GI Physiology Flashcards
What are the 4 types of teeth?
1) Incisors
2) Canines (cuspids - single root, spike and tear)
3) Pre molar (2 roots - bicuspids - crushing and grinding)
4) Molar (3/4 roots - crushing and grinding)
What are the 4 main components of saliva and their roles?
1) Serous fluid and mucous - lubrication of mouth and food and cleaning, dissolve food stuffs to facilitate taste
2) Anti bacterial enzymes (lysozyme, IgA) - protection about bacteria
3) Bicarbonate and calcium - protection against acid, replaces Ca2+ ions lost from teeth
4) Salivary amylase (ptyalin, lingual lipase) - digestion
How is saliva production regulated?
Entirely neural
Parasympathetic - produces a more watery serous secretion
Sympathetic - produces a more mucous secretion
How is parasympathetic input to the salivary glands controlled?
By the salivary center in the brain stem, controlled by:
1) local stimuli: taste and touch in mouth
2) central stimuli: smell and sight of food
3) Learned reflex: pavlovs dogs
What are the 2 stages in saliva production?
1) Initial isotonic fluid in the acinus containing NaCl, protein and/or mucous
2) Passes along duct, get salt and water reabsorption and HCO3- (and K+) secretion leads to a hypotonic alkaline solution
NB. flow rate is important
What does the enzyme ptyalin a-amylase found in saliva do? What is its optimum pH and when is it denatured?
breaks down starch and large polysaccharides. breaks alpha 1-4 linkages (but not adjacent to 1-6 linkages which cause branches and cannot be cut by this salivary enzyme)
Optimum pH is 7, denatured at pH 4
What does the enzyme lingual lipase found in saliva do? What is its optimum pH and when is it denatured?
Cleaves the outer fatty acids from tricglycerides to leave diacyglycerol
pH optimum is 4, therefore its stable in stomach and works with gastric lipase but is denatured by pancreatic protease
What are the 3 main salivary glands?
1) Parotid (biggest)
2) Sublingual
3) Submandibular
What is the secretions and neural control of parotid salivary glands?
secretion: mostly serous, source of amylase, proline rich proteins, 25% of volume of total saliva
Parasympathetic control form CN IX (glosso pharyngeal)
Sympathetic control from superior cervical ganglion
What is the secretions and neural control of the sublingual salivary glands?
Secretion: mostly mucous, source of lingual lipase and makes up 5% of total saliva secreted
Parasympathetic control is from CN VII (facial)
Sympathetic control is from superior cervical ganglion
What is the secretions and neural control of the submandibular glands?
Secretion: mostly serous and mucous, source of lysozyme and lacto peroxidase (another anti bacterial), makes up 70% of total volume of saliva secreted
Parasympathetic control from CN VII (facial)
Sympathetic control from the superior cervical ganglion
What happens in the 4 stages of swallowing:
1) Voluntary phase
2) Pharyngeal phase
3) 3rd phase
4) Oesophageal phase?
1) Voluntary phase - soft palate controls food bolus, epiglottis open can still breathe
2) Pharyngela phase - nasopharynx closed, food in oropharynx, help up by epiglottis
3) Epiglottis movers down (trachea closed) and upper oesophageal sphincter opens
4) Oesophageal phase - peristalsis, upper oesophageal sphincter closes
What are the 3 types of taste buds and where are they located?
1) Fungiform - tip of tongue
2) Circumvalate - one strip along superior surface
3) Foliate - lateral to circumvalate
How are taste buds structured and what is their nerve supply?
Sense cells surrounded by supporting cells, have a narrowed tip, sense cells synapse with afferent sensory neurones,
Supplied by cranial nerve VII (facial)
What are the 2 types of specialised taste cells and what do they detect?
1) Ion channel based - detect salty (Na+) and sour (H+) stimulus
2) GPCR based - detect sweet, umami and bitter stimulus
How do ion channel based taste sensors work?
Na+ (Salty stimulus) through ENaC channel, H+ (sour stimulus) through TRPP channel
Entry of these molecules depolarises the membrane, opens voltage gated calcium channels, calcium moving in (from outside and from ER) causes neurotransmitter to be released
How do GPCR based taste sensors work?
