Secretion and swallowing Flashcards
GI function - signals can be split into three phases - what are they?
Cephalic, Gastric, Intestinal
Signals can be physical (stretch receptors) or chemical (sight and presence of food)
Cephalic phase - thinking about food, body preparing for it
Gastric phase - when meal reaches stomach
Intestinal phase
Parasymp controlled, overlap between phases
What is secretion produced by?
Salivary glands
Gastric mucosa
Exocrine cells of pancreas
Liver (bile)
What are characteristics of saliva secretion?
High HCO3-
High K+
Hypotonic
alpha-amylase and lingual lipase
What factors increase and decrease saliva secretion?
Increase - Parasymp (primarily) and symp
Decrease - sleep, dehydration, atropine
What are functions and locations of saliva secretion?
Protection of teeth and gums, initial digestion of starches and lipids, dilution and buffering of ingested foods, lubrication of ingested foods with mucous
Sublingual - mostly mucous cells
Submandibular - serous and mucous
Parotid - serous
Three stages of salivation in salivary glands
Acini cells secrete primary secretion (isotonic)
Myoepithelial cells stimulated by neural input to eject saliva
Duct cells - secondary modification, reabsorb sodium and chloride and add potassium, HCO3- conc is altered depending on flow rate (inc HCO3- if high flow rate and vice versa)
What are the two salivation reflexes?
Simple (uncond) - pressure receptors - presence of food - impulses along afferent nerves - salivary centre in medulla
Acquired (conditioned) - thinking about food - cerebral cortex - salivary centre in medulla
From salivary centre impulses via extrinsic autonomic symp and parasymp stimulation - increased saliva
What are two phases of swallowing?
Oropharyngeal - mouth to oesophagus (oral phase, oral transit phase and pharyngeal phase)
Oesophageal - oesophagus to stomach - swallowing centre in brain triggers first peristaltic wave to move foood from beginning to end of oesopahgus - too fast for absorption. In peristalsis circular fibres behind bolus will squeeze blous down and longitudinal fibres in front of bolus shorten distance of travel. Secondary peristaltic wave more forceful than first as sticky food may be lodged in oesophagus - inc saliva production
Oesophageal secretion
Only secretes mucous for lubrication/protection
Simple mucous glands protect against mech damage
Gastric end has compound mucous glands to protect against chem damage
Satiety and hunger pangs
Satiety - receptive relaxation in stomach - fill up in large volumes before feel uncomfot`ble
Hunger pangs - caused by peristaltic waves when stomach empty, can be intense and tetanic for 2-3 mins, very intense when person is young and blood sugar is low
Secretory products of gastric cells and cells in gastric pit
Secretion of intrinsic factor and HCl and gastrin
Mucous neck cells - secrete mucous
Oxyntic/parietal cells - secrete Hcl and int factor
peptic/chief cells - secrete pepsinogen
When does inhibition of HCl secretion occur?
When HCl no longer needed to convert pepsinogen to pepsin, occurs after the chyme moves into SI and H+ buffering capacity of food is no longer a factor
Somatostatin:
Direct pathway - binds to receptors on parietal cells
Indirect pathway - inhibits histamine release from stomach and gastrin release from G cells
Pepsinogen secretion
Secreted by chief cells in oxyntic glands in response to vagal stimulation. H+ triggers local reflexes which stimulate chief cells to secrete pepsionogen
Actiavated to pepsin when it comes into contact with HCl
Intrinsic factor secretion
- Secreted by parietal cells
- Important for absorption of vit B12 by receptor mediated endocytosis
- Loss of secretion of intrinsic factor - pernicious anaemia (impaired RBC maturation)
Absorption ocurs in terminal ileum - cells only recognise B12 when it is bound to IF
Pancreatic secretion
Composed of enzymes and high HCO3- amounts
This bicarbonate neutralises the stomach H+
Enzymes digest carbs, proteins and lipids
Exocrine pancreas
Parasymp from vagus stimulates secretion
Symp inhibits secretion
Pancreatic enzymes stored in condensed zymogen granules until release
Formation of pancreatic secretion
Aqueous component and enzymes
Aq component released from centroacinar cells and ductal cells - fluid contains NA K, Cl and HCO3- - modified according to flow rate
Pancreatic enzymes
Released from acinar cells - amylase and lipases secreted as active enzymes, proteases secreted in inactive form and activated in duodenum
(3 types of enzymes)
3 major proteolytic enzymes are trypsinogen, chymotrypsinogen and procarboxypeptidase
Activation of pancreatic proteolytic enzymes
Trypsinogen is converted to trypsin when enteropeptidase is activated
1) The more trypsin we have got the more trypsin gets activated - pos feeback
2) The more trypsin we get the more activation of chymotripsinogen (to make more chymotrypsin) and the more activation of procarboxypeptidase (to make more carboxypeptidase)
Regulation of pancreatic secretion in 3 phases of digestion
Cephalic phase - mediated by vagus nerve (mainly enz secretion)
Gastric phase - distension of stomach and mediated by vagus nerve (enz secretion)
Int phase - accounts for 80% of pancreatic secretion and both enz and aqueous secretions are stimulated
Pancreatic response in int phase - acinar cells
CCK secreted in presence of AA’s, small peptides and fatty acids
This tiggers acinar cell is to produce enzymes
(ACh potentiates CCK action)
Pancreatic response in int phase - ductal cells
S-cells in duodenum secrete secretin (secretin release triggered by arrival of acidic chyme in duodenum) - triggers release of bicarbonate, neutralising the thing that was initial trigger
Gallbladder and bile secretion
Gall bladder stores bile which is continously produced by hepatocytes and flows to gallbladder through bile ducts
Concentrates bile - epithelial cells lining the gallbladder absorb ions and water iso-osmotically
Ejects bile - begins 30 mins after meal
What is bile?
Not enzymic
Essential for digestion and absorption of lipids
Bile salts emulsify lipids to prepare them for digestion and solubilise the products of digestion into packets called micelles
Mixture of bile salts, bile pigments and cholesterol
Regulation of bile secretion
In between meals - bile salts recirculated to liver via enterohepatic system therefore more bile produced, bile stored/concentrated in gall bladder
Cephalic phase (just before meal) - neural stimulation via vagus nerve to inc bile flow
During/after a meal - chyme in duodenum stimulates release of CCK and secretin
Secretions of large intestine
Alkaline mucous - high K+ and HCO3- and no digestive enzymes
Protection and lubrication
Neutralisation of H+ produced by gut bacteria
Distension/mech stimulation of wallls
Inc secretion triggered by Ach
Dec secretion stimulated by Adr and somatostatin
Extreme parasymp input gives excess mucous, defection (nervousness)
