Transcript Part III Intestinal and colonic motility

IV MOTILITY OF THE SMALL
INTESTINE
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Function of Intestinal Motility
(1)To mix chyme with digestive secretion
(2)To bring fresh chyme into contact with the
absorptive surface of the microvili
(3)To propel chyme toward the colon
2
Contractions of Intestinal Smooth Muscles
• Occur automatically in
response to endogenous
pacemaker activity.
• Rhythm of contractions is
paced by graded
depolarizations called
slow waves.
•Slow waves produced by interstitial cells of Cajal.
•Slow waves spread from 1 smooth muscle cell to another
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through nexuses.
Cells and Electrical Events in the
Muscularis
Insert fig. 18.16
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Contractions of Intestinal
Smooth Muscles
• When slow waves above threshold, it triggers APs by
opening of VG Ca2+ channels.
• Inward flow of Ca2+:
• Produces the upward depolarization phase.
• Stimulates contraction of smooth muscle.
• Repolarization:
• VG K+ channels open.
• Slow waves decrease in amplitude as they are
conducted.
• May stimulate contraction in proportion to the magnitude
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of depolarization.
+30
Vg K+ Channel
Vg Ca++ Channel
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Intestinal Smooth Muscle Action Potential
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Contractions of Intestinal
Smooth Muscles
• Parasympathetic NS, stretch and gastrin
increase the amplitude of slow waves.
• Stimulate APs.
• SNS decrease APs.
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Types of small intestinal movement
1. Tonic contraction: the base of the other contractions
2. Segmentation contractions
(1) def.
When a portion of the small intestine becomes distended
with chyme,
the stretch of the intestinal wall elicits a rhythmical
contraction and relaxation of localized circular muscles
spaced at intervals along the intestine,
(2) function:
mix the chyme with the digestive juice
increase its exposure to the mucosal surface
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Types of small intestinal movement
3. Peristalsis: propels the small intestinal contents towards the
large intestines
peristaltic rush：initiated by the harmful stimulation
4. MMC:
Occurs during fasting state
moves any undigested material still remaining in the
small intestine into the large intestine
prevents bacteria from remaining in the small intestine
long enough to grow and multiply excessively
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Segmentation: mix contents to promote
digestion & absorption
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Peristalsis
• Distinctive pattern of smooth muscle contractions
that propels foodstuffs distally through the
esophagus and intestines
• Mediated by….
• Local, intrinsic nervous system
• Ex: peristalsis is not affect to any significant
degree by vagotomy or sympathectomy
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Small Intestinal Motility
Peristalsis: movement along the tract
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Peristalsis
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Figure 24.4
peristalsis
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Peristalsis of the small intestine
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http://medweb.bham.ac.uk/research/toescu/Teaching/OverviewGITY2.html
Peristalsis – law of intestine
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Berne et al., 2004
Peristalsis – law of intestine
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Berne et al., 2004
Peristalsis – law of intestine
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Berne et al., 2004
Peristalsis – law of intestine
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Berne et al., 2004
Peristalsis – law of intestine
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Berne et al., 2004
Peristalsis – law of intestine
• Bolus of food →Mechanical distension and mucosal
irritation → stimulates afferent enteric neurons → 2 effects
1. Excitatory motor neurons above the bolus activated →
contraction of smooth muscle above the bolus
• Via Ach, substance P
2. Inhibitory motor neurons → stimulate relaxation of
smooth muscle below the bolus
• Via nitric oxide, vasoactive intestinal peptide and
ATP
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Control of Intestinal Motility – Neuronal
Mixing – segmentation
Frequency set by slow waves (12/minute duodunum)
additional control: myenteric plexus
Propulsion – peristalsis
Local reflex – stretch causes relaxation distal and
contraction proximal (Bayliss – Starling law of the
intestines)
Moves bolus through intestines
Intestino-intestinal reflex – extrinsic nerves
Local stretch in one area inhibits contraction in rest of
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bowel
Movement in Small Intestine
Peristalsis (ou propulsivos) e Segmentation (mixing)
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http://medweb.bham.ac.uk/research/toescu/Teaching/OverviewGITY2.html
Central and peripheral control of contractile patterns
Vagal
centre
Intestinal
wall
Vago-vagal reflexes
Interneurons
Integrating circuits
Sensory neurons
Motorneurons
Program circuits
Contractile
patterns
Enteric nervous system
Intestinal
lumenl
Peptide (CCK) Receptors
Glucose - Osmolality
Long chain fatty acids
Amino acids
Luminal stimuli elicit vago-vagal reflexes which activate integrating and
program circuits of the enteric nervous system.
These activate specific motorneurones responsible for specific contractile
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patterns.
Control of Intestinal Motility – Hormonal
Gastrin
CCK
Secretin
+ motility --
Glucagon
5-HT
VIP
Motilin
GIP
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Ileocecal
Valve
• What it is
• Opening to large intestines
• Function: (1) prevent the repulsion (2) control the emptying
• normally closed. Short-range peristalsis in terminal ileum
and distension relaxes IC sphincter --> small amount of
chyme is squirted into the cecum.
