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
How does Gastrin alter HCl secretion
It is secreted by G cells into circulation and has endocrine regulation of parietal cells-
Activates.CCKB receptors
Increases HCl secretion
What is the endocrine, paracrine & neurocrine regulation of HCl secretion
Endocrine- Gastrin
Paracrine- histamine
Neurocrine- acetylcholine
What phases does regulation of gastric secretion occur ?
1 cephalic
2 gastric
3 intestinal
Overview of cephalic phase
Stimuli involved in parasympathetic in brain - sight smell or taste of food stimulates the phase
Involves- Ach, Gastrin releasing peptide, histamine (secrete HCl), Gastrin (these stimulate parietal cells to secrete HCl)
Overview of gastric phase
The stimulus is food or fluid in stomach - the distention of stomach and digestion of food
Ie- the arrival of food in the stomach initiates the phase and accelerates gastric secretion in response to chemical stimuli and distention
Involves- mechanoreceptors (stretch), ENS reflex, peptones, Gastrin hormone
Overview of intestinal phase
Involves responses to products of protein digestion (chyme) in duodenum
Involves- enterogastric reflex, CCK, GIP
Then secretin
What inhibits Gastrin and thus HCl and how?
Somatostatin in response to low pH
-Directly inhibits via parietal cells
Indirectly inhibits by inhibiting ECL and histamine secretion, and inhibits G cells
How is churning and mixing motion slowed in the intestinal phase??
Enterogastric reflex (stimulated by chemoreceptors in response to chyme) inhibits the myenteric plexus
What does the presence of lipids and carbs in chyme do? (Intestinal phase)
Stimulates the production of cholescysokinin CCK and gastric inhibitory peptide (ie glucose dependent immunotropic peptide) in duodenum
How is secretin released? (Intestinal p)
Decrease in pH due to acidity of chyme stimulates production of secretin by S cells
What does secretin , GIP & CCK do?
Secretin inhibits Gastrin and thus HCl release
GIP and CCK inhibit secretions of chief cells, parietal cells, G cells and overall peristalsis in stomach
What are the 5 major actions of CCK?
1- contraction of gall bladder and ejection of bile
2- secretion of pancreatic enzymes
3- secretion of bicarbonate
4- trophic effects on exocrine pancrease&gall bladder
5- inhibition of gastric emptying
What does gastric inhibitory peptide do? (Hormone)
Stimulates insulin secretion
Inhibits gastric H+ secretion
What does secretin do (hormone)
Secreted by enteric endocrine S cells
