Theme 1: Lecture 5 - Salivary and gastric secretion Flashcards
What do secretions do
Lubricate, protect and aid digestion
Name 2 types of exocrine glands
- Salivary glands
- Gastric glands
Name a type of endocrine gland
enteroendocrine cells in the stomach and small intestine
What is the role of salivary secretions
- Lubrication
- Protection-oral hygiene
- Initiate chemical digestion
Name the 3 major salivary glands
- Parotid
- Submandibular
- Sublingual
Name 4 dispersed (minor) salivary glands
-Labial
-Buccal
-Palatal
-Lingual
(mucosa of the mouth and tongue)
Describe the parotid salivary gland
- Serous, watery secretions containing salivary amylase for starch digestion
- Sits anterior and inferior to the ear
Describe the submandibular salivary gland
- Mixed serous and mucus
- Sits inferior to the mandible
Describe the sublingual salivary gland
- Mucus: thicker mucus dominant secretions for lubrication
- Sits in the floor of the oral cavity
Describe the composition of saliva
- Water (99.5%)
- Electrolytes
- Enzymes
- Secretory IgA
- Mucin
- Organics urea and uric acid
Name the electrolytes in saliva
K+, HCO3-, Na+, Cl-, (PO4)3-
Name 5 enzymes found in saliva
- a-amylase (ptyalin)
- Lysozyme
- Lingual lipase (serous salivary glands of tongue)
- Lactoferrin
- Kallikrein
What is the function of water in saliva
Solvent dissolves food components to aid taste, swallowing, initiation of digestion, oral hygiene
What is the function of electrolytes in saliva
Buffer for acidic food contents
What is the function of a-amylase (ptyalin) in saliva
Hydrolysis of a-1,4 glycosidic bonds in starch to disaccharide maltose, trisaccharide maltotriose and a-dextrin, ~75%
What is the function of lysosome in saliva
Hydrolysis of peptidoglycans in wall of gram negative bacteria
What is the function of lingual lipase in saliva
Hydrolysis of lipid triglycerides to fatty acid and diglycerides (optimal in acidic pH)
What is the function of lactoferrin in saliva
Chelates iron to prevent microbial multiplication
What is the function of kallikrein in saliva
converts plasma protein a-2- globulin into bradykinin (vasoconstrictor)
What is the function of secretory IgA in saliva
prevents microbial attachment to epithelium
What is the function of mucin in saliva
lubrication
What is the function of organics urea and uric acid
Waste product removal for excretion
What are cells lining the acinar structure of the salivary glands
- Aciner cells
- Myoepithelial cells
- Ductal cells
What is the role of the acinar of salivary glands
- Functional unit of the salivary gland
- A small saclike cavity in a gland
Describe the properties of the salivary gland
- Large volume of saliva produced compared to mass of gland
- Low osmolarity
- High K+ concentration
Describe the formation of hypotonic saliva
Stage 1
-Acinar cells secrete isotonic saliva similar to blood plasma in electrolyte composition
Stage 2
-Ductal cells secrete HCO3- and K+ ions with reabsorption of NaCl and limited movement of water by osmosis
-produces HCO3- and K+ rich hypotonic saliva
Describe the electrolyte composition of saliva in relation to plasma
Na+ and Cl- < plasma
HCO3- and K+ > plasma
Does the composition of saliva change with flow rate
Yes
Describe the composition of saliva at a low rate of secretion
maximum reabsorption of electrolytes produces hypotonic saliva (lower concentration of osmotically active electrolytes)
Describe the composition of saliva at a high rate of secretion
reduced reabsorption of electrolytes produces alkaline, HCO3- rich saliva with increased osmolality closer to that of primary isotonic saliva
What is the dominant regulator of salivary secretion
Parasympathetic ANS
What is the stimulus for salivary secretion
Sight, thought, smell, taste (esp. sour acidic taste), tactile stimuli, nausea
What does the parasympathetic ANS do in terms of salivary secretion
- Increase salivary secretion
- vasodilation
- myoepithelial cell contraction
What are the inhibitors for salivary secretion
- Fatigue
- Sleep
- Fear
- Dehydration
What is the parasympathetic innervation for the sublingual gland
-Cranial nerve VII (facial nerve)
