Physiology of mastication, swallowing and GI tract motility

Question 1

What are the 4 basic digestive processes?

Answer 1

Motility, secretion, digestion and absorption

Question 2

Describe the motility of the GI tract.

Answer 2

Mechanical activity mostly involving smooth muscle (although the muscle is skeletal at the mouth, pharynx, upper oesophagus and external anal sphincter).
It consists of Propulsive movement, mixing movements and tonic contractions.

Question 3

Describe the secretion process in the GI tract.

What do digestive secretions contain?

Answer 3

Secretion occurs into the lumen of the digestive tract from the digestive tract and accessory structures in response to hormonal and neuronal signals. It is required for digestion and protection.
Digestive secretions contain water (a large volume extracted from plasma), electrolytes and organic compounds (e.g bile salts, enzymes, mucous etc).

Question 4

What is digestion?

Answer 4

The biochemical breakdown (or enzymatic hydrolysis) of complex foodstuffs into smaller, absorbable units.

Question 5

What is absorption?

Answer 5

The transfer of absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood or lymph. (products of digestion are initially absorbed into enteric cells, and then either the bloodstream or the lymph in the case of some fats).

Question 6

Describe the structure of the digestive tract wall.

Answer 6

Mucosa: Epithelial cells for absorption
Exocrine cells which secrete digestive juices
Endocrine gland cells which secrete hormones
Lamina propria
Muscularis mucosa

Submucosa: Connective tissue
Blood and lymph vessels
Submucosal plexus

Muscularis externa: Circular muscle
Myenteric plexus
Longitudinal muscle

Serosa: connective tissue

Question 7

Which reflexes mediate chewing?

Answer 7

Masseteric and diagastric

Question 8

What is the function of chewing?

Answer 8

1. It breaks down the food and mixes it with the saliva
2. Stimulates taste buds
3. Stimulation of taste buds triggers reflexes which increases salivary, gastric, pancreatic and bile secretion.

Question 9

What is the function of the palate?

Answer 9

It separates the mouth from the nasal passages and allows chewing and breathing to occur simultaneously.

Question 10

What is the function of the uvula?

Answer 10

It helps seal of nasal passages during swallowing.

Question 11

Name the 3 major pairs of salivary glands, and state what proportion of saliva comes from each.

Answer 11

Parotid- 25%
Submandibular -70%
Sublingual - 5%

Question 12

What are the functions of saliva?

Answer 12

Solvent: important for taste
Antibacterial: Contains lysozyme, lactoferrin (starves bacteria of iron) and immunoglobulins. This is important in preventing dental caries.
Lubricant: Important for swallowing and speech
Digestion of complex carbohydrates: contains amylase
Neutralisation of acid: contains bicarbonate
Facilitates sucking by infants : creates a fluid seal

Question 13

Name a condition in there is limited saliva production and the patient develops dental caries.

Answer 13

Xerostomia

Question 14

What are the two stages of saliva formation and where do they occur?

Answer 14

Primary secretion - by the acinus

Secondary modification - by duct cells

Question 15

What happens in primary secretion by the acinus?

Answer 15

Cells produce a primary secretion with Na, K, Cl and HCO3 content similar to plasma.

Question 16

What happens in secondary modification of saliva by the duct cells?
Why is there a diluting effect?

Answer 16

Cells modify the secretion by removing Na and Cl and to a lesser extent adding K and HCO3.
The duct cells are impermeable to water, and more ions are removed than are added, so there is a diluting effect.

Question 17

What compound is required for saliva production?

Answer 17

ATP

Question 18

What is the rate of saliva production when we are a) sleeping, b) awake by resting and c) actively salivating?

Answer 18

a) 0.05 ml /min
b) 0.5 ml/min
c) 5 ml/min

Question 19

Describe the composition of saliva, and how it changes with flow rate.

Answer 19

The NaCl content of saliva is much lower than that of plasma, which perhaps explains why we can detect a salty taste.
The bicarbonate content increases as flow rate increases, and decreases as flow rate decreases.
There is no glucose in saliva, which may explain why we can detect a sweet taste.

