Embryology, Anatomy & Physiology

Embryology

• Arises from the endoderm
• Ventral mesogastrium: Hepatoduodenal ligament
• Dorsal mesogastrium: Greater omentum and three ligaments
• Rotation: Rotates 90 degrees on the long axis during weeks 7-8
• Vagal nerves: Rotate around the stomach

Morphology

• Cardia: Most proximal part of the stomach
• Fundus: Most superior portion of the stomach
• Incisura: Defines the antrum
• Antrum: Constitutes 40% of the stomach
• Gastroesophageal (GE) Junction:
  • Located at the 7th chondrosternal junction
  • Positioned 2-3 cm below the diaphragmatic hiatus
• Pylorus:
  • Located at L1
  • Associated with the 9th costal cartilage

Arterial Supply

• Left Gastric Artery: Largest artery supplying the stomach
• Right and Left Gastric Veins: Drain into the portal vein
• Left Gastroepiploic Vein: Drains into the splenic vein
• Right Gastroepiploic Vein: Drains into the superior mesenteric vein (SMV)

Venous Drainage

• Refer to the provided image for a detailed illustration of the venous drainage of the stomach.

  

MCQ

Answer: C) Right Colic Vein

Explanation:

Henle's trunk, also known as the gastrocolic trunk, typically receives the right gastroepiploic vein (RGEV), superior right colic vein, and the anterior superior pancreaticoduodenal vein (ASPDV). The right colic vein is not a tributary of Henle's trunk, making it the correct answer.

Posterior Gastric Artery (Suzuka)

Origin and Path

• Arises from: Left Gastric Artery (LGA) or Splenic artery
• Location: Positioned posterior to the stomach

Areas Supplied

• Distal Esophagus
• Cardia
• Fundus

Lymphatic Drainage

MCQ 1

Answer: 4. Fundus drains into splenic nodes

Explanation: The fundus of the stomach drains into the splenic lymph nodes, making option 4 the correct statement. Other options contain incorrect statements regarding the anatomy and procedures related to the stomach.

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MCQ 2

Answer: 2. Left Gastric artery

Explanation: The Artery of Suzuka, also known as the Posterior Gastric Artery, most commonly arises from the Left Gastric Artery, making option 2 the correct answer.

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MCQ 3

Answer: 2. 80%

Explanation: A subtotal gastrectomy typically involves the removal of approximately 80% of the stomach, making option 2 the correct answer.

Innervation

Sympathetic Innervation

• Nerves: Splanchnic plexus
• Afferent Pathway:
  • Synapse in the dorsal root ganglia of thoracic spinal nerves (T8-10)
• Efferent Pathway:
  • Originates from T8-10
  • Synapse in the celiac ganglia
• Function: Nociceptive stimuli are transmitted via sympathetic nerves.

Parasympathetic Innervation

• Nerves: Vagi (Vagus nerves)
• Afferent Fibers:
  • 90% of the vagus nerve is composed of afferent fibers.
  • Afferent fibers synapse in the nucleus tractus solitarius (NTS).
• Efferent Fibers:
  • Efferent fibers synapse in the Auerbach and Meissner's plexus (Enteric Nervous System).
• Vagal Innervation:
  • The Criminal Nerve of Grassi is the first branch of the posterior vagus.
  • Missing the criminal nerve during surgery is the most common cause of recurrent ulcers.

Enteric Nervous System (ENS)

• Third Branch of the Autonomic Nervous System

Sympathetic Innervation

• Thoracic Spinal Nerves: T8-10

Vagal Innervation Overview

• Criminal Nerve of Grassi:
  • First branch of the posterior vagus
  • Missing this nerve is the most common cause of recurrent ulcers

MCQ

Answer: 2. 90% of vagus nerve is dedicated to efferent function

Explanation:

The vagus nerve is predominantly afferent, with about 90% of its fibers dedicated to sensory (afferent) functions, not efferent functions. Hence, statement 2 is incorrect, making it the correct answer for this MCQ.

Layers of the Stomach

Three Layers of Muscularis Externa

• Outer Layer: Longitudinal fibers
• Middle Layer: Circular fibers (only complete layer)
• Inner Layer: Oblique fibers

Additional Details

• Seromyotomy in Taylor's Procedure:
  • The outer two layers (longitudinal and circular) are divided.
• Muscularis Mucosa:
  • Contains rugal folds.
  • Involvement indicates invasive carcinoma.

