Secretion and swallowing

Question 1

GI function - signals can be split into three phases - what are they?

Answer 1

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

Question 2

What is secretion produced by?

Answer 2

Salivary glands
Gastric mucosa
Exocrine cells of pancreas
Liver (bile)

Question 3

What are characteristics of saliva secretion?

Answer 3

High HCO3-
High K+
Hypotonic
alpha-amylase and lingual lipase

Question 4

What factors increase and decrease saliva secretion?

Answer 4

Increase - Parasymp (primarily) and symp

Decrease - sleep, dehydration, atropine

Question 5

What are functions and locations of saliva secretion?

Answer 5

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

Question 6

Three stages of salivation in salivary glands

Answer 6

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)

Question 7

What are the two salivation reflexes?

Answer 7

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

Question 8

What are two phases of swallowing?

Answer 8

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

Question 9

Oesophageal secretion

Answer 9

Only secretes mucous for lubrication/protection
Simple mucous glands protect against mech damage
Gastric end has compound mucous glands to protect against chem damage

Question 10

Satiety and hunger pangs

Answer 10

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

Question 11

Secretory products of gastric cells and cells in gastric pit

Answer 11

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

Question 12

When does inhibition of HCl secretion occur?

Answer 12

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

Question 13

Pepsinogen secretion

Answer 13

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

Question 14

Intrinsic factor secretion

Answer 14

• 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

Question 15

Pancreatic secretion

Answer 15

Composed of enzymes and high HCO3- amounts
This bicarbonate neutralises the stomach H+
Enzymes digest carbs, proteins and lipids

Question 16

Exocrine pancreas

Answer 16

Parasymp from vagus stimulates secretion
Symp inhibits secretion

Pancreatic enzymes stored in condensed zymogen granules until release

Question 17

Formation of pancreatic secretion

Answer 17

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

Question 18

Pancreatic enzymes

Answer 18

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

Question 19

Activation of pancreatic proteolytic enzymes

Answer 19

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)

Question 20

Regulation of pancreatic secretion in 3 phases of digestion

Answer 20

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

Question 21

Pancreatic response in int phase - acinar cells

Answer 21

CCK secreted in presence of AA’s, small peptides and fatty acids
This tiggers acinar cell is to produce enzymes

(ACh potentiates CCK action)

Question 22

Pancreatic response in int phase - ductal cells

Answer 22

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

Question 23

Gallbladder and bile secretion

Answer 23

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

Question 24

What is bile?

Answer 24

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

Question 25

Regulation of bile secretion

Answer 25

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

Question 26

Secretions of large intestine

Answer 26

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)