886
UNIT 4
Maintenance of the Body
23
Migrating Motor Complex
True peristalsis occurs late in the
intestinal phase only afer most nutrients have been absorbed.
At this point, segmenting movements wane and the duodenal
mucosa begins to release the hormone
motilin
.
As motilin blood levels rise, peristaltic waves are initiated
in the proximal duodenum every 90 to 120 minutes and sweep
slowly along the intestine, moving 50–70 cm (about 2 f) beFore
dying out. Each successive wave begins a bit more distally, a
pattern oF peristaltic activity called the
migrating motor com-
plex (MMC)
. A complete “trip” From duodenum to ileum takes
about two hours. ±e process then repeats itselF, sweeping the
last remnants oF the meal plus bacteria, sloughed-off mucosal
cells, and other debris into the large intestine.
±is “housekeeping” Function prevents bacteria in the large
intestine From entering the small intestine. As Food again en-
ters the stomach with the next meal, segmentation replaces
peristalsis.
±e local enteric neurons oF the GI tract wall coordinate
these intestinal motility patterns and a variety oF effects occur
depending on which neurons are activated or inhibited. ²or ex-
ample, an ACh-releasing (cholinergic) sensory neuron in the
small intestine, once activated, may simultaneously send mes-
sages to several different interneurons in the myenteric plexus
that regulate peristalsis. As a result, the proximal area constricts
and Forces chyme along the tract, as the lumen oF the distal part
oF the intestine enlarges to receive it.
Ileocecal Control
Most oF the time, the ileocecal sphincter is
closed. However, two mechanisms—one neural and the other
hormonal—cause it to relax and allow Food residues to enter the
cecum when ileal motility increases:
±e
gastroileal reflex
(gas
0
tro-il
9
e-ul), a long reflex triggered
by stomach activity, increases the Force oF segmentation in
the ileum and relaxes the sphincter.
Gastrin released by the stomach increases the motility oF the
ileum and relaxes the ileocecal sphincter.
Once the chyme has passed through, it exerts backward pres-
sure that closes the valve’s flaps, preventing regurgitation into
the osmotic water loss From the blood into the intestinal lumen
would result in dangerously low blood volume. Additionally,
the low pH oF entering chyme must be adjusted upward and the
chyme must be well mixed with bile and pancreatic juice For diges-
tion to continue. ±ese modifications take time, and the pumping
action oF the stomach pylorus careFully controls Food movement
into the small intestine (see pp. 873–874 and ²igure 23.19) to pre-
vent the duodenum From being overwhelmed.
Since we cover the actual chemistry oF digestion and absorp-
tion in detail later, here we will examine how the motility oF the
small intestine mixes and propels Food and how this motility is
regulated.
Motility of the Small Intestine
Intestinal smooth muscle mixes chyme thoroughly with bile
and pancreatic and intestinal juices, and moves Food residues
through the ileocecal valve into the large intestine. In contrast
to the peristaltic waves oF the stomach,
segmentation
is the most
common motion oF the small intestine.
Segmentation
IF we examine the small intestine with X-
ray fluoroscopy afer it is “loaded” with a meal, it looks like
the intestinal contents are being massaged—alternately con-
tracting and relaxing rings oF smooth muscle simply move
the chyme backward and Forward a Few centimeters at a time
(see ²igure 23.3b). As with stomach peristalsis, intrinsic
pacemaker cells in the circular smooth muscle layer initiate
these segmenting movements. However, unlike the stomach
pacemakers, which have only one rhythm, the pacemakers in
the duodenum depolarize more Frequently than those in the
ileum. As a result, segmentation also moves intestinal con-
tents slowly and steadily toward the ileocecal valve at a rate
that allows ample time to complete digestion and absorption.
±e intensity oF segmentation is altered by long and short
reflexes, which parasympathetic activity enhances and sympa-
thetic activity decreases, and by hormones
(Table 23.3)
. ±e
more intense the contractions, the greater the mixing, but the
basic contractile rhythms oF the various intestinal regions re-
main unchanged.
Table 23.3
Control of Small Intestinal Motility
PHASE
 
STIMULUS
MECHANISM
EFFECT ON MOTILITY
Gastric
 
Gastric motility
and emptying
Long neural reflexes (gastroileal
reflex)
Activity in ileum
 
 
 
Gastrin
Segmenting movements
in ileum; relaxes
ileocecal sphincter
Intestinal
 
Distension of
small intestine
Long and short
neural reflexes
Strength of segmentation
 
 
Reduced intestinal
volume; fasting
Long and short
neural reflexes;
initiated by
blood levels of
motilin
Initiates migrating motor com-
plex (peristalsis); repeats until
next meal
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