small intestine - tuesday Flashcards
What are the two types of contractions that the GI muscle exhibits
tonic and phasic
what determines the type of contraction
the electrical properties of gut smooth muscle
what type of wave is the tonic contraction
long lasting (sphincters/antrum)
what type of wave is phasic contraction
slow waves
what can phasic contractions be depending upon ENS/paracrine/endocrine signaling events
- propulsive (peristalsis)
- mixing (segmentation)
what are the three autonomic innervations of the GI
- parasympathetic
- sympathetic
- enteric
what does parasympathetic innervation do to the GI
promotes passage of food through GI
rest and digest
parasympathetic innervation to the upper GI is via which nerve
vagus nerve
parasympathetic innervation to the lower GI is via which nerve
pelvic nerve
what does sympathetic innervation do to the GI
inhibits passage of food through GI via:
*closing of sphincters
*inhibition of enteric motor neurons
what nerves does sympathetic innervation to the GI arise from
splanchnic nerves
what is the function of the the enteric nervous systen
control GI movements, secretions, regional blood flow, fluid flow, etc.
what is the activity of the ENS modulated by
the sympathetic and parasympathetic nervous system, but the ENS is able to function without any external input
what two plexuses does the ENS house along its length
- outer plexus: myenteric
- inner plexus: submucosal
where does the myenteric plexus lie
between the inner circular and outer longitudinal layers of the muscularis externa
what does the myenteric plexus control
controls the overall muscle tone of the gut as well as its rhythmic contractions
what is peristalsis
- stimulated by distension of the gut
- inner circular layer of ME forms a contractile ring - squeezing, around the gut
- outer longitudinal layer contracts - pushing bolus caudally
- at same time, the distal gut relaxes allowing food to enter
where does the submucosal plexus lie
within the submucosal layer
what does the submucosal plexus oversee
localized secretions and absorptions of the epithelium in the lumen, regulation of blood flow, etc
what is the submucosal plexus responsible for
localized contractions of the muscularis mucosae and the villi of the gut
what are the interstitial cells of cajal the origin of
electrical slow waves: sweeping waves of partial membrane depolarization (due to calcium flux)
what do the interstitial cells of cajal serve as
pacemaker cells
what do interstitial cells of cajal form
a network surrounding smooth and longitudinal muscle
how are interstitial cells of cajal coupled to smooth muscle cells
via gap junctions
what are interstitial cells of cajal affected by
ENS
slow waves initiated by interstitial cells of cajal are necessary but…..
not sufficient to induce muscle contraction
what happens in the presence of norepinephrine to the slow wave
slow wave baseline is lowered, thus raising the threshold requisite for action potential
what happens in the presence of acetylecholine to the slow wave
the slow wave baseline is raise, enabling the crest to reach the critical point of depolarization
action potentials can trigger two major movements..
- segmentation
- peristalsis
what is the principal motor pattern
segmentation
what is segmentation/what does it do
- mix and circulates chyme
- regional gradient in frequency
what is the stimulus and what does it require
- distension
- requires a preprogrammed ENS
what is duration a function of
- nutrient content
- caloric density of the meal
what is short distances of movements
peristalsis
what is GI emptying
- exit rate from stomach much match digestion rate in SI
- liquids empty before solids
what period foes undigestible food empty
interdigestive period NOT the digestive period
coordination of stomach emptying with SI digestion occurs via the action of the ….
enterogastric reflex
ingesta that cannot be broken down into small enough particles remains in the stomach throughout the digestive period. these items are cleared during the interdigestive period by the ….
interdigestive (migrating) motility complex
not interdigestive in ruminants, where it’s related to feeding
how is the migratory motility complex characterized
by stong peristaltic contractions (~ hourly) through the distal stomach while the pyloric sphincter is relaxed
what will disrupt the activity of the interdigestive motility complex (in monogastrics)
feeding
digestive period motility
- nonpropulsive segmentation
- propulsive peristalsis over short distances
interdigestive motility
- strong peristaltic waves over long distances (migratory motility complex)
- may be involved in aboral localization of gut flora (bacterial conc. high in colon, low orally)
bicarbonate secreted by digestive glands must be ____ to maintain pH balance
recovered
K+ is primarily absorbed by ____ diffusion through a paracellular route
passive
why is all intestinal H2O absorption passive
because the absorption of osmotically active solutes
what are the 3 mechanisms of Na+ absorption
- co-transport (SI - jejunum)
- coupled sodium-chloride absorption (ileum/colon)
- diffusion
what are the 4 steps in the coupled sodium-chloride absortion mechanism of Na+ absorption
- HCO3 and H+ from H2CO3 via carbonic anhydrase go into lumen
- hydrogen exchanged for sodium
- accumulating bicarbonate exchnaged for Cl
- pumped out by Na/K- ATPase pump
what are 3 mechanisms of Cl- absorption
- coupled sodium-chloride absorption (ileum/colon)
- paracellular chloride absorption (SI)
- chloride-bicarbonate exchange
what happens during coupled sodium-chloride absorption
Cl- absorption
Cl accumulation from Cl/Na couple absorption diffuses out once favorable gradient is reached
what happens during paracellular chloride absorption
Cl- absorption
Cl slips though tight junctions to achieve electrical neutrality after Na cotransport
what happens during chloride-bicarbonate exchange
Cl- absorption
the Cl-/HCO3- exchanger (or antiporter) to neutralize acid in lumen (colon of ruminants post-fermentation)
what is the purpose of bicarbonate recovery
to maintain body acid-base balance
what are the 4 steps in bicarbonate recovery
- H+ and HCO3 are formed inside the cell from water and CO2
- H+ and Na+ are exchanged at apical membrane
- luminal H+ neuralizes HCO3
- intracellular HCO3 and Na+ are electrically balanced
what is the net effect ofr bicarbonate recovery
absorption of sodium bicarbonate NaHCO3
