Upper GI Tract Structure and Function

Question 1

Describe the anatomy of the oesophagus.

Answer 1

• Conduit between pharynx and stomach (25cm).
• Four layers: mucosa, submucosa, muscularis externa and adventitia
• Mucosa lined by stratified squamous epithelium (non-keratinised).

Submucosal mucous glands –> ducts –> lubrication.

Muscularis externa: upper 1/3 (superior) = skeletal muscle, lower 2/3 (inferior) = smooth muscle

Upper and lower oesophageal sphincters regulate movement of material into and out of oesophagus.

Question 2

Why do we chew?

Answer 2

• To prolong taste experience

- Defence against respiratory failure

Question 3

Describe both the voluntary and reflex mechanisms that controls chewing.

Answer 3

Voluntary: somatic nerves –> skeletal muscles of mouth/jaw

Reflex: contraction of jaw muscles –> pressure of food against gums, hard palate and tongue –> mechanoreceptors –> inhibition of jaw muscles –> reduced pressure –> contraction…

Question 4

Name the three paired salivary glands in the mouth.

Answer 4

• Parotid
• Submandibular
• Sublingual

Question 5

Describe the components of saliva.

Answer 5

• Water: 99%, dilutes particles. solvents
• Mucins: major protein component, + water = mucus, viscous solution - lubricant function
• Alpha amylase: catalyses breakdown of polysaccharides into disaccharides and glucose
• Electrolytes: tonicity/pH
• Lysozyme: bacteriocidal (cleaves polysaccharide component of bacterial cell wall)

Question 6

Describe the control of salivary secretion.

Answer 6

Parasympathetic;

• Cranial nerves VII (facial and IX (glossopharyngeal)
• Stimulate profuse watery salivary secretion

Sympathetic;

• Stimulate small volume, viscous salivary secretion
• High mucus content (alpha-1 adrenoreceptors)
• High amylase content (beta-2 adrenoreceptors)

Reflex control;
Presence of food in mouth –> chemoreceptors/mechanoreceptors (walls of mouth/tongue)

Question 7

Describe the phases of swallowing.

Answer 7

Oral Phase (voluntary);
- Bolus pushed to back of mouth by tongue .

Pharyngeal Phase;

• Presence of bolus –> sequence of reflex contractions of pharyngeal muscles.
• Co-ordinated by swallowing centre (medulla).
• Soft palate reflected backward and upward (closes off nasopharynx).

As bolus approaches oesophagus…
- Upper oseophageal sphincter (UOS) relaxes and epiglottis covers opening to larynx (prevents food entering trachea).

Once food has entered oesophagus…
- UOS contracts (prevents food reflux).

Oesophageal Phase;

• Propulsion of bolus to stomach.
• Peristaltic wave sweeps along entire oesophagus.
• Propelled to stomach in 10 secs.

As bolus nears stomach…
- Lower oesophageal sphincter (LOS) relaxes –> bolus enters stomach.

Question 8

Describe the receptive relaxation of the stomach.

Answer 8

• Initiated following relaxation of LOS and entry of bolus into stomach.
• Vagal reflexes –> relax Thin, elastic smooth muscle of gastric fundus and body
• Stomach volume: 50ml –> 1500ml (no change in pressure).

Question 9

Describe the functions of the stomach.

Answer 9

• Temporary store of ingested material.
• Dissolves food particles and initiate digestive process.
• Controls delivery of contents to small intestine.
• Sterilises ingested material.
• Produces intrinsic factor (Vitamin B12 absorption).

Question 10

Describe the anatomy of the stomach.

Answer 10

Oesophagus –> gastroesophageal (cardiac) opening (lower oesophageal (cardiac) sphincter) –> cardiac region –> fundus –> body (greater and lesser curvature) —> pyloric region –> pyloric opening (pyloric sphincter)

Question 11

Describe the histology of the stomach.

Answer 11

• Simple columnar epithelium with mucus-secreting cells on surface.
• Serosa: connective tissue outer layer.
• Muscularis externae: three layers: longitudinal (outer), circular (middle), oblique (inner)
• Submucosa and mucosa folded (= rugae) when empty –> stretch as stomach fills
• Lumenal surface: surface mucus cells –> gastric pits open into gastric glands, which contain mucus neck, parietal (secretes hydrochloric acid) and chief cells.