ANATOMY & PHYSIOLOGY
EMBRYOLOGY
- Primitive gut tube formed at 4 weeks.
- Midgut connected to the yolk sac by a thin stalk known as the vitelline duct.
- Endoderm forms the epithelium, while the mesoderm forms muscle and connective tissue.
Midgut Herniation & Rotation
- Herniation of the midgut occurs from the 5th week to the 7th week.
- Rotation during herniation:
- 90 degrees during herniation.
- 180 degrees during retraction.
Gut Epithelium Development
- Initially, the entire GIT is lined by simple columnar epithelium.
- 6th week: Proliferation of epithelium leads to occlusion of the lumen.
- 7th-8th week: Vacuoles develop and the lumen undergoes recanalization.
- Mesoderm pushes into the epithelium, leading to the development of villi.
- Mesoderm forms the submucosa and muscular layer.
Stem Cells & Epithelial Cell Development
- Stem cells reside within the crypts and divide into daughter cells:
- One daughter cell remains anchored to the crypt.
- The other daughter cell migrates to the villus.
- There are four major epithelial cells:
- Paneth cell
- Enteroendocrine cell
- Goblet cell
- Enterocyte
Digestive Function
- Digestive function develops only after exposure to food.
Arterial Development
- Celiac axis initially at C7, descends to T12.
- Superior Mesenteric Artery (SMA) initially at T2, later migrates to L2.
MCQs for Revision
1. During gestation, the entire GI tract of the embryo is lined by?
A) Simple columnar epithelium
B) Simple squamous epithelium
C) Ciliated columnar epithelium
D) Stratified squamous epithelium
Correct Answer: A) Simple columnar epithelium
2. False statement regarding embryology of the small intestine?
A) Fetal intestinal epithelium develops digestive function by 21 weeks
B) Epithelium develops from the endoderm
C) All layers of intestine except the epithelium develop from the mesoderm
D) The epithelium is covered by simple columnar epithelium initially
Correct Answer: A) Fetal intestinal epithelium develops digestive function by 21 weeks
(Digestive function develops only after exposure to food)
3. The initial embryological origin of the superior mesenteric artery is at the level of?
A) C7
B) T4
C) T2
D) T8
Correct Answer: C) T2
ANATOMY
Duodenum
- Length: Approx 25 cm long, divided into 4 parts.
- Longest part: 3rd part.
- Longest villi: Seen in the duodenum.
Comparison of Jejunum and Ileum Characteristics
| Characteristic | Jejunum (Ba & C) | Ileum (Bb, D, & E) |
|---|---|---|
| Color | Deeper red | Paler pink |
| Caliber | 2-4 cm | 2-3 cm |
| Wall | Thick and heavy | Thin and light |
| Vascularity | Greater | Less |
| Vasa Recta | Long | Short |
| Arcades | A few large loops | Many short loops |
| Fat in Mesentery | Less | More |
| Circular Folds (L. plicae circulares) | Large, tall, and closely packed | Low and sparse; absent in distal part |
| Lymphoid Nodules (Peyer patches) | Few | Many |
LAYERS OF THE SMALL BOWEL
- Mucosa
- Submucosa
- Strongest layer
- Contains Meissner's plexus.
- Muscularis Propria
- Two layers:
- Inner circular layer.
- Outer longitudinal layer.
- Contains Auerbach's plexus.
- Two layers:
- Serosa
- Single-cell mesothelial layer.
MCQs for Revision
1. False regarding the anatomy of the small intestine is?
A) Jejunum represents proximal 40% and the ileum the distal 60%.
B) Diameter of ileum is smaller than that of the jejunum
C) Plicae circulares aid in absorption of nutrients
D) Vasa recta of ileum are longer than that of jejunum
Correct Answer: D) Vasa recta of ileum are longer than that of jejunum
(Vasa recta of jejunum are longer than that of ileum)
2. Which part of the small intestine has the longest villi?
A) Duodenum
B) Jejunum
C) Ileum
D) Equal length in all parts
Correct Answer: A) Duodenum
(Longest villi are seen in the duodenum)
3. Which part of the duodenum is the longest?
A) First
B) Second
C) Third
D) Fourth
Correct Answer: C) Third
(The third part of the duodenum is the longest)