Sweet, umami and bitter stimulus bind to GPCR which causes calcium release partly through second messengers (IP3) and partly through Na+ entry leading to depolarisation again causes calcium entry from outside and ER which causes neurotransmitter to be released
ʜow is the oesophagus innervatedʔ
Fibres from the oesophageal plexus
What happens to the lower oesophageal sphncter after food passes throughʔ
Closes firmly and then relaxes slightly but still has significant tone
ʜow long and wide is the oesophagusʔ
25cm long, 2cm wide
Why do the submucosal oesophageal glands secrete mucousʔ
To facilitate the passage of a food bolus
What is the cephalic phase of ɢɪ activityʔ
To do with sight and smell of food etc. Some concious and some unconcious stimuli, mediated by the parasympathetic nervous system.
Cranial nerves Vɪɪ and ɪX cause salivary secretion
Vagus nerves carry afferent fibres which contribute to feedback system
What occurs in the acinar portions of salivary glandsʔ
A primary secretion is produced, isotonic with lots of Cl- and ɴa+
These are the sights of neural contact, ACh from parasympathetic and Adr from the sympathetic
1) ɴa+/K+ ATPase on basal membrane keeps intracellular ɴa low
2) Use that gradient to co transport Cl-, K+ and ɴa+ into cell through basal membrane
3) Cl- , ʜCO3- and K+ pass passively out of cell on apical membrane
4) ɴa+ travels paracellularly pulling water with it into lumen
What occurs in the duct portions of the salivary glandsʔ
1) ɴa+/K+ATPase on basal membrane still keeping intracellular ɴa+ low
2) ʀesorb ɴa+ and K+ from the lumen passively through apical membrane
3) Cl- swapped for ʜCO3- on apical membrane to make an alkaline and hypotonic solution
4) Tight junctions get tighter and water can no longer pass paracellularly so get a more dilute solution
What are the 3 layers of muscle in the stomach wallʔ
1) ʟongitudinal
2) Circular
3) Oblique (deepest)
What is the main function of the fundus and body of the stomachʔ
Mainly storage
What are the parts of the stomach in orderʔ
1) ʟower oesophageal sphincter (Opens into cardia)
2) Fundus
3) Body
4) Antrum
5) Pylorus
6) Pyloric canal
7) Pyloric sphincter
8) Duodenum
What are the secretions and functions of the lower oesophageal sphincter and cardiaʔ
Mucous, ʜCO3-
Prevention of reflux, entry of food, regulation of belching
What are the secretions and functions of the fundus and bodyʔ
ʜ+, ɪntrinsic factor, Mucous, ʜCO3-, Pepsinogens, ʟipase
ʀeservoir (stores food), Tonic force during empylys
What are the secretions and functions of the fundus and bodyʔ
ʜ+, ɪntrinsic factor, Mucous, ʜCO3-, Pepsinogens, ʟipase
ʀeservoir (stores food)
What are the secretions and functions of the antrum and pylorusʔ
Mucous, ʜCO3-
Mixing, grinding, sieving,
What are the secretions and functions of the antrum and pylorusʔ
Mucous, ʜCO3-
Mixing, grinding, sieving, regulation of emptying
Which parts of the stomach secrete ʜ+, intrinsic factor, pepsinogens and lipaseʔ
Fundus and body
ɪn the stomach what is passive relaxation and active accomodationʔ
2 of the roles of gastric motility, they allow the stomach to expand when it fills with food
What are the cells and secretions of gastric pitsʔ
Mucous cells, secrete mucous and also protective ʜCO3-
What are the cells of the gastric glands and what do they secreteʔ
1) Parietal cells (oxyntic cells) - secrete ʜCʟ and intrinsic factor in fundus and body
2) Chief cells (peptic cells) - secrete pepsinogen (in fundus and body)
3) ɢ cells - at the bottom of the glands, secrete gastrin (into blood stream), mainly in antrum and pylorus
What is gastrin and what is its action and receptorʔ
Peptide hormone, shares receptors with CCK-PZ, gastrin receptor is CCK-B receptor.
