Lecture 4 - Histology of the GI tract Flashcards
Basics of the GI tract: the layers of the GI tract
Lumen: The hole within the GI tract - where food travels through
Mucosa: first layer - protection, absorption, secretion
Submucosa: second layer - structural support
Muscularis: third layer - allows movement
Serosa/adventitia: last layer - attaches the gut tube to surrounding structures
Mucosa: what are the layers?
Composed of three layers:
* Epithelium
* lamina propria (connective tissue)
* muscularis mucosae (smooth muscle)
Submucosa: what does it contain?
Dense irregular connective tissue - containing blood vessels and nerves of the enteric plexus (Messiner’s plexus)
Muscularis externa: what is it and what does it contain?
Two layers of muscular tissue - inner circular tissue and outer longitudinal layer
Contains nerves of the enteric nervous system (Auerbach’s plexus)
Serosa/adventitia: what is the difference between both?
Serosa - Retroperitoneal structures
Adventitia - Intraperitoneal structures
Enteric nervous system:
Messiner’s plexus and Auerbach’s plexus
The Oesophagus: the layers and structures within it
- Mucosa - Stratified squamous epithelium with a lamina propria layer below it
- Submucosa - mucus-secreting glands below muscularis mucosae (ns if it counts as SM or M)
- Muscularis externa - unusual muscle layer
- Adventitia
The Stomach: the layers and structures within it
- Mucosa - Simple columnar epithelium with rugae and gastric glands/pits and a lamina propria layer below it
- Submucosa - lamina propria below muscularis mucosae (ns if it counts as SM or M)
- Muscularis externa - inner oblique, middle circular, and outer longitudinal muscles
- Serosa
The Duodenum: the layers and structures within it
- Mucosa - Simple columnar epithelium with goblet cells and villi/microvilli with a lamina propria layer below it
- Submucosa - Brunner’s glands below muscularis mucosae (ns if it counts as SM or M)
- Muscularis externa - ns, maybe similar to oesophagus?
- Serosa
The Jejunum: the layers and structures within it
- Mucosa - Simple columnar epithelium with goblet cells and villi/microvilli with Piliae Circulares and a lamina propria layer below it
- Submucosa - NO Brunner’s glands below muscularis mucosae (ns if it counts as SM or M)
- Muscularis externa - ns, maybe similar to oesophagus?
- Serosa
The Ileum: the layers and structures within it
- Mucosa - Simple columnar epithelium with goblet cells and villi/microvilli with Piliae Circulares (less than Jejunum) and a lamina propria layer below it
- Submucosa - Peyer’s Patches below muscularis mucosae (ns if it counts as SM or M)
- Muscularis externa - ns, maybe similar to oesophagus?
- Serosa
The large intestines: the layers and structures within it
- Mucosa - Simple columnar epithelium with goblet cells and crypts of Liberkuhn and a muscularis mucosae
- Submucosa - lamina propria layer sandwiched between two muscularis mucosae layers
- Muscularis externa - Taenia coli
- Adventitia - appendices epiploicae
The digestive system: basic concepts of the foregut, midgut and hindgut
Foregut: Spans from the lower oesophagus to the duodenum - Oesophagus, liver, stomach, biliary tree, gall bladder, pancreas, and first/second part of the duodenum
Midgut: Extends from the second part of the duodenum to the distal third of the transverse colon - Parts of the pancreas, duodenum; Jejunum, Ileum, appendix, caecum, ascending colon, hepatic flexure, transverse colon
Hindgut: Extends from the transverse colon to the anal canal - Distal third of the transverse colon, Splenic flexure, descending colon, sigmoid colon, rectum, upper third of the anal canal
Accessory organs for the GI tract
Teeth, tongue, salivary glands, Liver, Gall bladder, Biliary tree, and the Pancreas
Subdivisions of the small intestines
Duodenum, Jejunum, and the ilium
Subdivisions of the large intestines
- Caecum
- Ascending colon
- Hepatic flexure
