PHGY 210: Digestion 5

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

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

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)

Intrinsic frequency of diffrent cells is very different
-> it declines systematically from proximal to distal intestine

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

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**

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

Frequency and amplitude of contractions is greater in the proximal SI

There is a slow net aboral (proximal to distal) movement

Segmentation: standing rings of contraction

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)

Infrequent, irregular, weak, shallow
Travel only for short distances (.: need segmentation)

No

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

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

Longitudinal: contract-> make wall a telescope over the bolus
Circular: Relax -> this decreases resistance to flow

Longitudinal: Relax
Circular: Contract -> narrow lumen, create P system to push contents forward

No

Similar to SI, but:
Slower
More Sluggish
Irregular

In the small intestine

Yes: H2O and some ions, but NO NUTRIENTS

Mixing: ptomotes absorption of water and ions
Propulsion: slow (50-60 h, from proximal to distal colon)
Storage

Irregular BER

Mixing: transverse colon
Propulsion and storage: Descending colon

-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

Different reflexes

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)

Increases contractile activity in distal colon and push content in rectum

Induces increased activity in the distal colon

Increase activity in distal colon

Migrating Myoelectric (motor) Complex
Total of 90 min
Moves contents ahead dring interdigestive period (toward the colon)

60 min
-No spike potentials
-No contractions
->always have a BER, but nothing else in phase I

20 min
-Irregular spike potentials
Irregular contractions

10 min
Regular spike potential
Regular contractions

Entry of a new meal

Distal stomach and SI

NOT the CNS, ANS
Maybe gut peptides
**ENS: periodic activation of pattern-generating circuitry (ENS programmed durin phase I?)

Via the ENS with modulation by th ANS and gut peptides

Intake of a new meal

-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