The Colonic Phase of a Meal Flashcards
What are the main functions of the colon?
- Storage of waste
- Water absorption
What is the function of the ileocecal sphincter?
It prevents reflux back into the small intestine
What is the primary regulation of colon function?
Neural
What are local reflexes normally conducted by?
Enteric Nervous System
What are local reflexes?
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.
What is the gastrocolic reflex?
Distension of the stomach – increases colonic motility and mass movement of fecal material – involves 5-HT, Ach.
What is the orthocolic reflex?
Activated on rising from bed – promotes morning urge of defecation in some people.
What is the function of enteroendocrine cells?
Secrete Peptide YY (“Ileal brake”) – in response to lipid in the lumen.
What are the effects of Peptide YY?
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.
Why does Peptide YY respond to lipid?
Most of the lipids should have been digested in the small intestine and so presence of lipid means digestion needs to be slowed down.
What are haustrations?
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.
What are taeniae coli?
They are three non-overlapping bands of longitudinal muscles that produce long duration contractions
What are high amplitude propagating contractions?
They occur in healthy individuals 10 times/day from cecum to rectum and they clear the colon
How does parasympathetic innervation affect the colon?
It enhances motility
How does sympathetic innervation affect the colon?
Decreases motility
What is in the intestine but absent in the colon?
Villi
What cells are found in increased number in the colon?
Mucus producing goblet cells
What does the colon absorb that the small intestine does not?
It absorbs short-chain fatty acids - SCFA (e.g. butyrate) – salvaged from non- absorbed carbohydrates by colonic bacteria.
How are SCFAs absorbed in the colon?
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.
Why does the colon absorb these SCFAs but the intestine does not?
Bacteria in the colon can digest them, but we lack the mechanisms to do so
What is the channel that absorbs sodium in the colon?
ENaC (epithelial sodium channel)
What is the importance of the ENaC?
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.
What is the importance of the enteric bacterial ecosystem?
They work on both endogenous and exogenous substrates – release substances that destroy other types of bacteria.
How does defecation occur?
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.
What is one of the most common colon diseases?
Colon Cancer
What is Hirschsprung’s Disease?
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.
