VBMS 324 GI large intestine

entry into colon dogs,cats and ruminants

digesta enters colon via the ileocolic sphincter/valve

entry into colon horses, pigs, elephants, rats

ileum empties into cecum through ileocecal sphincter/valve

sphincter relaxation

caused by

• distention of the ileum

• gastric release

sphincter closure

triggered by filling of the cecum or large intestine which prevents backflow

pacemaker: proximal colon

initiates aboral mass movements

pacemaker: mid-colon

controls bidirectional mixing and peristaltic contractions

reverse peristalsis

slow wave activity moves contents towards cecum for enhanced microbial digestion (driven by mid-colon pacemaker)

migrating spike burst (MSB)

• propels large vol towards the rectum for defecation

• prevents buildup of fecal material

• coordinates with rectal filling to trigger defecation reflex

internal anal sphincter

non-voluntary, smooth muscle

external anal sphincter

striated muscle, voluntary control

defecation reflex: phase 1

peristalsis and rectal filing

• fecal material is moved into the rectum by peristaltic waves. waves modified by sacral parasympathetic efferent fibers

defecation reflex: phase 2

sensory Afferent detection

• sensory afferent fibers detect contact of feces with the internal anal sphincter

defecation reflex: phase 3

internal anal sphincter relaxation

• sacral parasympathetic fibers cause relaxation of the internal anal sphincter, allowing feces to move closer to external sphincter

defecation reflex: phase 4 

external anal sphincter contact

• fecal material contacts the external anal sphincter, signalling via sensory afferent fibers to spinal cord and brain

defecation reflex: phase 5

voluntary control

• external anal sphincter under conscious control, either relaxes to permit defecation or contracts to delay defecation

mucus

HCO3 and K

goblet cells

lubrication and barrier maintenance

ENS

ACh, VIP, and PGE2 stimulate release

pH in colon

drops significantly during fermentation

role of fermentation

• microbial fermentation breakdown fibrous carbs into volatile fatty acids (VFA)

• VFA provide energy and support gut health

location of fermentation

generally in colon and cecum of non-ruminants and in rumen of ruminants

substrates for fermentation

• structural carbs: cellulose 

• non-structural carbs: starches, sugars that escape digestion or overwhelm the system and escape digestion

anaerobic gut environment

oxygen free environment in the gut, especially hindgut

key products of fermentation

• VFA: acetate, propionate, butyrate, lactate

• Gases: CO2, H2, CH4

acetate (C2)

• gut microbes pathway: electrons go to H2 and CH4

• used by animal in: energy and lipid synthesis

propionate (C3)

• gut microbes pathway: internally balanced electrons

• used by animal: precursor for gluconeogenesis

butyrate C4

• gut microbes pathway: electrons reduce unsaturated long-chain fatty acids

• used by animal: energy for colonocytes and gut barrier maintenance

CO2 production

• pyruvate decarboxylation: pyruvate to Acetyl-coA + CO2

• decarboxylation in succinate to propionate conversion

H3 production

microbial reduction reactions: NADH+H to NAD +H2

methane formation

• methanogens use CO2 and H2 to produce CH4

• farts

celluloytic bacteria

ferment fiber (cellulose) major products are acetate and butyrate

amylolytic bacteria

ferment starches and sugars, major products are propionate and lactate

VFA are absorbed by…

colonocytes via SCFA/HCO3 exchangers and monocarboxylate transporters

VFA metabolism

• delivered to the liver via portal vein

• acetate turned to acetyl coA

• propionate to glucose

• butyrate used by colonocytes

absorption in colon

1. water absorbed through tight junctions

2. electrogenic and electroneutral; ENac drives absorption

3. SCFA enhanced absorption

4. water moves via aquaporins

what ensure colon hydration?

amylase-resistant starch containing ORS