The Colonic Phase of a Meal

Question 1

What are the main functions of the colon?

Answer 1

• Storage of waste

- Water absorption

Question 2

What is the function of the ileocecal sphincter?

Answer 2

It prevents reflux back into the small intestine

Question 3

What is the primary regulation of colon function?

Answer 3

Neural

Question 4

What are local reflexes normally conducted by?

Answer 4

Enteric Nervous System

Question 5

What are local reflexes?

Answer 5

Activated by the passage of a bolus of fecal material – stimulates short bursts of Cl- and fluid secretion – involves 5-hydroxytryptamine (5-HT) and Ach.

Question 6

What is the gastrocolic reflex?

Answer 6

Distension of the stomach – increases colonic motility and mass movement of fecal material – involves 5-HT, Ach.

Question 7

What is the orthocolic reflex?

Answer 7

Activated on rising from bed – promotes morning urge of defecation in some people.

Question 8

What is the function of enteroendocrine cells?

Answer 8

Secrete Peptide YY (“Ileal brake”) – in response to lipid in the lumen.

Question 9

What are the effects of Peptide YY?

Answer 9

It decreases gastric emptying and overall intestinal motility. It also reduces Cl- and thus fluid secretion by intestinal cells. By reducing fluidity of intestinal contents and inhibiting intestinal motility –peptide YY reduces propulsion of chyme.

Question 10

Why does Peptide YY respond to lipid?

Answer 10

Most of the lipids should have been digested in the small intestine and so presence of lipid means digestion needs to be slowed down.

Question 11

What are haustrations?

Answer 11

They are segmentation contractions that mix the contents. Short duration contractions (8 sec) of circular muscles at intervals which divides the colon into segments or haustra.

Question 12

What are taeniae coli?

Answer 12

They are three non-overlapping bands of longitudinal muscles that produce long duration contractions

Question 13

What are high amplitude propagating contractions?

Answer 13

They occur in healthy individuals 10 times/day from cecum to rectum and they clear the colon

Question 14

How does parasympathetic innervation affect the colon?

Answer 14

It enhances motility

Question 15

How does sympathetic innervation affect the colon?

Answer 15

Decreases motility

Question 16

What is in the intestine but absent in the colon?

Answer 16

Villi

Question 17

What cells are found in increased number in the colon?

Answer 17

Mucus producing goblet cells

Question 18

What does the colon absorb that the small intestine does not?

Answer 18

It absorbs short-chain fatty acids - SCFA (e.g. butyrate) – salvaged from non- absorbed carbohydrates by colonic bacteria.

Question 19

How are SCFAs absorbed in the colon?

Answer 19

SCFAs are absorbed in the luminal side in a Na+ - dependent fashion by symporters - sodium- monocarboxylate transporters (SMCTs).

This is driven by low intracellular Na+ established by basolateral Na+/K+ ATPase.

Question 20

Why does the colon absorb these SCFAs but the intestine does not?

Answer 20

Bacteria in the colon can digest them, but we lack the mechanisms to do so

Question 21

What is the channel that absorbs sodium in the colon?

Answer 21

ENaC (epithelial sodium channel)

Question 22

What is the importance of the ENaC?

Answer 22

It is the last mechanism to prevent the loss of water and this is not seen anywhere else in the GI tract. Water and Cl- ions flow passively via tight junctions as they follow the Na.

Question 23

What is the importance of the enteric bacterial ecosystem?

Answer 23

They work on both endogenous and exogenous substrates – release substances that destroy other types of bacteria.

Question 24

How does defecation occur?

Answer 24

High-amplitude propagating contractions produces mass movement of feces – rectum fills with fecal material. Filling of rectum relaxes internal anal sphincter – releases VIP, generates NO. However, defecation does-not occur since external anal sphincter is still tonically contracted.

Defecation occurs with the voluntary relaxation of external anal sphincter.

Question 25

What is one of the most common colon diseases?

Answer 25

Colon Cancer

Question 26

What is Hirschsprung’s Disease?

Answer 26

Condition where a segment of the colon is permanently contracted –causing obstruction. This is due to the failure of enteric nervous system to develop properly – the affected area completely lacks the plexuses of enteric nervous system and associated ganglia.