- Transverse colon
- Splenic flexure
- Descending colon
- Sigmoid colon
Embryonical digestive system
Primitive gut tube - the origin of the GI tract
Embryonic GI supply
Foregut: Coeliac trunk
Midgut: Superior mesenteric artery
Hindgut: Inferior mesenteric artery
Coeliac trunk: GI supply
Supplies the foregut:
* Oesophagus
* Stomach
* Pancreas (some parts)
* Gall bladder and biliary tree
* Liver
* Duodenum (1ˢᵗ and half of 2ⁿᵈ part)
Superior mesenteric artery: GI blood supply
Supplies the midgut:
* Duodenum (half of 2ⁿᵈ, 3ʳᵈ and 4ᵗʰ parts)
* Jejunum
* Ileum
* Pancreas (parts of)
* Appendix
* Caecum
* Ascending colon
* Hepatic flexure
* Transverse colon (first 3ʳᵈ)
Inferior mesenteric artery: GI blood supply
Supplies the hindgut:
* Transverse colon (distal 3ʳᵈ)
* Splenic flexure
* Descending colon
* Rectum
* Anal canal (upper 3ʳᵈ)
Venous drainage of the GI system
Portal system - formed from the joining of the splenic and inferior mesenteric veins:
Blood travels to the liver where it is filtered
After filtration, blood drains into the IVC
Lymphatic drainage of the GI system
Digestive organs -> local lymph nodes -> regional lymph nodes -> intestinal trunk -> Cisterna chyli -> Thoracic duct -> brachiocephalic/ left subclavian vein
Regional lymph drainage: foregut, midgut, and hindgut
Foregut - Coeliac nodes
Midgut - superior mesenteric nodes
Hindgut - inferior mesenteric nodes
Autonomic nervous system: its role in the digestive system
Parasympathetic - increases digestive function
Sympathetic - slows down digestive function
PNS digestive system innervation
Foregut and midgut - Vagus nerve
Hindgut - Pelvic splanchnic nerves
Where are nerve cell bodies of postganglionic neurones located in the digestive system?
In the walls of the GI tract in the myenteric plexus
SNS digestive system innervation
Preganglionic neuron starts in the spinal cord (between the thoracic and lumbar regions) in the lateral horn of grey matter, passing through the ventral root, into the ventral ramus, and then into the sympathetic chain.
In normal situations, the nerves synapse in the sympathetic chain either here or at the level of the organ they innervate and the postganglionic neuron returns back to the spinal cord through the ventral ramus and travel to the effector organ
In the digestive system, the nerves pass through the sympathetic chain and become a splanchnic nerve and will go to a prevertebral ganglion (in front of the vertebral column) where it will synapse and the postganglionic neuron will head to the target organ
Sympathetic nervous system innervation: foregut
Arise from T5-T9
Pass through the greater splanchnic nerve and synapsing at the coeliac ganglion
Sympathetic nervous system innervation: midgut
Arise from T10-T11
Pass through the lesser splanchnic nerve and synapsing at the superior mesenteric ganglion
Sympathetic nervous system innervation: hindgut
Arise from L1-L2
Pass through the lumbar splanchnic nerve and synapsing at the inferior mesenteric ganglion
What types of sensory information are transmitted from the digestive system?
Reflex afferents
Pain afferents
Reflex afferents
Transmitted to the CNS in parasympathetic pathways - vagus nerve
Pain afferents
Transmitted to the CNS in the sympathetic pathways - greater, lesser, and lumbar splanchnic nerves
Referred pain in the digestive system
Pain sensed by T5-T9 will be from dermatomes T5-T9 so pain is perceived in the epigastric region
Pain sensed by T10-T11 will be from dermatomes T10-T11 so pain is perceived in the umbilical region
Pain sensed by L1L2 will be from dermatomes L1-L2 so pain is perceived in the hypogastric region
Inflamed appendix: where will the pain be sensed?
Part of the midgut so T10-T11 will sense the pain and it will be perceived in the umbilical region
If inflammation increases further, the peritoneum lining the posterior abdominal wall will be affected, and pain will be sensed in the right iliac region