What is the parasympathetic innervation for the submandibular gland
-Cranial nerve VII (facial nerve)
What is the parasympathetic innervation for the parotid gland
-Cranial nerve IX (glossopharyngeal nerve)
Where do parasympathetic signals go to in the CNS in terms of salivary secretion
superior and inferiorsalivatorynuclei in the medulla
What else regulates saliva secretion apart from the parasympathetic ANS
Sympathetic neural stimulation but to a lesser extent than the parasympathetic NS
Describe the effect that the sympathetic nervous system has on saliva production
- Overall slight increase in secretion
- Produces a mucin and enzyme rich saliva
- Initial vasoconstriction (neurotransmitter noradrenaline stimulates b-adrenergic receptors)
- Later vasodilation (salivary enzyme kallikrein action on blood plasma protein alpha-2 globulin to form vasodilator bradykinin)
What is the sympathetic innervation of salivary glands
Via superior cervical ganglion
Name 2 diseases caused by salivary gland dysfunction
- Sjogren’s syndrome
- Xerostomia (dry mouth)
Describe Sjogren’s syndrome
- an autoimmune disease that destroys the exocrine glands
- commonly affects tear and saliva production
- dry eyes and dry mouth, known as sicca symptoms
Describe xerostomia (dry mouth)
- Patients lack adequate saliva
- dental caries (tooth decay) and halitosis common due to bacterial overgrowth
- difficulty speaking or swallowing solid food due to inadequate lubrication
Describe how gastric glands are formed
Gastric pits in the stomach mucosa branch into gastric glands
Name the exocrine cells in gastric glands
- Mucus neck cells
- Parietal cells
- Chief cells
Name the endocrine cells in gastric glands
- G cells
- D cells
- Enterochromaffin-like (ECL) cells
What do mucus neck cells secrete
thin mucus
What do parietal cells secrete
HCl and intrinsic factor
What do chief cells secrete
pepsinogen (also renin in neonates) and gastric lipase
What do G cells secrete
hormone gastrin (in the antrum)
What do D cells secrete
hormone somatostatin
What do enterochromaffin-like (ECL) cells secrete
histamine
What are the 2 major types of gastric gland
- Gastric or oxyntic glands
- Pyloric glands
Where are gastric or oxyntic glands located
In body and fundus of the stomach
Where are the pyloric glands located
In the antrum of the stomach
Describe gastric or oxyntic gland secretion
- Exocrine secretion of HCl, pepsinogen, intrinsic factor and mucus
- paracrine ECL cell secretion of histamine, paracrine D cell secretion of somatostatin
Describe pyloric gland secretion
- mucus and endocrine hormone gastrin
- paracrine/endocrine somatostatin
Describe the contents of gastric juice
- Water and electrolytes
- Mucus (glycoprotein mucin) from mucus neck cells
- Pepsinogen pro-enzyme, renin in neonates only and gastric lipase from chief cells
- HCl and Intrinsic factor from parietal cells
Function of water and electrolytes in gastric juice
medium for action of acid and enzymes, digestion of organic substances
Function of mucus (glycoprotein mucin) in gastric juice
protects surface epithelium from acid/pepsin
Function of pepsinogen pro-enzyme in gastric juice
Active pepsin form is an endopeptidase that cleaves peptide bonds (protein to smaller peptides)
Function of renin in gastrin juice in neonates
Coagulation of milk through casein proteolysis
Function of gastric lipase in gastric juice
triglycerides to fatty acid and diglycerides
Function of HCl in gastric juice
Converts pro-enzyme pepsinogen to pepsin, denatures proteins, kills microorganisms
Function of intrinsic factor in gastric juice
vitamin B12 absorption in the ileum, erythropoiesis in bone marrow, deficiency results in pernicious anaemia
Describe parietal cells
- Parietal cells have an intracellular branched canalicular structure and are packed with tubulovesicles in resting state
- These contain enzymes carbonic anhydrase and H+/K+-ATPase for acid secretion
- On stimulation of acid production tubulovesicles fuse with the canalicular membrane to form microvilli
- HCl is formed at these microvilli and secreted
Describe the secretion of HCl in gastric parietal cells
