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40 Cards in this Set

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What are the fcts of the upper small intestine?
Neutralization: Upper small intestine can't deal with the acidity of chyme
Osmotic equilibration: isotonicity reached in the duodenum (even though chyme= acidic)
Digestion
Absorption: only in the small intestine
What are the motor activities of the small intestine?
Effective mixing: mix chyme from stomach and alkaline solution and secretion coming into the SI
Slow propulsion: takes 2-6 hours to get from duodenum to the end of the SI
What governs intestinal contractions?
Electrical cahracteristics of smooth muscle
->Frequency: governed by BER
ERA: phase locked to BER
->Amplitude of contraction: Related to the number of spikes/burst of ERA (related to ACh release or strectch of muscle fibers)
How is the intestinal BER different from the distal stomach?
Intrinsic frequency of diffrent cells is very different
-> it declines systematically from proximal to distal intestine
Describe the f of cycles per minute in the SI.
Duodenum: max 12cycles/min
higher f of BER
Musculature of proximal SI is more excitable (need less f or less stretch to excite contractions)
-Ileum: ~8cycles/min
f of contractions dec as you go to the distal portion of the SI
Describe the mechanism regulating intestinal motility.
BER made by the muscle fibres in the SI has a declining f gradient (highest in the duodenum, lowest in the ileum)
-Gradient determined by series of pacemaker regions along the intestine, each with a slightly lower f than the previous one
-distribution of BER in time and space in the intestine created the distribution of spikes (ERA) and contractions
-.: proximal portions of the intestine exhibit more activity than the distal ones
**Maximal contractile activity in the SI cannot exceed the BER f of that gut segment**
Compare proximal and distal regions of the SI.
Proximal: more contractions that are also more vigorous. More resistance to flow upsream
-> f of BER is greater
-> Excitability of smooth muscle is greater (need less ACh or less spikes)
-> Thickness of smooth muscle is greater
Distal: fewer contractions and less vigorous
What is greater in the proximal SI?
Frequency and amplitude of contractions is greater in the proximal SI
What is the result of having more contractions that are also more vigorous proximally?
There is a slow net aboral (proximal to distal) movement
What is the most common type of contractile activity?
Segmentation: standing rings of contraction
What is segmentation?
1) Myogenic response to physilogical distension
2) ENS organizes tbe contraction over loner segments
3) ANS (parasym increases, symp decreases: amplitude of contraction) and H modulate (we don't need autonomic, but it can help)
Describe peristalsis in the intestine.
Infrequent, irregular, weak, shallow
Travel only for short distances (.: need segmentation)
Is there segmentation in the stomach?
No
What is intestinal peristalsis?
Series of local reflexes
Involves interaction of longitudinal and circular muscle (segmentation only uses circular muscle)
Maximum f cannot exceed f of BER
Integrity of ENS required for smooth peristaltic wave
Modulated by AS and Hormones
Describe the law of the intestine.
Relationship btw longitudinal and circular muscles
Radial stretch (inc diameter of lumen) -> stimulates ENS receptors -> activated ENS leads to neurally mediated contraction/relaxation of longitudinal/circular muscles
What happens to the longitudinal and circular muscles AHEAD of the bolus?
Longitudinal: contract-> make wall a telescope over the bolus
Circular: Relax -> this decreases resistance to flow
What happens to the longitudinal and circular muscles BEHIND the bolus?
Longitudinal: Relax
Circular: Contract -> narrow lumen, create P system to push contents forward
Does the colon absorb nutrients?
No
Describe colon motility.
Similar to SI, but:
Slower
More Sluggish
Irregular
Where are digestion and bsorption completed?
In the small intestine
Can anything be absorbed in the colon?
Yes: H2O and some ions, but NO NUTRIENTS
What are the fcts of the the colon?
Mixing: ptomotes absorption of water and ions
Propulsion: slow (50-60 h, from proximal to distal colon)
Storage
What governs segmentation and peristalsis?
Irregular BER
Where od mixing, propulsion and storage take place in the colon?
Mixing: transverse colon
Propulsion and storage: Descending colon
What happens after taking in a meal?
-Stomach becomes newly distended
-Gives rise to gastroileal reflex
->results in relaxation of ileal-cecum sphincter
->peristaltic activity in cecum (moves contents across ileal-cecum sphincter, from SI to colon)
-Gastro-colic reflex: elevate rectum
-> relaxes internal sphincter
->relax anal sphincter
-Evacuation of contents from colon-> relax external sphincter
-Ileocolic reflex: go to colon
What does distension of the stomach lead to?
Different reflexes
What happens in the interdigestive period?
GI motility is organized into intense *Pattern of cyclic myoelectric (motor) activity*
*a) Recurring at regular intervals (~90min)
*b) moving sequentially over distal stomach and small intestine up to distal ileum (~2-10cm/min)
What does the gastro colic reflex do?
Increases contractile activity in distal colon and push content in rectum
What does the ileocolic reflex do?
Induces increased activity in the distal colon
What does the gastroileal reflex do?
Increase activity in distal colon
What is MMC?
Migrating Myoelectric (motor) Complex
Total of 90 min
Moves contents ahead dring interdigestive period (toward the colon)
What happens in phase I of MMC?
60 min
-No spike potentials
-No contractions
->always have a BER, but nothing else in phase I
What happens in phase II of MMC?
20 min
-Irregular spike potentials
Irregular contractions
What happens in phase III of MMC?
10 min
Regular spike potential
Regular contractions
What stops MMC?
Entry of a new meal
Where is the MMC restricted to?
Distal stomach and SI
How is MMC initiated?
NOT the CNS, ANS
Maybe gut peptides
**ENS: periodic activation of pattern-generating circuitry (ENS programmed durin phase I?)
How is the MMC propagated?
Via the ENS with modulation by th ANS and gut peptides
How is the MMC interrupted?
Intake of a new meal
What are the fcts of the MMC?
-Housekeeping: cleanes distal stomach and SI during interdigestive periods and sweeps it into the colon
-Gastric emptying of large, non-digestible particles: pushed out of stomachand into the SI