GI Lecture 2 - Regulation of Salivary and Gastric Secretions

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  • Created by: LBCW0502
  • Created on: 28-09-19 14:43
What are the reflex pathways for salivary secretion?
Smell stimuli to hypothalamus and salivary centre, taste/touch to salivary centre. Sympathetic/parasympathetic activation. Response from glands (parotid, sublingual, submandibular)
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What is the reflex response set off by?
Taste, smell, touch receptors in the mouth and by chewing (conditioned reflexes may also be involved)
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Production of watery saliva is elicited by what?
Cholinergic, adrenergic and peptidergic stimulation
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Parasympathetic stimulation via chorda lingual nerve evokes what?
Marked fluid secretion with increased blood flow and oxygen consumption. Secretion (but not blood flow) blocked by atropine (VIP/SP co-release)
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Is salivation due to hydrostatic pressure gradient from blood to saliva?
No
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What does sympathetic stimulation cause?
Initial vasoconstriction (then vasodilation) with a viscous protein rich secretion via beta-adrenergic stimulation
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Describe features of the acinar cells
Glandular tissue specialised group of acinar cells arranged as end pieces surrounding small central lumen, opening into ductule-striated or intercalated duct which then converge into larger ducts and open into main excretory duct draining into mouth
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What are the two stage processes in which saliva is formed?
Isotonic primary fluid (plasma-like electrolyte composition) formed by acinar cells and modified in duct system by reabsorption of NaCl and secretion of K+ and HCO3-
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What are the salivary glands?
Compound organs that secrete electrolytes and proteins as a fluid into the oral cavity
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What is the function of the saliva?
To lubricate food for swallowing, glycoproteins and amylase for starch digestion
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What is the maximum rate of saliva flow in humans?
1mL/min.g (vascular is highly fenestrated, 5-10 fold increase in blood flow during neural stimulation, sufficient nutrient/water supply to sustain active secretion, blood flow 10x that of equal mass of contracting skeletal muscle)
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State the histological organisation of the digestive tract
Mucosa, submucosa, muscularis externa, adventitia/serosa
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What are the divisions of gastric mucosa?
Cardiac glands, pyloric glands and oxyntic glands
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Describe features of cardiac glands
Mucous secretion, near oesophageal end, tubular, highly branched, coiled glands with few or no peptic or oxyntic cells. Glands secrete some electrolytes
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Describe features of pyloric glands
Constitutes 15-20% of total gastric mucosal area, secretes alkaline mucous juice and some electrolytes as Ca phosphate, bicarbonates, NaCl, KCl, characteristic deep gastric pits
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Describe features of oxyntic glands
Occupy fundus and body of stomach, 75-80% of total gastric mucosa, numerous invaginations (gastric pits), 100 per sq.mm, 3-7 empty into each pit (35 million total). Key site of gastric HCl secretion
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What are the three regions of oxyntic glands?
Isthmus - parietal and surface mucous cells. Neck - parietal and mucous cells. Base - chief cells (secrete pepsinogen), some endocrine cells. Gastrin producing cells (G-cells) localised by in middle third of mucosa
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What is the role of blood flow in supply of nutrients for HCl secretion and buffering via HCO3?
Produces a bicarbonate wave and prevents autodigestion
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What is the gastric mucosal defence?
H+ entering mucous gel (pH 1-2). Surface mucous cells release HCO3- into mucous gel (pH 7). Bicarbonate can also be released into circulation
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What is the stomach?
An exocrine organ which secretes a large acid volume after a meal
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What does secretion contain?
Pepsin which initiates protein digestion (process continued by intestine by pancreatic enzymes)
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What is the gastric surface protected by?
A thin film of mucous which is produced constantly by surface epithelial cells
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Give an example of another exocrine product
Intrinsic factor, a glycoprotein that combines with vitamin B12 aiding absorption in the ileum
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Describe the entire surface of the glandular stomach
Simple columnar epithelium of surface mucous cells, which also line numbers tubular invaginations and gastric pits
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What happens during stimulation of HCl by ACh, His and gastrin?
Parietal cells undergo structural changes, forming a markedly increased tubulovesicular system for secretion of protons
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Compare a resting parietal cell and an activated parietal cell
Resting parietal cell - tubulovesicles. Activated - tubulovesicles fused to form intracellular canaliculus
