53 Colon Flashcards
How much nutrient absorption does the colon do?
Water absorption?
Small amount that was missed by small intestine
- Water reabsorption is larger role
The colon contains a unique biological ecosystem of _____________________.
Commensal bacteria
*How do the 4 layers of the GI tract differ in the colon?
In the 3rd (muscularis externa) layer, instead of the normal outer longitudinal muscle, contains 3 distinct bundles of muscle fiber known as tenia coli (“ribbons of the colon”)
The series of sacs in the colon (partially formed by the tenia coli) are known as _____________.
Haustra
Does the electrical activity of the small intestine propagate through the ileocecal valve into the colon?
No
In general, how is the ileocecal sphincter reflex mediated?
- Mostly neurally (not exclusive). Local reflexes + (2) long reflex arcs
- Sphincter is nlly closed
What NTs are used in local reflexes affecting the ileocecal sphincter?
What’s another stimulus for the reflex?
5-HT, ACh
- Passage of fecal bolus
What are the names of the 2 long reflex arcs?
- Gastrocolic reflex
2. Orthocolic reflex
Explain the gastrocolic reflex.
What NTs are involved?
Distension of the stomach causes an increase in colonic motility and mass movement of fecal matter
- Involves 5-HT, ACh
Explain the orthocolic reflex.
Activated on rising from bed – promotes morning urge of defecation in some people.
*What biological mechanism do we have for when the ileum is sending too much lipid into the colon?
L cells secrete peptide YY (“ileal break”)
- Released in response to lipids in lumen
- Decreases gastric emptying and overall intestinal motility
- 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. This allows more time for digestion and absorption in the small intestine
What are the names of the 2 distinct forms of colonic motility?
- Short duration
2. Long duration
How long are short duration colonic contractions?
Where do they originate?
What do they do?
- 8 sec
- Circular m.
- Divide colon into haustra - mix/circulate contents
How long are long duration colonic contractions?
Where do they originate?
What do they do?
- 20-60 sec
- Teniae coli
- Contents are moved back and forward between haustra, maximizing their contact time with epithelium.
What’s a normal, special type of long duration contractions (even longer) that occur in the colon?
How many per day?
What do they do?
High-amplitude propagating contractions
- Occurs 10 times/day from cecum to rectum
- Clears the colon
*What part of the colon does the vagus n. innervate?
Pelvic n.?
- Vagus: Cecum, ascending, transverse colon
- Pelvic: Descending, sigmoid colon, rectum, anus
Are the vagus and pelvic nn. parasymp or symp?
Parasymp
Name the 4 sympathetic plexi that control the colon.
- Superior mesenteric plexus
- Inferior mesenteric plexus
- Superior hypogastric plexus
- Inferior hypogastric plexus
What areas does the superior mesenteric plexus innervate?
What about the other 3 symp plexi?
- Sup mesenteric: Cecum, ascending and transverse colons
- Inf mesenteric, sup/inf hypogastric: Descending + sigmoid colons
*What is the special type of lipid that is absorbed in the colonic epithelium?
Short-chain fatty acids -SCFA (e.g. butyrate)
Salvaged from non-absorbed carbohydrates by colonic bacteria
How are SCFAs absorbed thru the apical colonic epithelial cell membrane?
Sodium-monocarboxylate transporters (SMCTs)
- A symporter (w/Na+)
How is Na+ absorbed into the apical colinic epithelial cell membranes in the distal colon?
What triggers this channel to open (generally)
ENaC (a sodium channel)
- Triggered to open hormonally and/or neurally
What is the function of reabsorbing Na+ in the distal colon (and Cl- following?)
Prevents excessive water loss in stool
What symptom would you expect to see w/reduced ENaC expression, e.g. in bowel inflammation?
Diarrhea (osmotic)
