Gastrointestinal tract Flashcards
What is the general structure of the GI tract wall?
Long muscular tube with variations on basic structure related to function
What is the mucosal layer made up of?
Epithelial cells
Lamina propria
Muscularis mucosae
What is the submucosal layer?
Collagen
Elastin
Glands
Blood vessels
What is the enteric nervous system?
Collection of nerve plexuses surrounding the GI tract including the pancreas and biliary system
Very complex and 1/3 of the divisions of the ANS
How is the GI tract intrinsically and extrinsically controlled by the nervous system?
Intrinsic- ENS primary mechansism of ganglia in submucosal and myenteric plexuses
Extrinsic- sympathetic and parasympathetic innervation
What is the structure of the contractile tissue of the GI system?
Unitary muscle- smooth muscle cells connected by gap junctions so there is a rapid spread of APs that allow coordinated contraction
What acts as the GI pacemaker?
Interstitial cells of cajal
How is food broken down in the mouth?
Teeth and tongue mechanically break down food to be ready for swallowing
Saliva lubricates and contains a-amylase which hydrolyses starch
What are slow waves in terms of the GI systems motility?
Subthreshold membrane depolarisation and then repolarisation
Produce weak/tonic contractions
What is the purpose of GI tract motility?
Regulation of antegrade and retrograde transport
Separation of low pressure organs by sphincters
What are mastication muscles innervated by?
Madibular nerve of cranial nerve V
What are the three phases of swallowing?
Oral
Pharygeal
Oesophageal
What is the function of the oesophagus in digestion?
Swallowing reflex- closes upper oesophageal sphincter and initiates primary peristaltic wave of coordinated sequential contractions
Continued distension stimulates secondary paristaltic wave by the ENS
Lower oesophageal sphincter opening mediated by the vagus nerve releasing VIP
What is the first phase of motility in the stomach?
Receptive relaxation in thin walled orad to receive food
What is the second stage of motility in the stomach?
3 muscular layers of the caudad region contract to mix food with gastric juice from mucosal glands through ANS control which forms chyme- HCl, pepsinogen, intrinsic factor and mucus
What is the third stage of motility in the stomach?
Gastric emptying through pyloric sphincter into the small intestine- fat content and H+ slow emptying
What is the function of the small intestine?
Digestion and absorption of nutrients- carbs, amino acids, fats, water and ions
Chyme is mixed with digestive enzymes and pancreatic solutions
What are the three parts of the small intestine?
Duodenum
Jejunum
Ileum
How is the surface area of the small intestine increased?
Plicae
Villi
microvilli
What is the function of the pancreas as an accessory organ?
1L of exocrine solution secreted every day which includes:
Solution rich in HCO3- secreted from centroacinar and ductal cells to neutralise H+ delivered from the stomach
Enzymes secreted from acinar cells
How does the ANS control the activity of the pancreas?
Parasympathetic NS stimulates secretion and sympathetic NS inhibits secretion
What are the three stages of pancreatic secretion?
Cephalic
Gastric
Intestinal
Where is bile produced and secreted?
Hepatocytes of the liver
Gall bladder stores, concentrates and ejects bile and duct secretes water + ions into it
What are amphipathic bile salts?
Major constituent of bile used for emulsification and solubilisation of lipids into micelles to aid their digestion and absorption
What is the function of CCK?
Secreted when chyme reaches the small intestine
Contracts gall bladder
Relaxes sphincter of oddi so that bile flows into the duodenum
What are the different types of contraction in the small intestine?
Perstaltic contractions- propel the chyme
Segmentation contractions- split and expose chyme to secretions through coordinated actions, opposing in circular and longitudinal layers
Where does material that isn’t absorbed go?
Passes through ileocaecal sphincter into the caecum of the large intestine
What are the three main functions of the large intestine?
Absorbs water and electrolytes (aldosterone increases Na+ absorption)
Makes and absorbs vitamins K and B
Forming and propelling faeces by fermenting indigestible food and bacteria to be excreted
What is the structure of the large intestine?
Surface columnar epithelial cells which absorb interspersed with crypts which secrete
Taenia coli- 3 bands of longitudinal muscle and haustra
What is secreted in the mouth?
a-amylase
ligual lipase
kallikrein
mucus
What is secreted in the oesophagus?
mucus
HCO3-
What is secreted in the stomach?