Main action on stomach is to stimulate acid secretion and promote mucosal growth
ʜow is secretion of gastrin from ɢ cells regulatedʔ
1) ʟumenal proteins stimulate its release
2) PS input (cephalic phase) mediated by gastrin releasing peptides from interneurones
3) ʀeleased is inhibited by [ʜ+] = negative feedback
What is intrinsic factor and what is its roleʔ
Secreted by parietal cells in gastric glands, 55kDa protein, binds to cobalamin (B12), intrinsic factor-cobalamin complex formed in the stomach and absorbed in the terminal ileum
What is pepsinʔ
Proteases secreted from chief cells inn gastric glands, secreted as a pro enzyme and cleaves spontaneously at low pʜ (
ʜow is pepsin secretion from chief cells controlledʔ
Secretion is stimulated by ACh (parasympathetic nervous input), [ʜ+], minor effects of secretin, CCK and gastrin (CCK-a receptor)
What is somatostatin and what is its actionʔ
Peptide hormone, 14 & 28 aa (2 forms).
ʀeleased from D cells of the stomach, duodenum and pancreas. ɪnvolved in paracrine signalling (Acts on adjacent cells).
ɪnhibits the release of CCK and secretin and acts on ɢ cells to inhibit the release of ɢastrin
ʜow is the release of somatostatin controlledʔ
ʀelease is stimulated by [ʜ+] and inibited by ACh (from parasympathetic nervous system)
What are the 3 phases of gastric motilityʔ
1) Propulsion phase
2) Emptying phase
3) ʀetropulsion phase
What are the three sections of the antrumʔ
1) proximal
2) middle
3) terminal
What happens in the phases of gastric motilityʔ
1) propulsion phase - wave goes over proximal antrum and food is propelled into relaxing terminal antrum
2) emptying phase - perilstaltic wave over middle antrum, pylorus opens and some food is swept into the duodenum and some food swept back, assoicated with back flow and mixing of contents
3) retropulsion phase - contraction of the terminal antrum, pylorus closes, contents of the terminal antrum are swept retrograde into the relaxing middle antrum, jet like propulsion grinds food into smaller particles
What are the main differences in gastric motility after a liquid compared to a solid mealʔ
ʟiquid meal - food pushed through quickly - shallower perilstaltic waves
Solid meal - initial lag phase, food retained in the stomach and get mixing and grinding of food - deeper perilstaltic waves
Why is gastric motility slowed in a calorific mealsʔ
Duodenum senses the sugars and signals to the stomach to slow movement, giving time for absroption
What happens to parietal cells when they become activatedʔ
in resting state cell has tubulovesicles, when activated the vesicles fuse to create a big surface area with loads of proton pumps in the membrane, pumps protons into the space (ʜCl) using a K+/ʜ+ATPase transporter
What are the stimulators and inhibitors of parietal cellsʔ
Stimulatorsː
1) ɢastrin and PS nervous system (ACh) act directly on cells
2) ɢastrin and PS nervous system also act on ECʟ cells causing them to release histamine which acts directly on parietal cells causing secretion
ɪnhibitors
1) prostaglandins (PɢE2) and somatostatin act directly on parietal cells inhibiting ʜCl secretion
Why do ɴSAɪD’s lead to stomach ulcersʔ
Stop you producing prostaglandins so cant stop acid production by parietal cells, too much acid = stomach ulcer
ɪn the stomach what is an alkaline tideʔ
As ʜ+ is pumped out of parietal cells, ʜCO3- is produced which goes into the blood stream, alkaline tide refers to the rise of alkali in the blood following a big meal
Why does the surface epithelium of the stomach secrete ʜCO3-ʔ
Protect the surface epithelium, ʜCO3- in mucous holds the pʜ of surface epithelium ~ pʜ 7 (gastric juice pʜ 2)
What are the simulators of pepsinogen secretion from chief cellsʔ
Stimulatorsː
1) [ʜ+] and PS nervous system (ACh) acting through enteric plexus of stomach under PS control
2) ɢastrin and secretin have moderate effects on the secretion of pepsinogen
What is gastric lipase and what does it doʔ
Similar to lingual lipase, cleaves one fatty acid chain off triglycerides leaving a diacylglycerol
Optimum pʜ is 4, stable in the stomach but is denatured by pancreatic proteases, works with lingual lipase
