Transport 1: Along the GI Tract

Question 1

Name 3 digestive functions of the stomach

Answer 1

1. Accommodation/storage
2. Mechanical and enzymatic breakdown
3. Slow delivery of chyme to duodenum

Question 2

Name a disorder of gastric motility/emptying

Answer 2

Gastric stasis / gastroparesis

- symptoms: nausea, bloating, vom, reflux, ab pain, loss of appetite

Question 3

Why is gastric emptying into the duodenum regulated?

Answer 3

• Coordinates mechanical/chemical breakdown with absorption
• Avoids swamping of duodenum
• Adequate neutralisation

Question 4

What is gastric emptying dependent on? (6)

Answer 4

1. Propulsive force generated by tonic contractions of prox stomach
2. Stomach’s ability to differentiate types of meals ingested and their components
3. Osmotic pressure of duodenal contents (hyperosmolar chyme decreases gastric emptying)
4. Vagal innervation upon over-distension of duodenum decreases gastric motility
5. Inhibiting hormones (somatostatin, secretin, cck, gip)
6. Injury to intestinal wall / infection -> decrease motility

Question 5

Describe gastric emptying of liquid, semi-solid and solid meals

Answer 5

1. Liquid meals - No lag phase, rapidly disperse, more nutrients = retained longer. Pass in spurts.
2. Semi-solids - Short lag phase, linear dispersion, solid part retained in prox stomach longer
3. Solids - Longer lag phase, linear dispersion, broken down to 1-2mm sizes

Question 6

Describe gastric emptying of fatty and indigestible foods

Answer 6

1. Fatty foods - Liquefy at body temp, float on top of liquid layer and empty slowly. Potent inhibitors of gastric motor events and gastric emptying
2. Indigestible solids - Not emptied in immediate post-prandial period, cleared by MMC or vomiting

Question 7

What is the myogenic control of gastric motility?

Answer 7

There is an intrinsic basic electrical rhythm, stomach muscle cells produce electric depolarisations from resting potential. Initiated at FUNUDS (vagal excitatory) and ripples move towards ANTRUM.

Slow waves from ICC - regular recurring migrating ripples.

Question 8

Which hormones mediate a DECREASE in funding motor activity?

Answer 8

GRP, CCK, Secretin, VIP, Somatostatin, Glucagon, Duodenal distension/acid

Question 9

Which hormone INCREASES fundic contractions?

Answer 9

Motilin

Question 10

What increases and decreases activity of intestinal motility?

Answer 10

• CCK, gastrin, serotonin, insulin & motilin INCREASE

- Secretin & glucagon DECREASES

Question 11

What negative feedback systems are in place to control gastric emptying?

Answer 11

• Antral over distension -> vago-vagal reflex

- Duodenal over distension/chem stimulation -> vago-vagal reflex + hormones

Question 12

What are the 3 phases of motor activity within the intestine?

Answer 12

Phase I: Quiet period
Phase II: Irregular propulsive contractions
Phase III: Burst of uninterrupted phasic contractions

Question 13

What is segmentation and where does it originate?

Answer 13

• Originates in the pacemaker cells - ICC
• Divides chyme, brings chyme in contact with intestinal walls - MIXING
• Causes localised, slow migration of chyme towards ileum/anus
• Duodenum/jejunum: 10-12 contractions/min
• Ileum: 8-9 contractions/min

Question 14

What is peristalsis?

Answer 14

Global movement of chyme towards large intestine; propulsive contractions! Oral -> anal. Mediated via ENS but can be enhanced or suppressed by CNS.

Ascending the bolus - circular contracts, longitudinal relaxes
Descending the bolus - circular relaxes, longitudinal contracts

Question 15

What is the MMC?

Answer 15

Occurs between meals when gut is ‘empty’ - Phase III of interest; burst of high frequency, large amplitude contractions that migrate the length of the intestine + die out.

Starts in lower portion of stomach. ‘INTESTINAL HOUSEKEEPER’

Question 16

What are the functions of MMC? (4)

Answer 16

1. Indigestible residues eliminated from stomach
2. Removes dead epithelial cells by abrasion
3. Prevents bacterial overgrowth
4. Prevents colonic bacteria from entering SI

Question 17

Describe the control of MMC

Answer 17

Smooth muscle cells of stomach produce slow waves, contractions coordinated by ENS/ICC. Initiated by VAGUS in upper tract. Some evidence suggests MOTILIN secreted from stomach/duo and feeding inhibits release of motilin.

Question 18

Describe the motility of the large intestine (2)

Answer 18

• Segmental/haustral contractions mix contents - key role for taenia coli long muscle.
• Peristalsis: slow compared to SI, distension initiates contraction
• Mass movement

Question 19

What is mass movement?

Answer 19

Powerful contraction of mid-transverse colon that sweeps contents into rectum (responsible for colonic evacuation)