GI: Colonic Phase Flashcards
What are the components of the colon?
- cecum
- ascending, transverse, descending colon
- sigmoid
- rectum
- anus
What are the functions of the colon?
- water absorption
- storage of waste products
- commensal bacteria
Describe how chyme is emptied from the ileum.
- short-range peristalsis propels chyme through the ileocecal sphincter
- once the sphincter is contracted and closed again, no reflux and no electrical connection
Describe the histology of the colon.
- absorptive columnar cells
- short microvilli, no villi
- goblet cells
- crypts of lieberkuhn glands
What muscles make up the colon?
- taeniae coli = 3 longitudinal bands
- circular muscle (when contracted you get haustra)
Describe local reflexes of colonic motility.
- short bursts of Cl and fluid secretions
2. mediated by 5-HT and ACh
What are the 2 long reflex arcs of colonic motility?
- gastrocolic
- orthocolic
Describe the gastrocolic reflex arc
- distension of the stomach
- increased colonic motility and mass movements
- mediated by parasympathetics (afferent) and 5-HT, ACh (CCK, gastrin)
Define the orthocolic reflex arc.
urge to defecate when you get out of bed
Describe the effects of peptide YY.
- slows gastric emptying
- reduces intestinal motility
- reduces Cl and water secretion
- reduces gastric propulsion
What causes the secretion of peptide YY?
- presence of lipids in the colonic lumen
- secreted by enteroendocrine cells in the terminal ileum and colon
Describe haustrations and their purpose.
- short duration contractions of the circular muscle
- segmental contractions that serve to mix the chyme
- increase exposure of chyme to epithelium to optimize water and salt absorption
- creates haustra
Describe long duration contractions and their purpose.
- initiated by taeniae coli
- mild propulsion of food forward or backward
Describe high-amplitude propagating contractions and their purpose.
- occur 10x/day
- clears the colon from the cecum to the rectum
- fills the rectum, but not necessarily associated with defecation
Describe parasympathetic innervation of the colon.
- enhances motility
- vagus = cecum, ascending, transverse
- pelvic = descending, sigmoid, rectum, anal canal
Describe sympathetic innervation of the colon.
- decreases motility
- superior mesenteric = cecum, ascending transverse
- inferior mesenteric = descending, sigmoid
Describe short chain FA absorption in the colon.
- SCFA is actively transported into the cell via SMCT (sodium monocarboxylate transporters)
- SCFA is used for intestinal epithelial metabolism
What is an example of an SCFA?
butyrate
Describe Na absorption in the colon.
- neurotransmitters activate apical ENaC channels to passively allow Na to enter the cell
- absorbed into the blood via Na/K ATPase
What is a possible cause of bowel inflammation and diarrhea?
reduced ENaC expression leads to water retained in the lumen
Describe Cl absorption in the colon.
- passively absorbed via tight junctions
Describe water absorption in the colon.
- passively absorbed via tight junctions
2. colon has a high water reserve, typically absorbs 1.8L
List some functions of the commensal bacteria system.
- digestion of endogenous and exogenous substances
- protects epithelial lining from bacterial invasion
- creates flatulence
What is the effect of broad-spectrum antibiotics on the commensal bacteria?
- wipes out protective bacteria
2. makes it easy for pathogenic bacteria to colonize