Lecture 7 Flashcards
What is the gastrointestinal (GI) tract?
A continuous muscular digestive tube. Functions change all the way along the tube.
What is the GI tract involved in?
Ingestion, Propulsion, Mechanical digestion, Chemical digestion, Absorption, Defaecation
What are the 4 main layers of the GI tract?
Lumen in the centre, Mucosa (in contact with lumen), Submucosa, Muscularis externa, Serosa
What are the key features of the mucosa?
Innermost epithelial layer, Secretion: mucus, enzymes and hormones, Absorption: digestive end products, Barrier function
What are the key features of the submucosa?
Connective tissue, Blood and lymphatic vessels, Neurones
What are the key features of the muscularis externa?
Smooth muscle (circular and longitudinal)
What are the key features of the serosa?
Protective outer layer –> called the visceral peritoneum
How does the innermost mucosal layer change as you move through the GI tract?
In esophagus: Mainly squamous epithelial cells; In stomach: Dark cells = secretory, Has invaginations/pits; In small intestine: Has villus folds to increase surface area for absorption; In large intestine: Increased musculature to push contents through lumen, Water absorption occurs here as well
What are some of the additional functional considerations of the GI tract?
Substances in GI tract lumen are essential outside of the body, Multiple sensors and receptors line the GI tract to monitor contents and respond to conditions, Controls: Intrinsic through endocrine secretion (local control), Extrinsic (CNS)
What types of hormones are identified from the gut? How many have been identified and how do they work?
Over 2 dozen GI hormones identified, All peptide hormones, Many are also produced in other places, Act in both a paracrine and endocrine way
Describe the structure of the mucosal layer of the stomach
Mucosal layer has pits, These are crypt-like structures that extend down into the mucosal layers, There is a change from the epithelial state to more glandular cells at the base of the pits, Chief cells predominate the pits, These produce pepsinogen, Parietal cells secrete HCl, Enteroendocrine cells secrete gastrin
Which endocrine cells are found in the stomach?
Chief cells, Enteroendocrine cells, Parietal cells
Why does the stomach mucosa need to be a low pH?
Pepsinogen is the inactive form of pepsin, To activate and form pepsin, need acidity, Parietal cells secrete HCl required to make stomach more acidic
What receptors do enteroendocrine cells express? What happens when these are stimulated?
They express G protein-coupled receptors (GPCRs), These are homologous to taste receptors, Ultimate effect of stimulating GPCR = calcium influx into cells, This stimulates docking of peptide hormone-containing vesicles in enteroendocrine cell to the cell membrane and allows release of hormone
How many peptide hormones can one enteroendocrine cell secrete?
Only 1 type (e.g. have a specific enteroendocrine cell that secretes gastrin)
What do the GPCRs on enteroendocrine cells (taste receptors) recognise?
Recognise digestion products of food, e.g products of proteins/lipids/carbohydrates
What do G cells secrete?
Gastrin
How does gastrin directly affect parietal cells? What is the effect of this?
Gastrin can bind to CCK-2R receptors on parietal cells, This causes parietal cell to alter its H+/K+ exchanger to pump more H+ into the lumen, Increases acidity, The increase in acidity negatively regulates gastrin by acting on G cells to inhibit gastrin secretion
How does gastrin indirectly affect parietal cells?
Gastrin released from G cells binds to and stimulates CCK-2R expressed on surface of ECL cells, Allows indirect action on parietal cells, Stimulates ECL cells to release histamine, Histamine travels to parietal cells and affects function, Allows indirect regulation of parietal cells
What is the trophic effect of gastrin?
Gastrin has a trophic function of stimulating growth, Also has effects on gastric motility
What are the 2 processes that regulate gastrin and parietal cells?
Neural inputs (Cholinergic activation of ECL and D cells in lumen body, Inhibition of D cells in lumen of antrum), Hormonal regulation (Somatostatin –> SS28 peptide produced from D cells)
What do D cells do?
Produce and secrete somatostatin
What is the structure of parietal cells?
Odd, canicular membrane shape, Loads of membrane-bound vesicles
How does the structure of parietal cells change under stimulation from gastrin/histamine?
When cells are stimulated by gastrin/histamine/ACh, huge movement of membrane occurs, Vesicles fuse which shifts the membrane, Causes shift in the membrane and shunts the H+/K+ exchangers to the membrane, Allows them to rapidly pump H+/K+ to increase acidity, Shift in membrane allows microvilli to expand and increase apical surface area
