GI Tract Flashcards
What are the 4 layers of the GI
Mucosa
Submucosa
Muscularis externa
Serosa
3 layers of the mucosa
1-mucous membrane - layer of epithelial cells that protect gut from abrasion, secrete substances, enteroendocrine cells that release hormones into lamina prioria & capillaries
2- lamina propria- connective tissues, contains blood capillaries and lymph vessels
3- musularis mucosa- thin layer of smooth muscle, controls folding of mucosa
what is sub mucosa
Contains blood and lymph vessels - a deep boundary- neurons are organised to form intrinsic or submucosal plexus
What is muscularis externa
Smooth muscle cells of the outer layer that run longitudinally, inner layer has circular arrangement
Produce waves of contraction (peristalsis or segmentation)
Myenteric plexus is located between thick and thin muscles
This network of neurons is connected to the submucosal plexus to form the ENS
What is serosa?
Loose connective tissue and membrane that protects the Tract when it moves
Forms a visceral (inner) peritoneum and is continuous with the parietal (outer) peritoneum
4 major activities of the GI tract
1 motility
2 secretions
3 digestion
4 absorption
How does the enteric nervous system receive input ?
1- from the autonomic nervous system (but can work independently)
2- from mechano-chemo receptors
How does enteric ns send input ?
To smooth muscles, secretory& endocrine cells
What are the 2 neuromodulators in GI tract
Acetylcholine
Noradrenaline
Role of acetylcholine?
- Contraction of smooth muscle in the wall
- relaxation of sphincters
- increased salivary, gastric and pancreatic secretion
Role of noradrenaline?
- relaxation of smooth muscle in wall
- contraction of sphincters
- increased salivary secretion
The role of saliva?
1- dilute, buffer and lubricate ingested food (very hypotonic, high K+ and bicarbonate) low Na and Cl
2- initial digestion of starches and lipids - eg
Amylase: enzyme in saliva and pancreatic fluid that converts starch and glycogen to simple sugars
Lingual lipase- digestive enzyme breaks down fatty acids
Role of the oesophagus
No absorptive or digestive function- just conveys bolus to stomach by secreting mucus to promote movement ie peristalsis
Functions of the stomach
The temporary storage of food - digests food chemically & mechanically
Also secretes enzymes for digestion and protection
Anatomy of the stomach
fundus, cardia, body and Antrum
3 layers of muscle of varied thickness eg thicker where contractions are needed
What is gastric motility (2 types)
1 receipting relaxation
2 mixing and digestion
What is receipting relaxation?
Orad region receives food bolus - temporary storage
What is mixing and digestion of gastric motility?
The caudad region has a thick muscular wall
Vigorous contraction from body to pylorus area - mix food and water with gastric secretory products (=chyme )
- grinding food to enhance digestion
- retropulsion
How is HCl secreted?
By parietal cells (oxyntic cells), predominantly in fundus and body of stomach
What is functions of HCl (5)
1- acidify gastric content to pH 1-2
2- breakdown of connective tissue and muscle fibres of ingested meat
3- activate pepsinogens
4- optimal conditions for activity of pepsins
5- defence mechanism against micro organisms that may cause infection
What substances alter HCl secretion?
Histamine
Acetylcholine
Gastrin
How does histamine alter HCl secretion
It is released by enterochromaffin like ECL walls.
Paracrine regulation of parietaL cells - activates H2 receptors and increases HCl secretion
How does acetylcholine alter HCl secretion
From the vagus nerve - neurocrine regulation of parietal cells
Activates h3 receptors to increase HCl secretion
They also have an indirect effect as they stimulate the release of histamine
How does acetylcholine have an indirect effect on HCl secretion
Stimulates release of histamine