Question 20

Give an overview of the reflex regulation of saliva production.

Answer 20

This is the neuronal control of saliva production.

There is the simple, unconditioned reflex and the acquired, conditioned reflex.

Question 21

Describe the simple, unconditioned reflex which regulates saliva production.

Answer 21

chemoreceptors and pressure receptors in the mouth are activated by the presence of food or another stimulus, impulses are then sent via afferent nerves to the salivary centre in the medulla. This sends out impulses via external autonomic nerves (both sympathetic and parasympathetic) to increase the production of saliva in the salivary glands.

Question 22

Describe the acquired, conditioned reflex which regulates saliva production.

Answer 22

Thinking about, smelling or seeing an appetising dish causes the cerebral cortex to send impulses to the salivary centre in the medulla. This then sends impulses via external autonomic (both sympathetic and parasympathetic) nerves, to increase the production of saliva.

Question 23

Describe the role of the autonomic nerves in the production of saliva.

Answer 23

Parasympathetic stimulation has the dominant role in normal saliva production.
Impulses travel via glossopharyngeal and facial nerves to produce a large volume of watery, enzyme rich saliva. This is mediated by M3 muscarinic receptors.
Sympathetic stimulation is dominant at stressful times (e.g. we have a dry mouth when nervous).
Impulses travel via postganglionic nerves from superior cervical ganglia to produce a small volume of thick, mucous rich saliva. This is mediated by Beta 1 adrenoceptors.

Question 24

What is another word for swallowing?

Answer 24

Deglutition.

Question 25

What are the 2 stages of deglutition?

Answer 25

The oropharyngeal stage and the oesophageal stage.

Question 26

Describe the oropharyngeal stage of swallowing.

Answer 26

Voluntary: Food bolus is formed in the mouth by the action of teeth and the tongue.
The tongue forces the bolus into the pharynx at the rear of the mouth.
Involuntary: Pressure stimulates pharyngeal pressure receptors.
Afferent impulse sent to the swallowing centre in the medulla.
Efferent signals initiate an all or nothing reflex sequence of muscle movements.
The upper oesophageal sphincter opens.
Food passes through the pharynx into the oesophagus.

Question 27

What happens to the larynx and epiglottis during swallowing?

Answer 27

The larynx elevates and the epiglottis tilts, to prevent the food from entering the trachea. The vocal cords also close across the glottis under the influence of laryngeal muscles to prevent food from entering the trachea.

Question 28

What does the swallowing centre in the brain do to prevent food from entering the trachea?

Answer 28

It inhibits the respiratory centre in the brain.

Question 29

How long does the oropharyngeal stage of swallowing take?

Answer 29

About 1 second.

Question 30

Describe the stages in the oesophageal stage of deglutition.

Answer 30

The swallowing stage in the medulla oblongata triggers a primary peristaltic wave and closure of the upper oesophageal sphincter.
The wave is mediated by skeletal muscle in the upper oesophagus and smooth muscle in more distal regions.
In primary peristalsis, there is orderly contraction of circular smooth muscle around the food bolus and at the same time, the longitudinal muscle in front of the food bolus shortens, shortening the distance the bolus has to travel.
The lower oesophageal sphincter opens 2-3 seconds after the initiation of a swallow (and closes after the bolus has passed into the stomach to prevent reflux).
Secondary peristalsis occurs when a bit of food gets stuck. Local pressure receptors trigger the secondary peristaltic wave (mediated by the enteric nervous system) and also trigger more production of saliva which is swallowed and helps dislodge the bolus.

Question 31

How long does the oesophageal stage of deglutition take?

Answer 31

40-10 seconds

Question 32

What sort of epithelium lines the oesophagus?

Answer 32

Stratified squamous epithelium, as this is best suited to protect against mechanical attack.
It also has a rapid turnover.

Question 33

What is secreted in the oesophagus, and what is its function?

Answer 33

The secretion is entirely mucous

This lubricates for the passage of food, and protects the epithelium from attack by acid and enzymes in gastric juice.

