PHYS Gut Secretions I - Week 4

Question 1

Acidity of salivary, stomach, pancreas, bile, SI & LI secretions.

Answer 1

7-8. 1-3.5, 8, 8, 8.

Question 2

Saliva composition

Answer 2

• H2O (97-99.5%)
• Na, K, Ca, Mg, PO4, HCO3, Cl
• Salivary amylase
• Mucin
• Immunoglobulins

Question 3

Functions of saliva

Answer 3

• Lubrication
• Digestion
• Protection
• Control of H2O intake
• Speech
• Absorption (e.g., of nitrates in certain medications)
• Taste sensation

Question 4

Salivary glands classifications & locations.

Answer 4

Saliva is secreted from salivary/buccal glands @ mucosa (epithelial lining of mouth) = intrinsic salivary glands.
Saliva is also secreted from the sublingual/parotid & submandibular glands @ mucosa = extrinsic salivary glands.

Question 5

Salivation process

Answer 5

Stimulus -> Olfactory/gustatory/visual/auditory cortex (higher centres) -> excite salivatory nucleus of the medulla -> activation of salivary glands -> salivation.

Question 6

Factors which inhibit salivation include:

Answer 6

Sleeping, stress

Question 7

Factors which increase salivation include:

Answer 7

Nausea, increased acidity of the oral cavity, hunger/approach of food.

Question 8

What is xerostomia?

Answer 8

Dry mouth.

Question 9

Xerostomia treatment

Answer 9

M receptor agonist.

Question 10

What is ptyalism?

Answer 10

Excessive saliva production.

Question 11

Ptyalism treatment.

Answer 11

M receptor antagonist.

Question 12

What is deglutition?

Answer 12

Process of bolus passing from mouth to stomach.

Question 13

3 phases of swallowing

Answer 13

1. Buccal phase (voluntary ) – skeletal muscles of tongue contract -> push bolus to top of pharynx -> bolus activates touch-sensitive/mechanoreceptors @ pharynx wall.
2. Pharyngeal phase (involuntary) – uvula rises & closes off nasal passages and epiglottis lowers & closes off trachea -> upper oesophagus relaxes -> oesophageal peristalsis.
3. Oesophageal phase (involuntary) – gastroesophageal/lower oesophageal sphincter relaxes -> bolus enters stomach.

Question 14

What part of the brain controls the buccal phase?

Answer 14

Cerebral cortex.

Question 15

What parts of the brain/nerves control the pharyngeal & oesophageal phases?

Answer 15

Swallowing centre @ medulla -> efferent outputs (via vagal nerve)
Both parasympathetic nerves (innervating the smooth muscle @ lower oesophagus & gastroesophageal sphincters) and somatic motor nerves (innervating the skeletal muscle @ pharynx, upper oesophageal sphincter) travel via vagus nerve.

Question 16

Common causes of dysphasia

Answer 16

• Diseases of the mouth, tongue, salivary glands
• Neuromuscular disorders
• Outpouchings on pharyngeal/oesophageal wall.

Question 17

Achalasia

Answer 17

Specific type of dysphasia, in which the lower oesophageal sphincter does not relax fully.

Question 18

Achalasia treatments

Answer 18

Balloon inflation of lower sphincter or botox injection.

Question 19

Where are gastric glands located?

Answer 19

Under gastric pits.

Question 20

What do gastric glands include:

Answer 20

• Mucous cells – secrete mucous to lubricate food
• Parietal cells – secrete HCl & intrinsic factor
• Chief cells – secrete pepsinogen (inactive form of pepsin)
• Enteroendocrine cells – release hormones
• ECL – secrete histamine
• G cells – secrete gastrin

Question 21

Importance of intrinsic factor secretion.

Answer 21

Required for uptake of B12. (B12 is required for RBC manufacturing/synthesis).

Question 22

Pepsin synthesis process.

Answer 22

pH2. Chief cell releases pepsinogen. Parietal cell releases HCl & intrinsic factor. HCl ensures pH to facilitate pepsinogen-pepsin conversion.

Question 23

How do parietal cells synthesise HCl in the gut?

Answer 23

CO2 diffuses from ECF/CO2 present in cell already is used in the reaction: CO2 + H2O  H2CO3 -> rapidly dissociated into H+ & HCO3- -> bicarbonate ions are exchanged for Cl- ions in veins draining into the stomach -> HCO3- enter veins draining into the stomach (forming alkaline tide).
H+ ions need to be actively pumped out of parietal cells via H/K ATPase pumps & Cl- ions leave via chloride channels.

Question 24

Receptors present on parietal cell membrane?

Answer 24

G receptors - respond to gastrin
H receptors - respond to histamine released by ECL cells
M receptors - respond to parasymp nerves
S receptors - respond to somatostatin

Question 25

Activation of which receptors on the parietal cell membrane leads to increased gastric acid secretions and how?

Answer 25

Activation of G, H, M receptors -> second messenger production within parietal cells -> H+ proton pump -> increased gastric acid secretions.

Question 26

Excessive gastric acid secretion can be treated via what 2 medications?

Answer 26

Anti-histamines or PPIs.

Question 27

What triggers the 3 x phases of gastric secretion?

Answer 27

Cephalic - occurs before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food; and the greater the appetite, the more intense is the stimulation.
Gastric – triggered by increased pH & stretching of the stomach.
Intestinal - duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes.