Salivation And Swallowing

Question 1

What processes happen in the mouth?

Answer 1

Salivation, mastication, bolus formation and swallowing.

Question 2

How does mastication take place?

Answer 2

The incisors cut food; the molars crush it. Mixed with saliva.

Question 3

Which muscle is heavily involved in mastication? What is it innervated by?

Answer 3

The masseter - trigeminal nerve.

Question 4

Why is production of saliva important?

Answer 4

Lubricates and moistens food, initiates carbohydrate digestion and it protects the oral environment. 1.5L is produced each day.

Question 5

How does saliva protect the oral environment?

Answer 5

Keeps mucosa moist, Cleans teeth, Slightly alkaline, neutralising acid produced by bacteria. Has a high [Ca2+].

Question 6

What is the clinical condition where no saliva is produced? What is its clinical significance?

Answer 6

Xerostomia. Teeth and mucosa will degrade very quickly but one can still eat moistened food.

Question 7

What is saliva composed of?

Answer 7

Water - more H2O than plasma

Electrolytes - less Na+/Cl-, more Ca2+/K+/I-

Alkali - more HCO3-.

Bacteriostats - e.g. I-

Mucus - mixture of mucopolysaccharides

Enzymes - salivary amylase

Question 8

How many salivary glands are there?

Answer 8

Three paired, ducted exocrine glands:
Parotid, Sub-maxillary, Sub-lingual

Question 9

What are the two prominent cells in exocrine glands?

Answer 9

Acinar cells and ductal cells.

Question 10

What do exocrine glands consist of?

Answer 10

The acini (lined with acinar cells) and a connected system of ducts (lined with duct cells) which lead to a single outlet.

Question 11

What is an acini?

Answer 11

In an exocrine gland, the acinar is the berry-shaped termination where the secretion is produced. They are also found in alveoli.

Question 12

What do each of the salivary glands produce?

Answer 12

Parotid - serous; watery - rich in enzymes Sub-lingual - mucous; viscous - lots of mucus Sub-maxillary - mixed; mixture of serous and mucus acini leading to common ducts

Question 13

What proportions of volume are secreted from each salivary gland?

Answer 13

Parotid (serous) - 25% Sub-lingual (mucous) - 5% Sub-maxillary (mixed) - 70%

Question 14

Saliva is hypotonic. What does this mean?

Answer 14

It is of a lower osmotic pressure than the extra-cellular fluid. This will mean it is less likely to take in water - it has a higher water content than ECF.

Question 15

How is hypotonic saliva manufactured?

Answer 15

Must start with ECF and remove solute to make it more dilute. This is because there is no cellular mechanism to actively secrete water.

Question 16

What is the composition of the fluid that the acinar cells secrete?

Answer 16

Isotonic to the ECF with enzymes present.

Question 17

Saliva then undergoes ductal modification. What happens here?

Answer 17

[Na+] and [Cl-] decrease; [HCO3-] is added.

Question 18

Why does water not follow the electrolytes out of the saliva?

Answer 18

The gaps between the duct cells are tight.

Question 19

What controls the volume of saliva?

Answer 19

Acinar secretion.

Question 20

What controls the composition of saliva?

Answer 20

Ductal modification.

Question 21

Is the rate of ductal modification limited?

Answer 21

Yes, there is a maximum rate. The more rapidly saliva is produced, the less modified it is n.b. will not apply to HCO3-

Question 22

How will ductal modification be affected by low flow rate (resting saliva)? What about a high flow rate (stimulated saliva)?

Answer 22

At a low flow rate, the duct cells have the opportunity to remove most Na+ ([Na+] = ~10mmol/L), however as the rate of ductal modification is saturable at a high flow rate the duct cells will remove the same amount (a lesser proportion) of Na+ ([Na+ = <80mmol/L]). These changes in flow will also mean [K+] changes (opposite direction to Na+, ranging from 8-20mmol/L).