• Distension of cecum contracts IC sphincter.
• Gastro-ileal reflex: enhances ileal emptying after eating.
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• The hormone gastrin relaxes ileocecal sphincter.
V. GASTROINTESTINAL MOTILITY
DURING FASTING STATE
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Gastric motility on fasting
“Migrating Motor Complex, MMC”
Occurs on fasting
To clear undigested food particles
Peristaltic contractions sweep down stomach and
duodenum – pylorus relaxes
Pattern of contraction approx every 90 min
Slow peristaltic waves sweeping whole of GI tract
Thought to be controlled by motilin
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The interdigestive motility consists of three phases
III
Interdigestive Cycles
Phases
II
I
Phase III
Stomach
Phase III
Pylorus
Duodenum
Accumulation
of residues
of chyme
Jejunum
Phase I
Contraction
of reservoir
Forceful
peristaltic
waves
Motor
quiescence
of stomach
and duodenum
III
Phase II
Sporadic
peristaltic waves
Segmenting
contractions
and single
peristaltic waves
Phase II
Motor
quiescence
Ileum
Phase I
Phase III
The phase III of the migrating motor complex (of dog)
Originates simultaneously at the stomach and duodenum
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Migrates within 90 to 120 minutes along the small intestine
MMC(migrating motor complex)
• PhaseⅠ: Almost have no contractions
• PhaseⅡ: have contractions, only have few
• PhaseⅢ: have continuous contractions
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Migrating myoelectric Complex
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Gastric phase III consisting
of 1 - 3 forceful contractions
Gastric phases III
Middle Antrum
0 mm
Pyloric diameter
6 mm
Duodenal bulb
Duodenum
1 min
A
P
P
P
Stomach is cleaned of residues
of chyme and secretions.
The antral waves are associated with a wide opening of the
pylorus and inhibition of duodenal contractions followed34by
duodenal peristaltic waves occurring at maximal frequency.
Phase III of the interdigestive” (MMC)
Jejunal phase III (MMC)
1 minute
oral
aboral
Aboral migration
of phase III
Velocity of the
peristaltic waves
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Rectangles: strain gauge transducers, Data of dog.
Phase III (MMC) of the small intestine
Intestinal phase III
oral
Successsive
peristaltic waves
Chyme
Slow aboral
migration of
phase III
aboral
The peristaltic waves clean the intestinal segment from chyme
which accumulates aborally.
Because the successive waves start and end further aborally
the
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phase III slowly migrates distally
Importance of MMC
1.Sweep the contents of the small intestine
towards the colon
Housekeeper of the small intestine
2.Inhibit the migration of colonic bacteria into
the terminal ileum
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Ingestion of a meal suppresses the interdigestive
motility and induces a fed motor pattern
Phase III
Meal
Fed motor pattern
Antrum
closed
Pylorus
open
Duodenum
5 min
Postprandial motility is characterised by
 a lower amplitude of the antral waves occurring at
maximal frequency,
 rhythmic pyloric opening and closure
 co-ordinated duodenal contractions occurring in sequence
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with the antral waves
VI MOTILITY OF THE COLON
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Large intestine
• Functions
• Absorption of water and electrolytes
• Storage of feces
• In non-ruminant herbivores, fermentative digestion
and absorption of nutrients
• Motility patterns
• Segmentation
• Antiperistalsis
• Mass movement
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Large intestinal
motility patterns
• Segmentation
• Antiperistalsis
Antiperistalsis moves large
intestinal contents cranially
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Segmentation in
large intestine
• Haustration: （结肠袋）
modified form of
segmentation in which
intense, local contraction of
circular muscle causes
large intestine to appear to
bulge into sacs
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Mass movement
• Occurs in colon; also known to occur in
equine cecum
• Period of intense propulsive activity that
moves entire contents of colon distally
toward rectum
• Contractions progress for long distance such that
long length of colon contracts as a unit
• Entry of fecal matter into recturn triggers
defecation reflex
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Defecation
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VII GI Reflexes
• Gastrocolic
• Increase in colonic activity after a meal
• Distention of the stomach stimulates evacuation of the
colon
• Enterogastric
• Distention and irritation of the small intestine results in
suppression of secretion and motor activity in the
stomach
• Colocolonic
• Propels stool caudally by proximal muscle constriction
and distal dilatation
• Mediated by myenteric plexus
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• Rectocolic
• Colonic peristalsis due to stimulation of rectum
• Mediated by pelvic nerve
• Intestinointestinal Reflex:
• When a part of the intestine becomes overdistended
or its mucosa becomes excessively irritated, activity
in other parts of the intestine is inhibited as long as
the distention persists.
• Gastroileal Reflex:
• An increase in ileal motility and opening of the ileocecal
valve when food enters the empty stomach
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