Phases of Gastric secretion
Gastric secretion is a continuous process
but the quantity of secretion varies depending upon time and the stimulus.
There are 3 phases of gastric secretion
- Cephalic phase
- Gastric phase
- Intestinal phase
The regulation of gastric secretion is
studied by experimental procedures, which are:
- Pavlov pouch
- Haidenhan pouch
- Bickel Pouch
- Farrel and Ivy Pouch
- Sham feeding
Pavlov pouch
It was designed by a Russian scientist Ivan Pavlov in
a dog experiment during his study of conditioned reflexes.
Preparation:
- Dog is anesthetized.
- Stomach is divided into a larger and a smaller portion by an incomplete incision.
- Cut edges are stitched but the connection is intact as shown in the picture below.
- Outlet is brought out through the abdominal wall for drainage of gastric secretion.
- Pavlov pouch receives parasympathetic fibers via Vagus nerve and sympathetic fibers through blood vessels
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| Pavlov pouch |
Use of Pavlov pouch:
- Used to demonstrate the different phases of gastric secretion.
- Used to demonstrate the role of Vagus in the Cephalic phase of gastric secretion.
Heidenhain pouch
- It is a modified Pavlov pouch
- The pouch is completely separated from the main portion of stomach without damaging the blood vessels.
- Heidenhain pouch has No parasympathetic supply but Sympathetic fibers are intact via the blood vessels
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| Heidenhain pouch |
Uses of
- To demonstrate the role of sympathetic nerve and hormonal regulation of gastric secretion after Vagotomy. (cutting of Vagus nerve)
Bickel Pouch
- Here even the sympathetic nerve fibers are cut by removing the blood vessels.
- Bickel pouch is totally a denervated pouch.
Uses of Bickel Pouch:
- To demonstrate the role of hormones in gastric secretion
Farrel and Ivy Pouch
- This is prepared by completely removing Bickel pouch from the stomach and transplanting it in the subcutaneous tissue of abdominal or thoracic wall in the same animal.
- New blood vessels develop after some days and sympathetic nerve supply is established.
Uses of Farrel and Ivy Pouch:
- To study the role of hormones during gastric and intestinal phases of gastric secretion
Sham Feeding
- It is aslo known as false feeding
- It was postulated by Ivon Pavlov
Preparation:
- Dog is anesthetized.
- A hole is made in the neck and esophagus is cut.
- Cut ends are drawn out through the hole in the neck.
- When the dog eats food, it comes out through the cut end of the esophagus.
- But the dog has the satisfaction of eating the food.
- A fistula is made in the stomach which opens to the exterior which is used to observe gastric secretion
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| Sham feeding |
Advantage of sham feeding:
- It is useful to demonstrate the secretion of gastric juice during cephalic phase of gastric secretion.
- After vagotomy, sham feeding does not induce gastric secretion, proves the role of vagus nerve during cephalic phase.
Phases of Gastric secretion
There are 3 phases of gastric secretion
- Cephalic phase
- Gastric phase
- Intestinal phase
- In humans a fourth phase called inter-digestive phase also exists
- Each phase is regulated by neural or hormonal mechanism or both
1. Cephalic phase
- This phase is elicited by smell, sight, thought, taste and chewing of food.
- This is called cephalic because the impulse to secrete gastric juice comes mainly from the brain (Cephalus).
- Secretion occurs even without food in stomach.
- This phases is mediated by Vagus nerve via Unconditioned and Conditioned reflex.
Experimental Evidence:
• Sham feeding
Unconditioned Reflex
- It is an inborn reflex
- Presence of food in mouth cause salivary and gastric secretion
Food in mouth
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Stimulation of taste receptors
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Sensory impulse carried by glossopharyngeal
and facial nerve
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Amygdla and appetite center in Hypothlamus
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Dorsal nucleus of Vagus
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Vagus innervate walls of stomach
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Secretion of gastric juice
(Neurotransmitter is ACh)
Conditioned Reflex
- This reflex is acquired by previous experience.
- Presence of food in mouth is not necessary to elicit this reflex.
- Sight, smell, hearing or thought of food cause salivary and gastric secretion.
Impulses from the special sensory organs
(eye, ear and nose)
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Afferent fibers
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Cerebral cortex
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Dorsal nucleus of Vagus
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Vagus innervate walls of stomach
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Stimulates the gastric secretion
2. Gastric phase
- As food enters the stomach there is secretion of gastric juice.