HCl
Pepsinogen
Intrinsic factor
mucus
What is secreted in the small intestine?
Mucus
HCO3-
Receives pancreatic juice + bile
What is secreted in the large intesine?
Mucus
What are the secretory cells in the body of the stomach and what do they secrete?
Oxyntic glands which are made up of: Epithelial cells- HCO3- Mucous neck cells- mucus Parietal cells- HCl and intrinsic factor Chief cells- pepsinogen
What are the secretory cells of the antrum of stomach and what do they secrete?
Pyloric glands which are similar but don’t have any parietal cells
G cells- gastrin
D cells- somatostatin
How is the secretion of gastric acid regulated?
Stimulation- histamine, ACh, gastrin
Inhibition- low pH, somatostatin, prostaglandins
Proteases
Pepsin, trypsin, chymotrypsin, elastase, carboxypeptidases
Carbohydrases
a-amylase, a-dextrinase, maltase, sucrase, trehelase, lactase
Products are absorbed into villus blood
Where are the products of carbohydrases, proteases and lipases/bile salts absorbed into?
Carbohydrases and proteases- villus blood
Lipases and bile salts- lacteals within villus
What lipases hydrolyse lipids in the jejunum and duodenum with the help of bile salts?
Pancreatic lipase, colipase, milk lipase and other esterases
What are the products of lipid digestion?
Cholesterol, lysophospholipids, monoglycerides and free fatty acids solubilised in mixed micelles
How are lipids absorbed?
Products are re-esterified with free fatty acids in the SER
Packaged into chylomicrons
Cross basolateral membrane and absorbed into lacteals so glycerol can enter the blood supply
What are the different cells of the endocrine pancreas and what proportions are they in?
Islets- 65% beta cells, 20% alpha cells and 10% delta cells
What do each of the cells in the endocrine pancrease secrete?
beta- insulin alpha - glucagon delta- somatostatin F (PP)- pancreatic polypeptide E- ghrelin Enterochromaffin cells- substance P
Cell to cell communication in the endocrine pancreas
Gap junctions between alpha and beta cells
Delta cells send dendrite like processes to beta cells
What types of neurons are islet cells innervated by?
Adrenergic
Cholinergic
Peptidergic
Sympathetic activation of beta cells
B-adrenergic stimulation increases secretion
a-adrenergic stimulation inhibits seretion
Parasympathetic activation of beta cells
Via vagus nerve, releases ACh
Increases insulin release
What humoral factors regulate the secretion of insulin?
GIP
Amylin
Somatostatin
What is the structure of insulin receptor?
Heterotetramer- 2 extracellular alpha chains and 2 membrane spanning beta chains
Intracellular tyrosine kinase activity
Intracellular signalling by insulin receptors
Receptor activated by insulin binding, autophosphorylates and phosphorylates several other proteins to activate or inhibit:
PKC, phosphatases, phospholipases, G proteins
Also stimulates gene transcription
How does insulin decrease blood-glucose concentration?
Increased glucose uptake into muscle and fat cells by GLUT4
Glycogenesis- promotes conversion of glucose into glycogen in the liver/ muscle
Lipogenesis- when glycogen stores are replenished glucose is converted into fat
Inhibits gluconeogenesis and glycogenolysis
What are the actions of insulin in the muscles?
Decreases blood amino acid concentration
Anabolic effect
Increasing uptake of amino acids and proteins
Stimulation of protein synthesis and inhibition of degradation
What are the actions of insulin in adipocytes?
Increase in GLUT4 expression for rapid glucose uptake
Glucose converted into FA and stored as triglycerides
Increase in lipoprotein lipase which liberates FA for triglyceride synthesis
Inhibition of mobilisation and oxidation of fat stores
Diabetes Mellitis
High blood sugar over prolonged periods
Type I- insulin dependent , pancreatic islet destruction
Type II- non-insulin dependent, low insulin production, peripheral insulin resistance
Symptoms of type I diabetes
Hyperglycaemia, increased fatty acid, ketoacid, and amino acid concentration in the blood
Osmotic diuresis
Polyuria
Hypotension
Hyperkalaemia- increased movement of K+ out of cells