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Describe features of gastric juice (1)
Complex solution of acidic component (HCl) secreted by parietal cells and alkaline component containing pepsinogen secreted from peptic cells plus electrolytes (Cl, Na, K) form other cell types
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Describe features of gastric juice (2)
Parietal cells postulated to secrete H+ at constant concentration of ~150 mEq/L with the variation of acid in gastric juice depending on rate of non-parietal cell secretion of alkaline components
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Describe features of gastric juice (3)
Mechanism of H+ secretion against large gradient (high in canaliculi) requires energy (luminal H+-K+ ATPase) generated by oxyntic glands by aerobic metabolism
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What is Soll's three receptor model?
Gastrin, ACh and histamine (at CCK2, H2 and M3 receptors) stimulate secretion of HCl
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What is the dimaline model? (1)
Gastrin binds to CCK2 on ECL-cell which causes release of histamine which binds to H2 receptor and stimulates HCl secretion (gastrin can also bind to CCK2 on parietal cell for HCl secretion). ACh binds to M3 receptor to stimulate HCl secretion
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What is the dimaline model? (2)
Physiological stimulation of gastric acid secretion
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Describe the control of G-cell function
Proteins and GRP stimulate secretion of gastrin into circulation (blood supply carries H+ away, helps antioxidant function). Presence of H+ stimulates D cell to secrete somatostatin which inhibits G-cell (prevents secretion of gastrin)
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Describe the gastric phase of gastric acid secretion
Food in the stomach stimulates gastric-acid secretion by two major mechanisms - mechanical stretch and presence of digested protein fragments
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Describe the intestinal phase of gastric-acid secretion
Protein digestion products - Intestinal G cell (gastrin), intestinal endocrine cell (entero-oxyntin), absorbed amino acids - all stimulate acid secretion from parietal cell
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Describe features of omeprazole (1)
Activated drug forms disulphide link with H+-K+ ATPase and blocks enzyme unreversibly. Inhibits gastric HCl secretion by 80% over 24h. Therapy for peptic ulcer and reflux oesophagitis
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Describe features of omeprazole (2)
Quicker relief/accelerated healing rates for peptic ulcers compared with H2-antagonists
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Provide a clinical summary for omeprazole (1)
Activated drug forms disulphide link with H+-K+ ATPase and blocks enzyme unreversibly. New enzyme is constantly being regenerated. Inhibits gastric HCl secretion by 80% over 24h. Therapy for peptic ulcer and reflux oesophagitis
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Provide a clinical summary for omeprazole (2)
Long term maintenance treatment is not currently recommended for peptic ulcer through patients with reflex oesophagitis can be continued at a dose of 20 mg OD. Quicker relief from symptoms compared to H2-antagonists (with accelerated healing rates)
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What are the drug interactions with omeprazole?
Can delay elimination fo diazepam, phenytoin and warfarin
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What are the adverse drug reactions for omeprazole?
Generally well tolerated. Nausea, headache, diarrhoea, constipation, flatulence. Skin rashes (mild/transient/no consistent relationship with treatment). High doses - carcinoid tumours of gastric mucosa (in animal studies but not reported in patients)
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Describe the trend for acid secretion and gastric pH in response to a meal in healthy human beings
Acid secretion increases then decreases after 1.5h. pH increases rapidly then decreases after 1h
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What are the phases of gastric acid secretion?
Acid secretion divided into basal (fasting) and stimulated (post-prandial) phases. Three phases regulating HCl secretion - cephalic, gastric, intestinal
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Describe the trends for the inhibition of gastric acid secretion following the infusion of fat into the jejunum
Saline in jejunum increases rapidly. Fat in jejunum increases gradually but remains low
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What are the stimulatory and inhibitory processes on the GI system? (1)
Stimulatory - parasympathetic, hypoglycaemia, psychic stimuli, taste, smell etc, CNS, gastrin released into vascular system, contact/distention in stomach
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What are the stimulatory and inhibitory processes on the GI system? (2)
Inhibitory - sympathetic, pain, psychic stimuli, CNS, SIH, secretin, GIP released into vascular system, receptors for H+, osmolarity, neutral amino acids (indicate CNS to stop signals)
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Other cards in this set

Card 2

Front

What is the reflex response set off by?

Back

Taste, smell, touch receptors in the mouth and by chewing (conditioned reflexes may also be involved)

Card 3

Front

Production of watery saliva is elicited by what?

Back

Preview of the front of card 3

Card 4

Front

Parasympathetic stimulation via chorda lingual nerve evokes what?

Back

Preview of the front of card 4

Card 5

Front

Is salivation due to hydrostatic pressure gradient from blood to saliva?

Back

Preview of the front of card 5
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