Stimulus
- Distention of stomach
- Mechanical stimulation by bulk of food
- Chemical stimulation by contents of food
Regulated by both nervous and hormonal
control
Regulatiory mechanisms
Nervous mechanism
- Local Myenteric reflex and Vagovagal reflex
Hormonal mechanism
- Gastrin
Nervous mechanism
Local myenteric reflex:
- Elicited by stimulation of Myenteric nerve plexus in stomach wall
- Nerve fibers release ACh
- Stimulates gastric glands – secretion of large quantity of gastric juice
- Stimulates G cells to secrete gastrin
Vagovagal reflex:
- Entrance of bolus into stomach – stimulation of sensory
- (afferent) nerve endings of Vagus
- Impulse transmitted to dorsal nucleus of Vagus
- Efferent impulses through the motor fibers of Vagus secretion of gastric juice
- Both afferent and efferent – Vagus – Vagovagal reflex
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| Vasovagal reflex |
Hormonal mechanism
- Gastrin is secreted by the G cells (pyloric glands)
Mechanism of action:
- Local nervous reflex: Vagovagal reflex – by releasing gastrin releasing peptide, a neurotransmitter which stimulates G cells to secrete gastrin
- Gastrinstimulates secretion of pepsinogen and HCl by gastric glands
Experimental evidences
Nervous mechanism
- Pavlov pouch
Hormonal mechanism
- Heidenhain pouch
- Bickel pouch
- Farrel and Ivy pouch
3. Intestinal phase
- It starts after the chyme enters the intestine
- It is also called post gastric phase
- Initial part of intestinal phase is stimulatory to gastric secretion, but later part is inhibitory
Initial stage of Intestinal Phase:
Chyme that enters the intestine
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Stimulation of duodenal mucosa
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Release gastrin
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transported to stomach by blood
▼
increases gastric secretion
Later stage of Intestinal Phase:
Decrease or complete stoppage of gastric
secretion – Inhibited by 2 factor:
- Enterogastric reflex
- Gastrointestinal hormones
Enterogastric reflex
- Inhibits the gastric secretion and motility
- Due to the distention of intestinal mucosa by chyme
- Chemical or osmotic irritation of intestinal mucosa by chyme
- Mediated by
- Myenteric nerve (Auerbach) plexus
- Vagus
Gastrointestinal hormones
Presence of chyme in the intestine cause
stimulation GI hormone secretion
Hormones inhibit the gastric secretion and
gastric motility
- Secretin
- Cholecystokinin
- Gastric inhibitory peptide (GIP
- Vasoactive intestinal polypeptide (VIP)
- Peptide YY
- Somatostatin
Experimental evidence
- Bickel pouch
- Farrel and Ivy pouch
Interdigestive phase
- It is secretion of small amount of gastric juice in between the meals.
- It is mainly due to the hormone gastrin.
Experimental evidences
- Demonstrated by Farrel and Ivy pouch
Factors influencing gastric secretion during interdigestive phase
- Alcohol
- Caffeine
Applied aspects
- Gastritis
- Peptic Ulcer
Gastritis
- Inflammation of gastric mucosa is known as gastritis.
- It can be acute or chronic
Acute gastritis:
- Inflammation of superficial layers of mucus membrane
Chronic gastritis:
- Inflammation of deep layers, results in atrophy of gastric mucosa damaging chief cells and parietal cells causing decrease in gastric juice secretion.
Causes of Acute Gastritis
- Infection with bacterium Helicobacter pylori.
- Excess consumption of alcohol
- Excess administration of Aspirin and other nonsteroidal antiinflammatory drugs (NSAIDs).
- Trauma by nasogastric tubes.
- Repeated exposure to radiation.
Causes of Chronic Gastritis
- Chronic infection with Helicobacter pylori.
- Long term intake of excess alcohol.
- Long term use of NSAIDs.
- Autoimmune disease.
Features of Gastritis:
- Non-specific
- Common feature is abdominal upset or pain felt as a diffused burning sensation often referred to epigastric pain
- Other features
- Nausea
- Vomiting
- Anorexia (loss of appetite)
- Indigestion
- Discomfort or feeling of fullness in the epigastric region
- Belching
Peptic Ulcer
Ulcer:
- Erosion of the surface of any organ due to shedding or sloughing if inflamed necrotic tissue lining the organ is known as ulcer.
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| Ulcer |
Peptic ulcer:
- Ulcer in the wall of stomach or duodenum, caused by digestive action of gastric juice.
- If found in stomach: Gastric ulcer.
- If found in duodenum: Deuodenal ulcer.
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| Gastric and Duodenal ulcer |
Causes:
- Increased peptic activity due to excessive secretion of pepsin in gastric juice
- Hyperacidity of gastric juice
- Reduced alkalinity of duodenal content
- Decreased mucin content in gastric juice or decreased protective activity in stomach or duodenum
- Constant physical or emotional stress
- Food with excess spices or smoking (classical causes of ulcers)
- Long term use of NSAIDs (see above) such as Aspirin,
- Ibuprofen and Naproxen
- Chronic inflammation due to Helicobacter pylori.
Features:
- Most common feature is severe burning pain in epigastric region
- In gastric ulcer pain occurs while eating or drinking
- In duodenal ulcer, pain is felt 1 or 2 hours after food intake and during night
- Other symptoms accompanying pain are:
- Nausea
- Vomiting
- Hematemesis (vomiting blood)
- Heartburn (burning pain in chest due to regurgitation of acid from stomach into esophagus)
- Anorexia (loss of appetite)
- Loss of weight