Motility of the Gut and the Digestion and Absorption of Nutrients Flashcards
What neural system control to GI tract?
Enteric nervous system.
Interstitial Cells of Cajal act as pacemakers in the GI tract.
What are the effects of vagal stimulation, CCK, GIP and secretin on gastric motility?
Vagal stimulation - increases motility
CCK, GIP and secretin all decrease motility of the gut.
Outline GI smooth muscle contractile activity.
Visceral smooth muscle show rhythmic cycles of activity controlled by the Interstitial cells of Cajal.
Peristalsis - waves of contraction
Segmentation - mass contraction
Sphincter tone and relaxation
How is gastric emptying regulated?
Neuronal enterogastric reflex.
Hormonal (enterogastrone) mechanisms
These inhibit gastric secretion and duodenal filling.
How does secretin effect gastric activity?
Stimulates HCO3- secretion.
Inhibits secretion of gastric acid in the stomach.
How does Cholecystokinin (CCK) effect gastric activity?
Stimulates gall bladder contraction - releases bile into duodenum.
Inhibits gastric emptying
How does gastric inhibitory polypeptide (GIP) effect gastric activity?
Inhibits gastric contractions.
How does motilin effect gastric activity?
Motilin is produced by Mo cells in the small intestine.
Increases gastric motility
and intestinal motility.
How is gastric activity regulated neuronally?
Short reflexes in the enteric nervous system inhibit gastric secretion/contraction.
Long reflexes via the CNS alter autonomic nerve activity by targetting chemoreceptors and stretch receptors.
Name the 2 plexuses in the enteric nervous system.
Myenteric plexus
Submucosal plexus
What is the function of the enteric nervous system?
Can regualte GI function independently of the CNS.
Separate division of the autonomic nervous system - contains complete reflex pathways.
What is the function of microvilli?
Increase surface area of absorption in the small intestine.
Fenestrated capillary network lies under epithelial layer to bring blood suply, nutrients and water.
Contains lacteal which takes chylomicrons to thoracic duct.
How are carbohydrates digested?
In the mouth - physical breakdown (chewing). Chemical breakdown - amylase secretion from salivary gland.
Pancreatic secretion - alpha-amylase into duodenum.
Small intestine secretes 1,6-glucosidase which breaksdown starch.
How is glucose absorbed from the intestinal lumen?
SGLUT-1 transporter protein.
Na+-coupler co-transporter.
Sodium moves down a concentration gradient into epithelial cell, glucose transported simultaneously
How are proteins digested?
Stomach - HCl activates pepsinogens forming pepsin which breakdown proteins.
Duodenum - Trypsin, chymotrypsin and HCO3- breakdown polypeptides to tri- and di-petides.
Small intestine - dipeptidase and amino peptidases are membrane bound and break down tri- and di-peptides into single amino acids.
How are amino acids absorbed from the intestinal lumen?
H+ and Na+ co-transporters in epithelial membrane.
Na+ gradient is maintained by Na+/K+ ATPase.
How is vitamin A (retinol and derivatives) absorbed?
Vitamin is transported in chylomicrons (it is fat-soluble).
These are taken up by the liver for further hydrolysis of retinyl esters.
How is vitamin E absorbed?
Vitamin E is lipid soluble so is absorbed in the form of alpha and gamma tocopherol - incorporated in chylomicrons and VLDL.
These pass via lymph into blood stream then into liver.
How is cobalamin (vitamin B12) absorbed?
Cobalamin is bound to brotein in food.
Acidic pH release cobalamin from protein.
Gastric glands secrete haptocorrin - binds to cobalamin.
Intrinsic factor secreted by parietal cells and binds to cobalamin. Forms IF-CBL complex.
Enterocyte in lieum absorbs the IF-CBL complex.
How is calcium absorbed across the intestinal epithelium?
H+/Ca2+ exchanger transports Ca2+ from epithelial cell into the blood stream.
Free [Ca2+] in intestinal lumen = 1mM
Free [Ca2+] in blood = 1.2mM