- H+/K+-ATPase proton pump drives active secretion of H+ into lumen of stomach
- Carbonic anhydrase (CA) catalyses formation of HCO3- producing H+ ions
- HCO3- exchanged for Cl- (alkaline tide - gastric venous blood becomes alkaline postprandially)
- Cl- diffuses into lumen of stomach
What stimulates gastric acid secretion
- ACh: acetylcholine release from vagus
- Gastrin from G cells
- `Histamine from ECL cells
What inhibits gastric acid secretion
- Somatostatin from D cells (paracrine and endocrine): inhibits adenylate cyclase (AC)
- Mucosal prostaglandin antagonists for H receptor (NSAIDs inhibit prostaglandin formation and increase gastric acid secretion)
Describe the 2 biochemical pathways that cause H+ to be released from parietal cells
- Gastrin and ACh activate phospholipase C which activates inositol triphosphate (IP3) which activates Ca2+ stimulating H+
- Histamine binds to the H2 receptor which activates adenylate cyclase which activates cAMP stimulating release of H+
Name 3 drugs that cause inhibition of gastric acid
- Omeprazole
- Cimetidine
- Atropine
How does omeprazole inhibit gastric acid
Proton pump inhibitor inactivates H+/K+ -ATPase
How does cimetidine inhibit gastric acid
H2 receptor antagonist inhibits stimulus for acid secretion
How does atropine inhibit gastric acid
inhibits muscarinic receptors and vagal stimulation of acid secretion
What stimulates the secretion of the hormone gastrin
Vagus, distention, peptide
What does the hormone gastrin do
- parietal cell secretion of HCl
- Chief cell secretion of pepsinogen
- lower oesophageal sphincter contraction
- Increased motility of stomach
- Relaxation of pyloric sphincter
How is the secretion of gastrin from G cells inhibited
- H+ stimulates D cells to secrete somatostatin
- Somatostatin inhibits secretion of gastrin from G cells
Describe a type of gastric parietal cell dysfunction
- Autoimmune atrophic gastritis is an antibody mediated destruction of gastric parietal cells, which causes hypochlorhydria (insufficient acid secretion), and a deficiency of intrinsic factor IF
- The loss of IF results in vitamin B12 mal-absorption and pernicious anaemia
What are the 3 phases of gastric secretion
- Cephalic
- Gastric
- Intestinal
Describe the cephalic phase of gastric secretion
Vagus stimulates parietal, chief cell production of gastric juice and hormone gastrin secretion
Describe the gastric phase of gastric secretion
- Local nervous secretory reflexes, Vagal reflexes and gastrin-histamine stimulation
- stimulate parietal, chief, mucus secretion, antral G cells (gastrin stimulates parietal cells directly and indirectly via ECL histamine release)
Describe the intestinal phase of gastric secretion
- Excitatory: Chyme with pH >3, peptides stimulates gastric secretions via vagus and gastrin
- Inhibitory: Chyme with pH< 2, distention, protein breakdown products, hypo/hyper-osmotic products inhibit gastric secretions via cholecystokinin, secretin, gastric inhibitory polypeptide
Why is the gastric mucosa not damaged
- Surface mucous glands secrete viscous mucus layer of mucopolysaccharides /proteins
- Mucus viscosity generates mucosal barrier (Mucin has basic side chains and HCO3- secreted from surface epithelial cells. Both neutralise H+ ions)
- Tight junctions stop acid damaging underlying tissue
- Net result - unstirred layer is ~pH7, pepsinogen not activated, prevents enzymatic and chemical damage
Name a disease caused by dysfunction of the gastric mucosa
Gastritis
What is gastritis
Inflammation of the gastric mucosa
What is gastritis caused by
- Most commonly caused by an infection by the bacteria Helicobacter pylori (Primary cause of peptic ulcer disease)
- Gram negative bacteria produce urease which forms ammonia from urea, Ammonia neutralizes bactericidal acid and is toxic to mucosal barrier
- Also caused by smoking, alcohol, nonsteroidal anti-inflammatory drugs (NSAIDs) (inhibit cyclooxygenase to reduce protective prostaglandin synthesis), chronic stress
What happens after acute damage of the gastric mucosa
rapid regeneration is via a process called restitution - rapid division of stem cells located in the neck of gastric glands
