GIT Flashcards
Swallowing phases
1) Buccal phase - voluntary
Upper oesophagus sphincter contracts
Tongue pushes up against hard palate, forcing bolus into oropharynx
2) Pharyngeal phase - involuntary
Uvula and larynx rise up to prevent aspiration
Tongue closes off oral cavity, soft palate closes off nasal cavity, epiglottis closes off larynx
Upper oesophagus sphincter relaxes
3) Oesophageal phase - involuntary
Constrictor muscle of pharynx contracts to force food inferiorly
Upper oesophagus contracts again to prevent food regurgitation
What is dysphagia and who is more at risk
Dysphagia (difficulty swallowing) can occur at any stage
Higher risk in elderly, those with xerostomia, and neural problems
Have a basic understanding of the tissue layers that make up the GIT and their specific roles
Serosa/adventitia
Muscularis externa (longitudinal fibres)
Myenteric/Auerbach’s plexus
Muscularis externa (circular fibres)
Submucosal/Meissner’s plexus
Submucosa
Mucosa [epithelium, LP, muscularis mucosae]
6 stages of digestion
1) ingestion
2) mixing/propulsion
3) mechanical digestion
4) chemical digestion
5) absorption
6) excretion
Be able to describe each of the main regions of the GIT in terms of structure and function
Oral cavity - bite, chew, swallow
Oesophagus
- Stratified squamous epithelium
- transport
Stomach
- Pylorus: extremely thick circular layer of muscularis externa
- Mechanical and chemical digestion
Small intestine - mechanical and chemical digestion, absorption
Large intestine
- Colon absorbs water of faeces
- Appendix has crypts but no villi, extensive MALT
- From SI > pyloric valve > caecum > ascending colon > transverse colon > descending colon > signmoid colon > rectum
Rectum/anus - storage and excretion
Stomach cell types
Isthmus - A) Mucous cells - mucous surface cells - insoluble, thick gel adheres to epithelium
Neck- Mucous neck cells - soluble thinner mucus, mixes with lumen’s contents
B) Parietal cells - secrete HCl for digestion + intrinsic factors for vitB12 absorption
Base - C) Chief/zymogenic cells - inactive pepsinogen in granules that convert to pepsin in presence of HCl, site of protein synthesis and export
D) Enteroendocrine cells
Fundus - serotonin (contributes to satiation)
Body - histamine (controls acid)
Pylorus - gastrin and somatostatin (regulates secretion of HCl)
how does stomach avoid digesting itself
⇒ TO avoid digestion of gastric mucosa (protective mucus is only at epithelial surface), hydrogen and chloride ions only combine in the lumen to form HCl + pepsinogen only converts to pepsin after mixing with HCl in lumen.
Small intestine epithelial cell types in order of crypt depth
Enterocytes - microvilli for absorption
Goblet cells - secrete mucus for lubrication and protection from acidic contents
Enteroendocytes - open and closed type, secrete regulatory factors
Paneth cells - MALT aka gut-ALT here produce lysozymes
Enteroendocyte-produced regulatory factors
S - secretin - secretion of pancreatic and biliary bicarbonates and water
K - glucose-dependent insulinotropic peptide/gastric inhibitory peptide - inhibits secretion of gastric acid
I - CCK, cholecystokinin - stimulate gall bladder contract and release of pancreatic enzymes and bile, stimulate secretion of gastric acid
CCK secretion is stimulated by fats and ingested proteins (and not carbs)
Causes delayed gastric emptying, allowing fats to enter SI slower so bile has time to emulsify lipids
Mo - motilin - increase gut motility
EC - Substance P - increase gut motility
D - somatostatin - inhibit neurotransmitter secretion
G - gastrin - 1) contracts lower oesophageal sphincter and relaxes pyloric sphincter, allowing food to drop into stomach 2) stimulate secretion of HCl from parietal cells + secretion of pepsinogen from zymogenic cells to digest the food
M cells - transport antigens across intestinal epithelium
Ito (Stellate) cells - store vitamin A
Small intestine submucosal cell types
Duodenal glands - submucosal (Brunner’s) mucous glands protect duodenum mucous membrane from gastric acid
Peyer’s patches - MALT/GALT only for intestine.
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Muscularis: 1) Ring-like contractions and relaxation - mixing. Pacemaker cells 2) Propagating contractions propel chyme forward hormone-controlled
Can define the different types of motility in the intestines
Haustral contractions
Non-propulsive, swishes chyme back and forth, for mixing and facilitate absorption of substances, Initiated by autonomous rhythmicity of pacemakers
Segmental (one section that is relaxed gradually contracts, while previously contracted area relaxes to form a new sac)
Non-haustral - Mass movements
Peristalsis - contraction from oral to anal direction
Be able to identify accessory structures of the GIT and describe how they facilitate digestion and non-digestive functions
Gall bladder
Stores bile for release in gut when necessary
Concentrates bile by wter absorption
Bile facilitates digestion: 1) emulsification, bile salts adsorb to surface of hphobic triglyceride droplets 2) micelles where bile salts & lecithin aggregate to form a hphilic shell around triglycerides
Stimulating bile secretion:
1) neural - vagal nerve 2) hormonal - secretin produced by duodenum is carried to liver, CCK cholecystokinin is carried to GB causing GB contractions and sphincters relax 3) chemical - bile salts increase bile secretion
Pancreas
Exocrine
Dark-staining clusters of acini of cells, forming lobules
Pancreatic juice: bicarbonates and digestive enzymes, storage of proteases as inactive zymogens
- Proteases, amylase and lipases enter small intestine via pancreatic duct
Activated by vagus nerve, CCK (induced by triglycerides), and secretin (induced by acid in duodenum)
Endocrine
Pale-staining islets of Langerhans
Insulin and glucagon
Histological structure of each layer
1) Mucosa
Epithelium
Stratified squamous → tough for protection
Later, simple squamous → absorptive enterocytes, goblet, paneth, enteroendocrine & exocrine cells
Tight junctions btw cells prevent simple diffusion of most cells
Aq solution from glands moisten surface
Fast cell renewal
Lamina propria
Thin loose CT
Blood vessels, nerves,
Cells of MALT for defence
Muscularis mucosae
Thin layer of smooth muscle
Causes folds in mucosa, increased surface area for absorption
Assists local movements
2) Submucosa
Thin loose CT layer- BV, glands, lymphatic tissue
Submucosal (Meissner’s) plexus - Enteric nerve plexus
3) Muscularis externae
Skeletal muscle: mouth, pharynx, upper oesophagus, anus
Smooth muscle:
inner circular fibres
myenteric/Auerbach’s plexus controls gut motility
outer longitudinal fibres
4) Serosa OR Adventitia
Serosa: covers all organs and cavity walls not open to outside of body. Thin CT layer covered with simple squamous epithelium, secretes slippery fluid
Adventitia: covers organs not associated with peritoneal cavity; open to outside. Fibrous CT