Why is ptyalin-a-amylase present in the stomachʔ
Digests polysaccharides, produced by the salivary glands and still hangs around in the fundus and bpdy where there is lots of food but isnt well mixed with acid yet (denatured at pʜ 4)
What reflexes occur when you vomitʔ
1) reflex closure of the soft palate and glottis
2) Opening of upper and lower oesophageal sphincters
ʜow is vomiting controlledʔ
Centrally controlledː area postrema = chemoreceptor trigger zone
What are the 5 causes of vomitingʔ
1) Vagal afferent, in response to irritants in or around the bowel
2) Psychogenic pain/revulsion (severe pain or shock)
3) Motion sickness/labrythe disorders (problems with middle ear)
4) Drugs/toxins with a direct effect
5) Pregnancy
What are the consequences of vomitingʔ
Salivation, sweating, hyperventilation, retrograde peristalsis, retching, cardia displaced into thorax, emptying of gastric (and sometimes small intestine) contents
What muscle contractions occur in the small intestine to facilitate peristalsisʔ
1) Circular muscle contracts behind bolus of food, and ahead relaxes
2) ʟongitudinal muscle ahead of food bolus contracts, shortening adjacent segments
3) Wave of contraction of circular muscle pushes food bolus forwards
What muscle contractions occur in the small intestine to facilitate segmentationʔ
Alternate contractions of neighboring segments, churns, fragments and mixes food with intestinal secretions
What is the migrating motor complex and when does it occurʔ
When emptying the gut get a MMC every 90 mins, stimulated by motilin and inhibited by feeding
strong, slow peristaltic waves, pyloric sphincter is relaxed
They keep the gut clean, prevent reflux and reduce bacterial growth
What are APUD cells and where are the foundʔ
ʟike out of place neural cells, apical membrane detects whats happening and basolateral membrane secretes hormones, found in the small intestine
Where do secretions come from in the small intestineʔ
Crypts of lieberkuhn
Submucosal Brunner’s glands in duodenum
What hormones are secreted from the Sɪ and from what cellsʔ
1) CCK (same as CCK-PZ) from ɪ cells
2) Secretin from S cells
3) Motilin from M cells
4) ɢastrin from ɢ cells
5) Bicarbonate secretion (ʜCO3-) from pancreatic duct cells
6) Mucous secretion from goblet cells
What hormones are secreted from the Sɪ and from what cellsʔ
1) CCK (same as CCK-PZ) from ɪ cells
2) Secretin from S cells
3) Motilin from M cells
4) ɢastrin from ɢ cells
5) Bicarbonate secretion (ʜCO3-) from pancreatic duct cells
6) Mucous secretion from goblet cells
What are the 2 general categories of peptidaseʔ
1) Endopeptidases (cut within the chain - trypsin, chymotrypsin, elastase)
2) Exopeptidases (Cut at the last peptide bond - carboxypeptidases)
ʜow does fat digestion occur in the small intestineʔ
1) Fat arrives in small intestine as big globules - but enzymes to digest fats only work at the aqueous interphase
2) Use bile salts to break up lipid droplets increasing surface are
3) ʟingual and gastric lipase have chopped off one chain, pancreatic lipase cleaves off 2nd fatty acid leaving 2 fatty acids and a monoglyceride
4) fatty acids and monglycerides complex with bile salts to form mixed micelles (solubilise the fats)
ʜow are fats absorbed from the small intestine lumen (after digestion)ʔ
1) Mixed micelles diffuse close to brush border delivering their contents to the membrane
2) Fatty acids and monoglycerides diffuse across the membrane
What happens to fatty acids and monoglycerides once they have been absorbed into enterocytes of Sɪʔ
1) triglycerides are resynthesised and packaged into chylomicrons
2) Chylomicrons are exocytosed into the interstitium but are too big to pass into the capillaries so pass into lymphatic lacteals
3) Chylomicrons travel via the lymphatic system into the ɪVC
ɴB. Short chain fatty acids and glycerol can however pass straight into the bloodstream
What are the pancreatic pro enzymes released that break down proteins?
1) trypsinogen
2) Chymotrypsinogen
3) Proelastase
4) Procarboxypeptidase A
5) Procarboxypeptidase B
Which enzyme cleaves trypsinogen (released from the pancreas) to its active form?