MCQ

Answer: a. Three layers of muscularis externa

Explanation:

The stomach's muscularis externa has three distinct layers: the outer longitudinal, middle circular (the only complete layer), and inner oblique fibers. Hence, option "a" is the correct answer.

Physiology of Stomach

Cell Types, Locations, and Functions

	Location	Function
Parietal Cells	Body	Secretion of acid and intrinsic factor
Mucus Cells	Body, Antrum	Secretion of mucus
Chief Cells	Body	Secretion of pepsinogen
Surface Epithelial Cells	Diffuse	Secretion of mucus, bicarbonate, and prostaglandins (?)
Enterochromaffin-like Cells	Body	Secretion of histamine
G Cells	Antrum	Secretion of gastrin
D Cells	Body, Antrum	Secretion of somatostatin
Gastric Mucosal Interneurons	Body, Antrum	Secretion of gastrin-releasing peptide
Enteric Neurons	Diffuse	Secretion of calcitonin gene-related peptide, others
Endocrine Cells	Body	Secretion of ghrelin

Gastric Pit Structure

• Gastric Pits:
  • Longer, tubular in the body and fundus
  • Short, branched in the cardia and antrum
• Cell Locations:
  • Chief Cells: Located at the base (proximal region)
  • Parietal Cells: Located in the isthmus

MCQ

Answer: d) Pylorus

Explanation:

Parietal cells, responsible for secreting hydrochloric acid and intrinsic factor, are found in the body and fundus of the stomach but are absent in the pylorus.

Mucosal Protective Factors

MCQ

Answer: d. Predominant effect of somatostatin is endocrine

Explanation:

Somatostatin's effects are primarily paracrine, meaning it acts locally near its site of secretion, rather than through endocrine mechanisms that involve systemic circulation.

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Gastrin

Types of Gastrin

• G-34: Big Gastrin
• G-17: Little Gastrin (Predominant form in the stomach)
• G-14: Mini Gastrin

Characteristics

• Produced in: Antrum (Endocrine secretion)
• Predominant Form in Circulation: G-34, which has a longer half-life
• Structure: 5 amino acid sequence at the C-terminal end

Receptors

• CCK-B Receptors: Present on cells that respond to gastrin

Regulation

• Stimulants: Luminal peptides, distension, acetylcholine, calcitonin
• Inhibitors: Acid, somatostatin

Notes

• Parietal Cells: Do not have CCK-B receptors

Causes Of Hypergastrinemia:

MCQ 1

Answer: 4. Gastrin has predominant endocrine action

Explanation:

Gastrin is primarily an endocrine hormone, meaning it is secreted into the bloodstream to exert its effects on distant target cells, such as stimulating acid secretion by parietal cells.

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MCQ 2

Answer: 2. H. pylori infection

Explanation:

While H. pylori infection is associated with peptic ulcers, it does not typically cause hypergastrinemia. Conditions like Gastric Outlet Obstruction (GOO), Short bowel syndrome, and Zollinger-Ellison Syndrome (ZES) are more closely associated with hypergastrinemia.

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MCQ 3

Answer: 1. Produced by G cells in the fundus

Explanation:

Gastrin is produced by G cells located in the antrum of the stomach, not in the fundus. The other statements are true regarding the half-life, predominance of G-34 in circulation, and the inhibitory effect of acid on gastrin secretion.

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MCQ 4

Answer: 2. Cardia

Explanation:

The cardia region of the stomach does not play a significant role in gastric acid secretion. The fundus and antrum are more involved in this process, while the pylorus regulates the passage of stomach contents into the duodenum.

Somatostatin

Key Points

• Structure:
  • 14, 18 Amino Acids: Exists in two forms, with the 14 AA isoform being predominant in the stomach.
• Action:
  • Predominantly Paracrine: Acts locally near its site of secretion.
  • Principal Stimulus: Luminal acidification when pH < 3.
  • Inhibition: Acetylcholine (Ach) inhibits somatostatin secretion.
  • Role in H. pylori Induced Ulcer: Loss of D cells leads to reduced somatostatin, contributing to ulcer formation.
  • Tonic Inhibition: Maintains tonic inhibition of acid secretion in the resting state.