Question 23

What is the mechanism of acinar secretion?

Answer 23

Cl- are secreted directly into the lumen of the duct - water and Na+ will follow.

Question 24

Visualise the ion channels on the basolateral and apical surfaces of the duct cell. What does it look like?

Answer 24

Question 25

What is the mechanism of ductal secretion?

Answer 25

Basolateral side
Na+-K+-Pump: lowers [Na+] in duct cell, increases [K+]
K+-Cl- Symporter: effluexes [K+] and [Cl-] - key for AE
NHE (Na+-H+-Exchanger) - effluxes [H+] (H+ + HCO3- <–> CO2 + H2O - catalysed by carbonic anhydrase).

Apical side:
AE (Anion exchanger) - [Cl-] must move into the duct cell, [HCO3-] otherway = alkali saliva: ideal
Passive transport of Na+ and K+

Question 26

What is salivary secretion controlled by?

Answer 26

Nervous system - Autonomic Nervous System…

Sympathetic - Superior cervical ganglion - reduces blood flow to the glands, thus less salive = dry mouth.

Parasympathetic - Glossopharyngeal Nerve (IX Cranial Nerve), Otic ganglion (Parotid); Facial Nerve (VII Cranial Nerve), Submaxillary ganglion (Sub-lingual and maxillary).

Question 27

With regards to salivary secretion, what afferent inputs affect the parasympathetic output? Where do these afferent inputs go to?

Answer 27

Taste receptors (especially acid) in the tongue and mouth. the nose (e.g. smells), conditioned reflexes (e.g. Pavlov’s dogs).

These afferent inputs will be sent to centres in the medulla.

Question 28

What receptors are the parasympathetic system mediated by? What drugs might affect saliva production?

Answer 28

Muscarinc receptors - sensitive to ACh. Acts on acinar cells to promote formation of primary secretion, duct cells to promote HCO3- secretion. Atropine-like drugs may affect these processes.

Question 29

How else can the sympathetic nervous system affect saliva?

Answer 29

Aldosterone, released from the adrenal cortex, will upregulate the Na+-K+-Pump. This will cause saliva to become even more hypotonic.

Question 30

When does the swallowing reflex take place?

Answer 30

Normally when food has reached the larynx.

Question 31

What are the phases of swallowing?

Answer 31

Voluntary phase - separation of bolus, moves into pharynx

Pharyngeal phase - pressure receptors (palate and anterior pharynx) send afferent input to brain stem swallowing centre. (Inhibits respiration, raises larynx, closes glottis, opens UOS)

Oesophageal phase - Upper 1/3 of oesophagus is voluntary muscle (rest is smooth). A rapid peristaltic wave is coordinated by extrinsic nerves in the swallowing centre, transferring food to the stomach in ~9s. … LOS opens.

Question 32

What is dysphagia? How does this differ from odynophagia?

Answer 32

Difficulty in swallowing, a brainstem function. Absent in brain death - must be checked in head injuries (risk of aspiration). Odynophagia is the sensation of pain whilst swallowing.

Question 33

What can cause dysphagia?

Answer 33

Motility problems, e.g. achalasia - failure of smooth muscle to relax. Primary (oesophageal) cause.

Obstruction or compression of oesophagus, e.g. tumours. Secondary (non-oesophageal) cause.

Question 34

Broadly, what are the two types of dysphagia? How would you investigate in each case?

Answer 34

Difficulty swallowing food - Oesophageal dysphagia
Barium swallow/endoscopy

Difficulty swallowing liquids - Oropharyngeal dysphagia
Flexible endoscopy - allows you to view the entire oesophagus and trachea.

Question 35

What is aspiration?

Answer 35

When food or drink is swallowed and enters the trachea or lungs.

Question 36

What is dysphagia of liquids commonly associated with and why?

Answer 36

Strokes - the patient may have reduced sensation and not realise they have aspirated (silent aspiration).