Upper GI structure and function

Question 1

1. Why chew?

2. How is chewing controlled?

Answer 1

1. Prolong taste experience. Defence against respiratory failure.
2. Voluntary: somatic nerves controls skeletal muscles of mouth/jaw so can make it move/stop.
   Reflex: contraction of jaw muscles due to pressure of food against gums, hard plate & tongue activates mechanoreceptors within them which inhibits jaw muscles, the jaw slacks. When the pressure taken off (mechanoreceptors not firing anymore) the muscles contract & the jaw comes up again.

Question 2

Which pair of glands secrete saliva?

Answer 2

Parotid
Submandibular
Sublingual
These are exocrine glands.

Question 3

What is saliva made of and why?

Answer 3

1. Water: 99% of secreted fluid. Softens, moistens, dilutes particles. Solvent
2. Mucins: Major protein component. Mucins + H2O = mucus. Viscous solution - lubricant function.
3. a-amylase: Catalyses breakdown of polysaccharide (starch & glycogen) into disaccharide (maltose) + glucose.
4. Electrolytes: Tonicity/pH
5. Lysozyme: Bactericidal - cleaves polysaccharide component of bacterial cell wall.

Question 4

What controls salivary secretion, is the effect stimulatory or inhibitory?

Answer 4

Parasympathetic & sympathetic NS. Both stimulatory.

Question 5

1. Which parasympathetic nerves control salivary secretion?
2. Which sympathetic nerves control salivary secretion?
3. What triggers reflex control?

Answer 5

1. Cranial nerves VII (facial) & IX (glossopharyngeal). Stimulation profuse watery salivary secretion. Parasympathetic more important.
2. Via the superior cervical ganglion that releases noradrenaline. Stimulation leads to small volume, viscous salivary secretion. High mucus content (a1 adrenoreceptors). High amylase content (b2 adrenoreceptors).
3. Presence of food in mouth activates chemoreceptors/pressure receptors within walls of mouth/tongue.

Question 6

What 2 basic types of acinar epithelial cells exist within glands?

Answer 6

Serous cells which secrete a watery fluid (sort of devoid of mucus).
Mucous cells which produce a mucus-rich secretion.

Question 7

Oesophagus:

1. What lubricates oesophagus?
2. What kind of muscles are found in muscularis externa layer?
3. What regulates movement into & out of oesophagus?

Answer 7

1. glands found in submucosa.
2. upper 1/3 (superior) = skeletal muscle. lower 2/3 (inferior) = smooth muscle.
3. upper & lower oesophageal sphincters.

Question 8

How does swallowing occurs from mouth to oesophagus?

Answer 8

Through phases:

1. Oral phase (voluntary): Bolus pushed to back of mouth by tongue.
2. Pharyngeal phase: Presence of bolus leads to sequence of reflex contractions of pharyngeal muscles. This is co-coordinated by swallowing centre (medulla). Soft palate reflected backward & upward (closes off nasopharynx).
3. As bolus approaches oesophagus: upper oesophageal sphincter (UOS) relaxes & epiglottis covers opening to larynx (prevents food entering trachea).
4. Once food has entered oesophagus: UOS contracts (prevents food reflux).

Question 9

How does food get from oesophagus to stomach?

Answer 9

1. Oesophageal phase: Propulsion of bolus to stomach. Peristaltic wave sweeps along entire oesophagus. Propelled to stomach in 10 secs.
2. As bolus nears stomach: LOS relaxes & bolus enters stomach.
3. Receptive relaxation of stomach: Initiated following relaxation of LOS & entry of bolus into stomach. Vagal reflexes leads to relaxation of thin, elastic smooth muscle of gastric fundus & body. Stomach volume increases from 50ml to 1500ml but with no change in pressure.

Question 10

What are the functions of stomach?

Answer 10

• Temporary store of ingested material.
• dissolve food particles & initiate digestive process.
• control delivery of contents to small intestine
• sterilise ingested material
• produce intrinsic factor (needed for B12 absorption).

Question 11

Describe histological layers of stomach.

Answer 11

• Serosa = connective tissue outer layer.
• Muscularis externae = 3 layers: longitudinal (outer), circular (middle), oblique (inner).
• Submucosa & mucosa folded (= rugae) when empty, stretch as stomach fills.
• Luminal surface = surface mucus cells leads to gastric pits leads to gastric glands which contain mucus neck, parietal & chief cells.