Question 34

Why does no absorption take place in the oesophagus?

Answer 34

The transit time is too rapid to permit absorption to take place.

Question 35

Stimulation from which nerve causes the stomach to relax to accommodate food from the oesophagus?

Answer 35

The vagus nerve

Question 36

What is the capacity of the stomach?

Answer 36

50- 1000ml

Question 37

What type of compound can begin to be broken down in the stomach?
Which contents of stomach secretions can enable this to happen?

Answer 37

Protein.

HCl and pepsin.

Question 38

What is produced when the stomach mixes the food with gastric secretions?

Answer 38

Chyme

Question 39

Where is gastric juice secreted from?

How much is secreted a day?

Answer 39

It is secreted from gastric pits in the gastric mucosa.

About 2 litres a day is secreted.

Question 40

Describe the fundus.

Answer 40

It is next to the oesophagus and has a thin smooth muscle layer.
It receives food but there is little mixing in the fundus.
There is little food stored in the fundus- it is usually a pocket of gas.

Question 41

Describe the body of the stomach.

Answer 41

This is the middle section of the stomach.
It also has a thin smooth muscle layer.
There is little mixing in the body, but food is stored here.

Question 42

Describe the antrum of the stomach.

Answer 42

This is next to the duodenum. It has a thicker smooth muscle layer and is highly contractile.
There is a lot of mixing in the antrum.

Question 43

Is there any absorption through the stomach wall?

Answer 43

There is very little absorption, with the exception of highly lipophilic substances.

Question 44

Where are the pacemaker cells in the stomach located and what are they called?

Answer 44

They are located in the fundus.

They are called interstitial cells of Cajal.

Question 45

What do the pacemaker cells in the stomach do?

Answer 45

They establish a basal electrical rhythm (BER) that spreads over the stomach from the fundus to the pyloric sphincter. The BER occurs continuously (3 per minute) and may or may not generate smooth muscle contraction.

Question 46

What is required to get the basal electrical rhythm to threshold?

Answer 46

An external stimulus, such as the presence of food, which stimulates pressure receptors which sense the distension.

Question 47

How does the BER spread across the stomach?

Answer 47

The muscle cells are joined by gap junctions, which allows the electrical signal to travel from cell to cell.

Question 48

Why does all of the chyme not leave the stomach with each contraction?

Answer 48

Because the pyloric sphincter has constant tone and is semi-closed, so it does not allow all of the chyme to leave with each contraction. Some is therefore pushed back and further mixed with the gastric acid.

Question 49

What determines the escape of chyme through the pyloric sphincter?

Answer 49

The strength of the antral wave. This itself depends on several factors which are either gastric or duodenal.

Question 50

What are the gastric factors controlling stomach emptying?

Answer 50

The volume of chyme in the stomach: the rate of emptying is proportional to this, as the distension increases motility due to stretch of smooth muscle, stimulation of intrinsic nerve plexuses, increased vagus nerve activity and gastrin increase.
The consistency of chyme: emptying is facilitated by finely divided, thick liquid chyme. This affects emptying as the consistency of chyme affects how easily it can pass through the pyloric sphincter.

Question 51

What are the duodenal factors affecting gastric emptying?

Answer 51

Fat- is particularly potent and a delay in gastric emptying is required for digestion and absorption in the small intestine.
Acid - time is required for neutralisation of the acid by bicarbonate secreted from the pancreas. This is important for optimal function of pancreatic digestive enzymes.
Hypertonicity- products of carbohydrate and protein digestion are osmotically active and draw water into the small intestine. There is therefore a danger of reduced plasma volume and circulatory disturbances (known as dumping syndrome).
Distension.

Question 52

How do the duodenal factors delay gastric emptying?

Answer 52

They drive neuronal and hormonal responses.
Neuronal response: this is the enterogastric reflex. It decreases antral peristaltic activity through signals from the intrinsic nerve plexuses and the autonomic nervous system.
Hormonal response: the release of enterogastrones (e.g. secretin and cholecystokinin) from the duodenum inhibits stomach contraction.