MCQ 1

Answer: 4. All of the above

Explanation:

Somatostatin is secreted by D cells, is stimulated by luminal acid, and directly inhibits acid secretion by parietal cells, making all the statements true.

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MCQ 2

Answer: 4. Jejunum

Explanation:

Somatostatin is secreted by D cells in the antrum, duodenum, and pancreatic islets, but not by cells in the jejunum.

Intrinsic Factor

Key Points

• Produced by: Parietal cells
• Type: Glycoprotein
• Function:
  • Binds Vitamin B12 in the duodenum after R factor (Haptocorrin) is lysed.
  • Optimum binding pH: 7.
• Resilience: Unaffected by Proton Pump Inhibitors (PPI).

Pepsinogen

Key Points

• Activation:
  • Undergoes autocleavage at pH 1.5-2.
  • Denatured at pH 7.2.
• Stimulants: Acetylcholine (Ach), Gastrin, Gastrin-Releasing Peptide (GRP).
• Types:
  • Group I: Found in the body and fundus, secreted by chief and mucous neck cells.
  • Group II: Present in all regions, including the proximal duodenum, and is active over a wide range of pH. It is especially useful in achlorhydria.

Ghrelin

Key Points

• Origin: Peptide hormone produced by oxyntic glands in the stomach.
• Distribution: Found in the proximal bowel and pancreas.
• Forms:
  • Desacyl Ghrelin: Comprises up to 90% of circulating ghrelin, with little biological activity.
  • Active Form: Undergoes acylation at serine-3, responsible for most hormonal activity.
• Function:
  • Increases vagal tone, leading to histamine release and augmented gastric acid production.
  • Peaks before meals and drops postprandially, playing a role in basal acid secretion.
  • Stimulant of gastric motility.
  • Promotes appetite and feeding, and increases adiposity.

MCQ

Answer: A. Ghrelin

Explanation:

Ghrelin is known as the "hunger hormone" because it stimulates appetite and feeding behaviors, and is also involved in gastric motility and acid secretion.

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Bombesin (Gastrin-Releasing Peptide - GRP)

Key Points

• Source: Neurons in the body and antrum of the stomach.
• Function:
  • Stimulates both D and G cells.
  • Rapidly cleared from circulation by neuropeptidase.
• Administration Effects:
  • Peripheral: Stimulates gastric acid secretion.
  • Central: Inhibits acid secretion and stimulates the sympathetic nervous system.

Regulation of Acid Secretion

Key Points

• Basal Acid Output (BAO):
  • Represents 10% of Maximum Acid Output (MAO).
  • Dependent on vagus nerve and acetylcholine (Ach).
• Maximum Acid Output (MAO):
  • Dependent on gastrin.
• Receptor Mediated Actions:
  • Ach: Acts on M3 receptors.
  • Histamine: Acts on H2 receptors.
  • Gastrin: Acts on CCK-B receptors.
  • Somatostatin: Acts on SSTR2 receptors.
  • Enterochromaffin-Like (ECL) Cells: Play a central role in acid secretion regulation.

Phases of Acid Secretion

Cephalic Phase

• Characteristics:
  • Most intense but of shorter duration.
  • Discovered by Ivan Pavlov.
  • Accounts for 20-30% of total acid output.
  • Can be abolished by vagotomy.

Gastric Phase

• Characteristics:
  • Longest and most sustained phase.
  • Accounts for 60-70% of total acid output.
• Mechanisms:
  • Vago-Vagal Reflex: Important for gastric phase function.
  • Pyloro-Oxyntic Reflex: Contributes 30-40% to acid output.

Intestinal Phase

• Characteristics:
  • Contributes to 10% of meal-stimulated acid secretion.
  • Triggered by partly digested chyme in the proximal intestine.

MCQ

Answer: 2. Gastric phase is the most intense

Explanation:

The gastric phase is the longest and most sustained phase, but it is not the most intense; the cephalic phase is more intense, though shorter in duration. The gastric phase contributes the most to the Maximum Acid Output (MAO) and involves concurrent, not consecutive, processes. Vagotomy is ineffective in completely abolishing meal-stimulated acid secretion due to other mechanisms in place.