Introduction to Gastrointestinal Physiology Flashcards
What are the components of the GI tract in order?
Oral cavity, pharynx, oesophagus, stomach, duodenum, jejunum, ileum, ascending large intestine, transverse large intestine, descending colon, rectum and anal canal. Accessory organs are the teeth, tongue, salivary glands, pancreas, liver and gall bladder.
What is the general structure of the GI wall from the inside out?
the mucosa, the submucosa, the submucosal plexus, the muscularis externa (with the myenteric plexus between its two layers) and the serosa.
What is the structure of the mucosa?
From the inside out, consists of a layer of epithelial cells, surrounded by the lamina propria (their basement membrane), and the muscularis mucosa, consisting of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle.
Describe the structure of the submucosa
made up of loose connective tissue with collagen and elastin fibrils, blood vessels, lymphatics and in some regions submucosal glands.
Describe the structure of the muscularis externa
inner layer consists of circular smooth muscle, the outer is longitudinal. the inner is 3-5x thicker than the outer. At intervals along the tract, inner layer is thickened and modified to form a sphincter, controlling rate of movement of GI contents from one part of the gut to another.
What is the role of the myenteric plexus?
largely motor in function, regulates the motility of the GI tract. Hirschsprung’s disease, missing ganglion cells in the myenteric plexus cause loss of motility and severe constipation.
What is the role of the submucosal plexus?
AKA Meissner’s plexus. Chiefly concerned with controlling secretory activity and blood flow to the gut. Receives signals from the intestinal epithelium and stretch receptors in the gut wall.
What are APUD cells and where are they found?
scattered throughout the mucosa, these are amine precursor uptake and decarboxylation cells, involved in the synthesis and secretion of GI hormones.
Describe the entero-endocrine cells of the stomach
G cells secrete Gastrin
Describe the entero-endocrine cells of the Pancreas
alpha and beta cells in the islets of langerhans secrete glucagon and insulin respectively
Describe the entero-endocrine cells of the duodenum
S cells secrete secretin and K cells secrete GIP
Describe the entero-endocrine cells of the Ileum and Colon
L cells secrete GLP-1&2 and PYY
How does eating modulate the splanchnic circulation?
Postabsorbative, the splanchnic vessels receive about 25% cardiac output, but during digestion and absorbtion this increases considerably, due to CCK and gastrin secretion, with fatty acids and glucose also acting as vasodilators.
Describe the organisation of the splanchnic circulation
The celiac artery splits into the gastric and splenic arteries, supplying the stomach and the splenic and pancreatic capillary beds respectively. The superior and inferior mesenteric arteries supply the submucosal layer in a capillary network. Venous blood from the GI organs supplies ~70% of blood to liver via portal vein, liver is oxygenated by the hepatic artery. Hepatic vein > vena cava.
Describe the functions of saliva
Lubricates swallowing, aids speech, contains salivary amylase beginning starch digestion. Dissolves substances for taste. IgA and lysozyme, bacteriostatic actions for oral comfort and reduced infection risk. xerostomia, impairment of salivation, increases risk.
Where in the mouth are salivary glands found?
three main pairs: parotid (majority fluid, serous watery), submandibular and sublingual (serous and mucoprotein)
Describe the innervation of the salivary glands
Noradrenergic sympathetic fibres supply the acinar cells and blood vessels. Postganglionic parasympathetic fibres from ganglia near the glands supply the secretory and duct cells.
Describe the structure of the salivary glands
(Acinus -> intercalated duct -> interlobular duct) lobule -> myoepithelial cells line the interlobular ducts which drain into a main duct.
How is saliva produced?
Isotonic primary secretion is formed by active transport of electrolytes into the acinus followed by osmosis of water. Epithelial cells lining the ducts then actively reabsorb Na+ and Cl- as saliva flows past - rate determines osmolality.
What are the effects of parasympathetic stimulation of the salivary glands?
Muscarinic receptors mediate the production of water saliva rich in amylase and mucins. Bicarbonate secretion is stimulated while sodium reabsorption and K+ secretion is inhibited. These effects are inhibited by atropine, but increased blood flow to glands isn’t.
What are the effects of sympathetic stimulation of the salivary glands?
output of amylase is enhanced, but vasoconstriction and reduced blood flow decrease net salivation.
What is Sjögren’s syndrome and what are its effects?
Sjögren’s syndrome is an autoimmune disorder where the body attacks moisture-producing glands, leading xerostomia and poor oral health.
Describe the innervation of the GI tract
Autonomic: Parasympathetic increases exocrine secretion, gut motility and GI blood flow. Sympathetic does the opposite but has no real effect on exocrine secretion. The ENS mediates ANS activity - postganglionic nerves from part of its network - but can function independently.
Describe the parts of the stomach
cardia surrounds the cardiac orifice. From top to bottom, the stomach regions are the fundus, the body, the pyloric antrum (main endocrine area), and the pylorus surrounded by the pyloric sphincter.
