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SECTION V
Gastrointestinal Physiology
response, defecation after meals is the rule in children. In
adults, habit and cultural factors play a large role in determin-
ing when defecation occurs.
CHAPTER SUMMARY
■
The regulatory factors that govern gastrointestinal secretion
also regulate its motility to soften the food, mix it with secre-
tions, and propel it along the length of the tract.
■
Two major patterns of motility are peristalsis and segmentation,
which serve to propel or retard/mix the luminal contents, re-
spectively. Peristalsis involves coordinated contractions and
relaxations above and below the food bolus.
■
The membrane potential of the majority of gastrointestinal
smooth muscle undergoes rhythmic fluctuations that sweep
along the length of the gut. The rhythm varies in different gut
segments and is established by pacemaker cells known as inter-
stitial cells of Cajal. This basic electrical rhythm provides for
sites of muscle contraction when stimuli superimpose spike
potentials on the depolarizing portion of the BER waves.
■
In the period between meals, the intestine is relatively quiescent,
but every 90 min or so it is swept through by a large peristaltic
wave triggered by the hormone motilin. This migrating motor
complex presumably serves a “housekeeping” function.
■
Swallowing is triggered centrally and is coordinated with a peri-
staltic wave along the length of the esophagus that drives the
food bolus to the stomach, even against gravity. Relaxation of
the lower esophageal sphincter is timed to just precede the arriv-
al of the bolus, thereby limiting reflux of the gastric contents.
Nevertheless, gastroesophageal reflux disease is one of the most
common gastrointestinal complaints
■
The stomach accommodates the meal by a process of receptive
relaxation. This permits an increase in volume without a signif-
icant increase in pressure. The stomach then serves to mix the
meal and to control its delivery to downstream segments.
■
Luminal contents move slowly through the colon, which en-
hances water recovery. Distension of the rectum causes reflex
contraction of the internal anal sphincter and the desire to def-
ecate. After toilet training, defecation can be delayed till a con-
venient time via voluntary contraction of the external anal
sphincter.MULTIPLE-CHOICE QUESTIONS
For all questions, select the single best answer unless otherwise directed.
- In infants, defecation often follows a meal. The cause of colonic
contractions in this situation is
A) histamine.
B) increased circulating levels of CCK.
C) the gastrocolic reflex.
D) increased circulating levels of somatostatin.
E) the enterogastric reflex.
2. The symptoms of the dumping syndrome (discomfort after
meals in patients with intestinal short circuits such as anastomo-
sis of the jejunum to the stomach) are caused in part by
A) increased blood pressure.
B) increased secretion of glucagon.
C) increased secretion of CCK.
D) hypoglycemia.
E) hyperglycemia.
3. Gastric pressures seldom rise above the levels that breach the
lower esophageal sphincter, even when the stomach is filled with
a meal, due to which of the following processes?
A) peristalsis
B) gastroileal reflex
C) segmentation
D) stimulation of the vomiting center
E) receptive relaxation
4. The migrating motor complex is triggered by which of the
following?
A) motilin
B) NO
C) CCK
D) somatostatin
E) secretin
5. A patient with achalasia would be expected to exhibit a decrease
in which of the following?
A) esophageal peristalsis
B) expression of neuronal NO synthase at the esophageal/
gastric junction
C) acetylcholine receptors
D) substance P release
E) contraction of the crural diaphragm
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Gastrointestinal Physiology.
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