4. The strongest layer of the small intestinal wall is?
A) Mucosa
B) Submucosa
C) Muscularis propria
D) Serosa
Correct Answer: B) Submucosa
(Submucosa is the strongest layer of the small intestinal wall)
PHYSIOLOGY
Hormonal Control of Small Intestinal Motility
| Hormone | Actions |
|---|---|
| Gastrin | Increased contraction rate |
| CCK | Smooth muscle contraction; increased mixing of intestinal contents; increased intestinal transit |
| Motilin | Increased intestinal transit |
| VIP | Duodenal contractions; increased motility |
| Secretin | Reduced contractility, more pronounced in duodenum |
| Glucagon | Inhibitory effect globally |
Hormonal Control of Small Intestinal Secretion
| Hormone | Effects |
|---|---|
| Serotonin | Increased Ca²⁺ influx, cAMP and cGMP generation, opening of Cl⁻ channels |
| VIP | Increased Ca²⁺ influx, cAMP and cGMP generation, opening of Cl⁻ channels |
| Cholera toxin | Increased serotonin levels, increased cAMP levels |
| Pituitary adenylate cyclase-activating protein | Homology to VIP; receptors in ileum |
| Substance P | Opens water channels; affects CFTR |
| L-Tryptophan | Metabolic precursor of serotonin |
| PGE₁ and PGE₂ | Prosecretory |
| Cholecystokinin | Prosecretory |
| Guanylins | Prosecretory |
| Galanin | Prosecretory |
| Neuropeptide Y (peptide YY) | Increased absorptive time, decreased gastric and pancreatic secretions |
| Antisecretory factor | Antisecretory globally in small intestine |
Vitamins and Method of Intestinal Absorption
| Vitamin | Method of Absorption |
|---|---|
| Fat soluble: A, D, E, K | Chylomicrons |
| Vitamin C (ascorbic acid) | Na⁺-dependent brush border carriers |
| Biotin | Na⁺-dependent brush border carriers |
| Nicotinic acid | Passive diffusion |
| Folic acid | Na⁺-independent brush border carriers |
| B₂ (riboflavin) | Na⁺-dependent brush border carriers |
| B₁ (thiamine) | Na⁺-independent brush border carriers |
| B₆ (pyridoxine) | Passive diffusion |
| B₁₂ (cobalamin) | Translocation with intrinsic factor |
MCQs for Revision
1. Which of the following is not a promotility hormone?
A) Gastrin
B) VIP
C) Glucagon
D) CCK
Correct Answer: C) Glucagon
(Glucagon has an inhibitory effect globally, while the others promote motility)
2. Which of the following is an antisecretory hormone for small intestinal secretion?
A) VIP
B) Serotonin
C) Neuropeptide YY
D) Cholecystokinin
Correct Answer: C) Neuropeptide YY
(Neuropeptide Y increases absorptive time and decreases gastric and pancreatic secretions)
3. Which of the following vitamins is absorbed in the small intestine by passive diffusion?
A) B₆
B) Folic acid
C) Vitamin K
D) Vitamin B₁₂
Correct Answer: A) B₆
(Vitamin B₆ is absorbed by passive diffusion)
Physiologic Functions of Gastrointestinal Hormones
Secretin
| Category | Details |
|---|---|
| Origin/Distribution | S cells, Duodenum, small and large intestine |
| Target Organ | Pancreas, Stomach, Liver, Esophagus, Colon |
| Function | - Stimulates pancreatic exocrine secretion |
| - Inhibits gastric secretion and motility | |
| - Stimulates gastric pepsin release | |
| - Stimulates biliary secretion (bicarbonate, chloride, and water) | |
| - Decreases bile salt concentration | |
| - Decreases LES tone and colonic motility | |
| Clinical Significance | - Diagnosis of Zollinger-Ellison syndrome |
| - Pancreatic stimulation in MRCP/ERCP | |
| - Localization of gastrinomas |
Cholecystokinin (CCK)
| Category | Details |
|---|---|
| Origin/Distribution | I cells, Duodenum, and small intestine |
| Target Organ | Gallbladder, Pancreas, Esophagus, Small bowel, Stomach, Brain |
| Function | - Stimulates gallbladder contraction |
| - Stimulates pancreatic enzyme secretion | |
| - Relaxes the sphincter of Oddi | |
| - Stimulates pancreatic growth | |
| - Decreases LES tone | |
| - Inhibits gastric emptying, induces satiety, slows intestinal transit | |
| Clinical Significance | - CCK antagonists used in obesity, anorexia, and bulimia treatment |
| - Treatment of peptic ulcer disease (PUD) |
Gastric Inhibitory Polypeptide (GIP) & Glucagon-like Peptide-1 (GLP-1)
| Category | Details |
|---|---|
| Origin/Distribution | K cells (GIP), L cells (GLP-1), Duodenum and proximal small intestine |
| Target Organ | Pancreas, Heart, Bone, Stomach, Hypothalamus |
| Function | - Enhances insulin secretion |
| - Regulates stomach emptying | |
| Clinical Significance | - Potential treatment target for DM, HTN, myocardial infarction, osteoporosis, Parkinson’s, Alzheimer’s |
Somatostatin
| Category | Details |
|---|---|
| Origin/Distribution | D cells, δ cells, Intramural enteric nervous system of duodenum and small bowel |
| Target Organ | Stomach, Duodenum, Small bowel, Pancreas, Gallbladder, Colon |
| Function | - Inhibits motility and secretion of gastrin and pepsin |
| - Inhibits absorption of amino acids, water, electrolytes | |
| - Inhibits secretion of pancreatic enzymes and biliary secretions | |
| - Inhibits gallbladder contractions, prolongs large bowel transit time | |