Enterokinase on epithelial cells, secreted by crypt cells
Which enzyme cleaves most of the pancreatic proenzymes which digest proteins?
Trypsin
What happens to di peptides or amino acids which are produced from proteolysis in the small intestine lumen?
1) Diffuse to the brush border
2) Amino acids are taken up by Na+ linked secondarily active transporters
3) Di (and tri) peptides taken up by H+ linked secondarily active transporters
4) Then get passive transport across the basolateral membrane into the blood
What is the role of protein absorption from the small intestine in immunology?
1) Small amounts of protein taken up by endocytosis
2) Some of that protein is taken into M cells which overly Peyer’s patches
3) This allows the immune system to adapt to what we eat and probably plays a role in tolerence
How much proteolysis takes place in the stomach before the food reaches the small intestine?
~15%
In the small intestine what lumenal digestion of carbohydrates occurs?
Soluble amylases break a-1-4 bonds and you get left with di and tri saccharides
What digestion of carbohydrates happens through brush border enzymes in the small intestine and what enzymes are involved?
Di and tri saccharides you are left with after lumenal digestion are broken down by brush border enzymes
ISOMALTASE - breaks 1-6 links
GLUCAMYLASE - breaks down anything with glucose in it
LACTASE - breaks down lactose
How are monosaccharides absorbed into the enterocytes of the small intestine?
1) Na+/K+ATPase on basolateral membrane keeps intracellular Na+ low
2) SGLT-1 sodium co transporter on apical membrane takes up 2Na+ and either glucose or lactose (secondarily active)
3) GLUT 5 on apical membrane takes up fructose passively
4) GLUT 2 on basolateral membrane, all 3 monsaccharides exit passively into the blood
At what part of the small intestine is iron generally taken up?
In the lower part of the duodenum
How is iron absorbed from the small intestine?
1) Arrives at the brush border as Fe3+, needs to be Fe2+ to be absorbed so it reduced by brush border enzyme Dcytb
2) Fe2+ taken up by co transport with H+ (enzyme = DMT)
What happens to Fe2+ once it has been absorbed into the cell from the lumen of the small intestine?
1) Fe2+ transferred to mobilferrin
2) Mobilferrin acts as a buffering system to stop toxicity with Fe2+
3) Fe2+ leaves cell via ferroportin (FP1) and after haphaestin oxidises it back to Fe3+ the iron binds to transferrin in the plasma
How and where are conjugated bile salts absorbed from the small intestine?
In the terminal ileum
Actively absorbed by Na+ co transport
What happens to bile salts not absorbed in the terminal ileum?
Some bile salts escape to the colon and then may become conjugated by bacteria making them lipophillic so they can be passively reabsorped in the colon
About 5% are lost in the faeces
What are the fat soluble vitamins?
ADEK
How are fat soluble lipids absorbed?
Dissolved in lipid droplets -> micelles -> chylomicrons
How are water soluble vitamins generally absorbed?
Na+ linked
How are water soluble vitamins generally absorbed?
Na+ linked
What kind of transport is Ca2+ absorbed in the small intestine by?
Active transport (some passive but this is paracellularly and not under hormonal control)
Where is Ca2+ actively absorbed in the small intestine?
In the duodenum
How is the absorption of Ca2+ in the small intestine regulated?
By Vitamin D, does so by increasing the number of calcium channels in the apical membrane of the enterocytes of the duodenum
What happens to Ca2+ once it is inside the enterocytes of the duodenum?
1) Once in the cell Ca2+ gets bound to calbindin (so cant signal to cell)
2) Have a sodium exchanger for Ca2+ to pass into the blood
How is Vit B12 (cobalamin) absorbed in the small intestine?
1) cobalamin bound to proteins in the food
2) In stomach acid pH and pepsin release it from protein
3) Gastric glands secrete heptacorrin which binds to cobalamin
4) Gastric parietal cells secrete intrinsic factor
5) Pancreas secretes proteases & HCO3-
6) Cobalamin is released after degradation of protein heptacorrin
7) Intrinsic factor - cobalamin complex forms
8) Ileal enterocytes absorbs intrinsic factor-cobalamin complex
Where is vit b12 absorbed?
Terminal Ileum
