| GIP | Decreases |
| CCK | Decreases |
| Sympathetic stimulation | Decreases |
Gastric Motility and Emptying
| Type of Contraction | Description | Function |
|---|---|---|
| Mixing waves | Gentle, rhythmic | Mix food with gastric juice |
| Peristaltic waves | Stronger, less frequent | Force chyme toward pyloric sphincter |
| Pyloric pump | Peristaltic waves | Regulate gastric emptying |
Factors Affecting Gastric Emptying:
| Factor | Effect |
|---|---|
| Volume of food | Larger volume → faster emptying |
| Composition | Carbohydrates > proteins > fats |
| Duodenal acid | Slows emptying |
| Duodenal fats | Slows emptying |
| Duodenal hypertonicity | Slows emptying |
Formula / Mathematical Form
Not applicable to this topic.
Solved Example
Not applicable to this topic (descriptive topic).
Important Points
- 1. Gastric juice secretion is regulated by both nervous and hormonal mechanisms.
- 2. Approximately 2–3 litres of gastric juice are produced each day.
- 3. The vagus nerve stimulates gastric secretion through acetylcholine.
- 4. Acetylcholine stimulates parietal cells, chief cells, and endocrine cells.
- 5. Gastrin is released by endocrine cells in the pyloric region.
- 6. Gastrin travels through the blood to parietal cells.
- 7. Gastrin stimulates additional gastric juice secretion.
- 8. The greatest volume of gastric secretions is initiated by the presence of food in the stomach.
- 9. Primary stimuli for gastric secretion: distention of the stomach, amino acids, and peptides.
- 10. Peristaltic waves force chyme toward the pyloric sphincter.
- 11. The pyloric sphincter regulates the passage of chyme into the duodenum.
- 12. Other hormones that regulate gastric secretion: secretin, GIP, CCK.
Remember
- - Vagus nerve = main nerve for gastric secretion
- - Acetylcholine = neurotransmitter that stimulates secretion
- - Gastrin = hormone that stimulates secretion
- - G cells = endocrine cells that secrete gastrin
- - Parietal cells = secrete HCl and intrinsic factor
- - Chief cells = secrete pepsinogen
- - Endocrine cells = secrete gastrin
- - Cephalic phase = sight, smell, taste, thought of food
- - Gastric phase = food in stomach; greatest volume
- - Intestinal phase = chyme in small intestine; inhibits secretion
- - Mixing waves = mix food with gastric juice
- - Peristaltic waves = force chyme toward pyloric sphincter
Common Mistakes
- 1. Thinking only hormones regulate gastric secretion — Both nervous and hormonal mechanisms are involved.
- 2. Confusing gastrin with gastric juice — Gastrin is a hormone; gastric juice is the secretion.
- 3. Believing the vagus nerve only affects the heart — It also controls gastric secretion and motility.
- 4. Mixing up parietal and chief cells — Parietal cells secrete HCl; chief cells secrete pepsinogen.
- 5. Thinking gastrin is released by parietal cells — Gastrin is released by endocrine cells (G cells).
- 6. Forgetting the role of acetylcholine — Acetylcholine stimulates secretion.
- 7. Confusing cephalic and gastric phases — Cephalic is before food enters the stomach; gastric is when food is in the stomach.
- 8. Thinking the pyloric sphincter is always open — It is usually partially closed.
- 9. Believing gastric secretion is constant — It varies with the phases of digestion.
- 10. Confusing secretin with gastrin — Secretin inhibits gastric secretion; gastrin stimulates it.
Quick Revision
Gastric juice secretion is regulated by nervous and hormonal mechanisms. The vagus nerve releases acetylcholine, which stimulates parietal cells, chief cells, and endocrine cells. Gastrin is released by endocrine cells (G cells) in the pyloric region and travels through the blood to parietal cells, stimulating HCl secretion. The greatest volume of gastric secretion occurs during the gastric phase when food is in the stomach. Primary stimuli include distention, amino acids, and peptides. Mixing waves mix food with gastric juice; peristaltic waves force chyme toward the pyloric sphincter. Other hormones (secretin, GIP, CCK) inhibit gastric secretion.
60-Second Revision
- Gastric juice = 2–3 L/day
- Vagus nerve → acetylcholine → stimulates secretion
- Gastrin = hormone from G cells → stimulates HCl secretion
- Cephalic phase = sight, smell, taste, thought
- Gastric phase = food in stomach; greatest volume
- Intestinal phase = inhibits secretion
- Mixing waves = mix food
- Peristaltic waves = push chyme
- Pyloric sphincter = regulates emptying
- Warning: Both nervous and hormonal mechanisms regulate gastric secretion
NEXT TOPIC
- Chapter:
- 1. Digestive System of Man
- Topic:
- 1.5 SMALL INTESTINE
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1.4 Role of the Nervous System and Gastrin Hormone On the Secretion of Gastric Juice
Sticky Notes · Quick Revision
Key Point 1
- Gastric juice = 2–3 L/day
Key Point 2
- Vagus nerve = stimulates secretion
Key Point 3
- Acetylcholine = stimulates parietal, chief, and endocrine cells
Key Point 4
- Gastrin = hormone; stimulates HCl secretion
Key Point 5
- G cells = endocrine cells in pyloric region
Key Point 6
- Cephalic phase = sight, smell, taste, thought
Key Point 7
- Gastric phase = food in stomach; greatest volume
Key Point 8
- Intestinal phase = inhibits secretion
Key Point 9
- Mixing waves = mix food
Key Point 10
- Peristaltic waves = push chyme
Key Point 11
- Pyloric sphincter = regulates emptying
Key Point 12
- Secretin, GIP, CCK = inhibit gastric secretion
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