Transport along and across the GI tract

Question 1

Describe how the proximal stomach relaxes to accommodate food by reduction of ongoing cholinergic activity and activation of “n.a.n.c” inhibitory (VIP/NO) systems.

Answer 1

Gastric emptying is dependant on propulsive force generated by the tonic contractions of the proximal stomach

Question 2

Understand the meaning of “reciprocal control” in the context of the control of gastric motility

Question 3

Describe the pattern of emptying of liquids and solids from the stomach

Answer 3

Liquids pass in spurts, solids are broken down to 1-2mm sizes. Large indigestible materials are cleared by MMC or vomiting.

First there is a lag phase, where material must be grinded for emptying. Liquids have no significant lag phase and leave the stomach faster than solids. Larger particles take longer to be emptied.

Question 4

Describe the regulation of gastric emptying and particularly the involvement of neural (vagal) and hormonal inhibitory feedback from the small intestine

Answer 4

Somatostatin, secretin, CCK, VIP, duodenal distension and duodenal acid inhibit emptying by decreasing fundic motor activity.

Motilin increases fundic contractions.

Localised distension of the duodenum decreases motility. Hormonal and nervous factors initiate and maintain peristalsis and mixing. CCK, gastrin and motilin increase intestinal motility. Secretin decreases the activity.

Antral over-distension - Involves vago-vagal reflex

Contraction of antrum allows squirting of food into duodenum. When there is acidic chyme, the pyloric sphincter contracts so no more chyme enters the canal.

Question 5

Know that meals with a high lipid content or a high osmolarity empty relatively slowly from the stomach.

Answer 5

A higher osmotic pressure of duodenal contents decreases gastric emptying. High lipid content means fattier foods that take longer to be emulsified, so emptying occurs slowly.

Question 6

Describe the difference between propulsive (peristaltic) and segmenting (mixing) contractions

Answer 6

Segmentation mixes chyme with pancreatic secretions, and the chyme move towards the ileum. In contact with intestinal walls.

Peristalsis is a type of GI motility that is characterised by rhythmic contractions of the longitudinal muscles in the GIT

Peristaltic (propulsive) contractions spread the food out allowing digestive enzymes to mix with it, but primarily push the food towards the anus (global movement)

Segmenting (mixing) contractions primarily churn the food, but also propel it towards the anus - some localisation; allows enzymes to be mixed with the chyme so that digestion can occur; also bring the chyme close to the gut mucosa

Question 7

Describe the circuit for the small intestinal peristaltic reflex

Answer 7

Presence of food particles in lumen stimulate sensory neurons, stimulating the vagal centre, vago-vagal reflex, stimulation of inter neurons within the enteric nervous system, then motoneurons to contractile patterns.

Question 8

Describe the motor functions of the colon (storage and propulsion)

Question 9

Describe the involvement of small and large intestinal motility in the regulation of transit and its relationship to intestinal absorptive mechanisms and the genesis of diarrhoea and constipation

Question 10

Describe the gross and microscopic structure of the small and large intestine relating structure to function for the villi, brush border, neuronal plexuses, lacteals, tight junctions and lateral intercellular spaces

Answer 10

Villus cells - Highly vascularised, lots of blood vessels, thin, moist, allow absorption of substances, secretion of gastric juice

Crypt cells responsible for signalling and host defence, regrowth of epithelial cells. Also contain stem cells.

Question 11

Describe the digestion of carbohydrates, proteins and fats by the exocrine secretions of the gut. In particular, be able to give examples of the effects of enzymes from the stomach, pancreas and brush border

Answer 11

Carbohydrate digestion - Only monosaccharides, complex CHO reduced to disaccharides by amylases, specific brush border enzymes convert disaccharides to monosaccharides.

Question 12

Describe how glucose is transported across the small intestine

Question 13

Describe the transport of amino acids, di- and tri-peptides across the small intestine

Question 14

Describe the transport of fats across the small intestine

Question 15

Define steatorrhoea