Upper GI tract structure & function

Question 1

Why do we chew?

Answer 1

Prolong taste experience

Break up food so we dont choke and have resp failure

Question 2

How is chewing controlled?

Answer 2

Both somatically and through reflexes…

Somatic nerves - control skeletal muscles of the mouth and jaw

Reflex - this works in a cycle:

• Contraction of jaw muscles –> pressure of food on gums, hard palate and tongue
• Activates mechanoreceptors which inhibit jaw muscle contraction
• Relaxation –> mouth opens causing a loss of pressure –> which causes contraction again

Question 3

What is saliva made up of?

Answer 3

• Water (99%) containing:
• Mucins
• a-Amylase (salivary amylase)
• Electrolytes
• Lysozyme

Question 4

What are mucins?

Answer 4

Major protein component of saliva

Mucin + water = mucous

Forms a viscous solution which lubes everything up

Question 5

Why is there A-amylase in saliva?

Answer 5

Catalyses breakdown of polysaccharide (starch, glycogen) into disaccharide (maltose) + glucose

Basically just starts digestion of carbs a wee bit earlier

Question 6

Why are there electrolytes in saliva?

Answer 6

Maintain the tonicity/pH of the mouth

This is important for maintaining our enamal and dentition etc

Question 7

What is lysozyme and why is it in saliva?

Answer 7

Enzyme which acts against bacteria (bacteriocidal)

Cleaves polysaccharide component of bacterial cell wall

Question 8

Where is saliva secreted from?

Answer 8

Parotid glands

submandibular glands

sublingual glands

(3 pairs of glands)

Question 9

Which salivary gland is largest?

Answer 9

Parotid gland

Its duct enters from the hard pallet of the mouth

Question 10

What is the general structure of salivary glands?

Answer 10

Gland composed of ‘alveoli’ of exocrine cells

3 types:

• Mucous alveolus
• Serous alveolus
• Mixed alveolus

Question 11

What regulates salivary secretion?

Answer 11

Controlled by both Sympathetic and parasympathetic nervous systems (both stimulatory) and also by Reflexes

ANS control (as with all things) is in response to fight/flight or rest/digest states

Question 12

How does parasympathetic stimulated work for salivary glands?

Answer 12

Cranial nerves:

• Facial nerve (VII)
• Glossopharyngeal nerve (IX)

Stimulation => profuse watery salivery secretion

(ideal for lots of chewing)

Question 13

How does sympathetic stimulation of salivary glands work?

Answer 13

Secretion of small volume, viscous salivary secretion (dry mouth). Why?

Sympathetic stimulation (via superior cervical ganglion) causes release of NA which binds to a₁ adrenoceptors and B₂ adrenoceptors

Effect:

• a₁ adrenoceptors => High mucus content
• B₂ adrenoceptors => High amylase content

Question 14

How does reflex control of salivary secretion work?

Answer 14

Presence of food in mouth stimulates chemoreceptors and pressure receptors on the walls of mouth/tongue

(i assume this causes a similar effect to parasympathetic stimulation)

Question 15

What are the four layers of the oesophagus?

Answer 15

Mucosa

Submucosa

Muscularis externa

Adventitia

Question 16

How long is the oesophagus?

Answer 16

25cm

Question 17

What type of epithelium is found on the mucosa or the oesophagus?

Answer 17

Non-keritonised Stratified squamous

Question 18

What is found in the submucosa in the oesophagus?

Answer 18

Sub-mucosal mucous glands

Their secretion reaches the oesophagus through ducts which pass through the mucosa

Question 19

What types of muscle are found in the muscularis externa in the oesophagus?

Answer 19

Upper 1/3 (superior) = skeletal muscle

Middle 1/3 = transition to smooth muscle (mixture of both)

Lower 1/3 (inferior) = smooth muscle

(Some lectures etc say the middle third is just smooth muscle)

Question 20

What happens in the oral (voluntary) stage of swallowing?

Answer 20

Tongue moves bolus (food) to back of mouth

Question 21

What happens in the pharyngeal phase of swallowing?

Answer 21

• Presence of bolus initiates sequence of reflex contractions of pharyngeal muscles
• This is co-ordinated by swallowing centre (medulla)
• The Soft palate reflects backward and upward to close off the nasopharynx

Question 22

What happens in swallowing when the bolus approaches the oesophagus?

Answer 22

• Upper oesophageal sphincter (UOS) relaxes and opens
• Epiglottis covers the opening to the larynx

Question 23

What happens once the bolus has entered the oesophagus?

Answer 23

Upper oesophageal sphincter (UOS) closes to prevent any reflux

Peristaltic waves in the oeophagus propell the bolus towards the stomach in about 10seconds

Question 24

What happens once the bolus reaches the bottom of the oesophagus?

Answer 24

Lower oesophageal sphincter (LOS) relaxes so bolus enters stomach

LOS relaxation and entry of bolus to stomach causes vagal reflex to relax the elastic smooth muscle in the fundus and body of the stomach (so it expands)

Question 25

By how much can the stomach expand when food enters it?

Answer 25

50ml => 1500ml with no change in pressure

Question 26

Do you know the anatomy of the stomach?

Answer 26

Question 27

What are the 4 layers of the stomach?

Answer 27

Mucosa

Submucosa

Muscularis externa

Serosa

Question 28

What makes up the muscularis externa in the stomach?

Answer 28

three layers:

• longitudinal (outer)
• circular (middle)
• oblique (inner)

Question 29

What feature of the mucosa and submucosa of the stomach allows it to stretch?

Answer 29

Mucosa and Smucosa are folded into Rugae

These stretch out as it fills

Question 30

What is the lumenal surface of the stomach like?

Answer 30

Contains surface mucous cells and gastric pits

(see gastric secretion)