| Clinical Significance | - Treatment of cirrhosis, variceal bleeding, peptic ulcer disease, pancreatic fistulas, pancreatitis |
Motilin
| Category | Details |
|---|---|
| Origin/Distribution | Nonargentaffin cells, Small bowel mucosa |
| Target Organ | Stomach, Pancreas, Gallbladder |
| Function | - Prolongs large bowel transit time |
| - Initiates phase III contraction of MMC | |
| - Stimulates gastric emptying and pepsin secretion | |
| - Activates pancreatic enzyme secretion | |
| - Stimulates contraction of the gallbladder | |
| Clinical Significance | - Motilin receptor agonist used in the treatment of gastroparesis |
5-Hydroxytryptamine (5-HT)
| Category | Details |
|---|---|
| Origin/Distribution | Enterochromaffin cells, Duodenum, Proximal small bowel |
| Target Organ | Stomach, Small bowel |
| Function | - Induces stomach relaxation |
| - Increases intestinal motility | |
| Clinical Significance | - 5-HT antagonists used as antiemetics |
| - 5-HT receptor agonists used for treatment of diarrhea, IBS, and other GI disorders |
Nitric Oxide
| Category | Details |
|---|---|
| Origin/Distribution | Myenteric plexus of the bowel, Duodenum, Small bowel |
| Target Organ | Esophagus, Stomach, Small bowel, Gallbladder |
| Function | - Relaxes LES and smooth muscle cells of GI tract and biliary system |
| Clinical Significance | - Potential application in peptic ulcer disease (PUD) as a protector of gastric mucosa |
Vasoactive Intestinal Peptide (VIP)
| Category | Details |
|---|---|
| Origin/Distribution | Nerve plexus of the bowel |
| Target Organ | Stomach, Small bowel, Gallbladder, Biliary system, Liver |
| Function | - Inhibits pepsin production |
| - Enhances secretion of water and electrolytes | |
| - Inhibits intestinal absorption | |
| - Relaxes gallbladder | |
| - Glycogenolytic effect in the liver | |
| Clinical Significance | - Potential benefit in treatment of IBD, immunosuppression, asthma, and COPD |
Neurotensin
| Category | Details |
|---|---|
| Origin/Distribution | N cells, Duodenum, Small bowel, Brain |
| Target Organ | Stomach, Pancreas, Colon, Hypothalamus |
| Function | - Decreases stomach motility |
| - Stimulates pancreatic secretion | |
| - Stimulates CNS and colonic contraction | |
| Clinical Significance | - Potential role in autism, schizophrenia, and Parkinson's disease |
Substance P
| Category | Details |
|---|---|
| Origin/Distribution | Terminals of sensory nerves of the bowel |
| Target Organ | Stomach, Small bowel, Gallbladder, Pancreas, Biliary system |
| Function | - Increases motility of GI tract |
| - Stimulates CCK release | |
| - Increases contraction of gallbladder, reduces pancreatic flow | |
| Clinical Significance | - Blocks Substance P receptors to treat inflammatory diseases |
Gastrin-Releasing Peptide
| Category | Details |
|---|---|
| Origin/Distribution | Postganglionic fibers of the vagus nerve |
| Target Organ | Stomach, Small bowel, Hypothalamus |
| Function | - Modulates acid, gastrin, and peptide secretion in GI tract |
| - Stimulates CCK release | |
| - Regulates circadian signals | |
| Clinical Significance | - Involved in the carcinogenesis of various tumors (lung, colon, etc.) |
Ghrelin
| Category | Details |
|---|---|
| Origin/Distribution | P/D1 cells in gastric mucosa and pancreas |
| Target Organ | Stomach, Hypothalamus |
| Function | - Increases gastric acid secretion |
| - Accelerates food intake | |
| Clinical Significance | - Potential role in ghrelin antagonists for obesity treatment |
Endorphins and Enkephalins
| Category | Details |
|---|---|
| Origin/Distribution | Myenteric plexus |
| Target Organ | Stomach, Small bowel, Colon |
| Function | - Decrease motility |
| Clinical Significance | - Antagonists used for postoperative ileus treatment |
Peptide YY and Pancreatic Polypeptide
| Category | Details |
|---|---|
| Origin/Distribution | Endocrine cells, L cells of the ileum and colon |
| Target Organ | Stomach, Small bowel, Pancreas, Biliary system |
| Function | - Stimulates secretion of stomach, pancreas, and liver |
| - Increases motility of GI tract | |
| - Induces satiety | |
| Clinical Significance | - Investigated for weight-loss and in Peptide YY role in postoperative ileus |
Melatonin
| Category | Details |
|---|---|
| Origin/Distribution | EC cells, Bowel, Pineal gland |
| Target Organ | Duodenum, Small bowel |
| Function | - Stimulates secretion of HCO₃⁻ |
| - Antioxidant | |
| Clinical Significance | - Potential role in treating GI conditions like ulcers and gastritis |
Gastrin
| Category | Details |
|---|---|
| Origin/Distribution | G cells of Stomach |
| Target Organ | Stomach, Small bowel, Pancreas, Biliary system |
| Function | - Stimulates acid secretion |
| - Promotes gastric motility | |
| - Relaxes ileocecal valve | |
| - Stimulates pancreatic enzyme secretion | |
| - Stimulates gallbladder emptying | |
| Clinical Significance | - Pentagastrin is used in diagnosing hypergastrinemia |