Motility of Small Intestine
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| MOTILITY AND DYSMOTILITY OF SMALL INTESTINE |
The digestive status (fed versus fasting) is a key component of small bowel motility. Fasting small intestinal motility follows four cyclic phases, referred to as the migratory motor complex. The migratory motor complex consists of waves of electrical activity that sweep through the intestine every 90 to 120 minutes. In addition to facilitating the transport of indigestible substances from the stomach to the colon, the migratory motor complex also transports bacteria from the small intestine to the large intestine and inhibits the reflux of colonic bacteria to the terminal ileum. It has thereby been termed the “intestinal housekeeper.”
Phase
I of the migratory motor complex lasts 5 to 20 minutes and is characterized by
a prolonged period of quiescence. Phase II lasts 10 to 40 minutes and is
defined by an increased frequency of random contractions. Phase III lasts 3 to
6 minutes and is characterized by bursts of prolonged high-amplitude
contractions. In phase IV, there is a rapid decrease in contractions. In the
duodenum, phase III contractions have a frequency of 1 to 12 contractions per
minute and last for at least 3 minutes. The velocity progressively decreases
from the proximal duodenum to the distal jejunum. Alterations in the migratory
motor complex have been implicated in small intestinal bacterial overgrowth,
IBS, functional dyspepsia, gastroparesis, Chagas disease, intestinal
pseudoobstruction, obesity, anorexia nervosa, and aging.
The
postprandial phase is defined as the time from meal intake until the return of
phase III of the migratory motor complex. When nutrients enter the small bowel,
transit is initially rapid and chyme is distributed throughout the bowel.
During digestion, transit slows down to promote absorption by increasing
contact time of the chyme with the small bowel wall. Pressure waves after a
meal are similar to those occurring during the migratory
motor complex, but they propagate on average half the distance of phase III
pressure waves. Most postprandial pressure waves propagate less than 2 cm and
serve to mix and grind nutrient chyme. Flow rates during this period are highly
variable and rely on caloric content and the nature of the meal. In addition,
the enteric nerves, hormonal function, and level of paracrine mediators,
including gastrin, cholecystokinin, neurotensin, peptide YY, pancreatic
polypeptide, and motilin play a role.
The
stationary pressure waves of the postprandial period favor absorption, as does
phase I of the migratory motor complex. The stationary pressure waves work in
two ways: (1) by stirring intestinal contents and (2) by
providing propulsive pressure waves to spread and expose the chyme to a larger
absorptive surface. This type of activity is also referred to as rhythmic
segmentation. Intestinal peristalsis is generated by the contraction of the
muscularis propria, made up of outer longitudinal and inner circular layers,
forming a continuous tube that lengthens,
shortens, twists, and constricts so that the enclosed contents are constantly
agitated and propelled. A meal generally traverses the small bowel in
approximately 5 hours, a period that is shortened by the intake of another
meal.
The
initiation and cycling of the migratory motor complex is under the control of
the enteric nervous system. Smooth muscle cells of the gastrointestinal tract
undergo periodic depolarization of their mem- brane potential. These are called
slow waves, and they are generated by the interstitial cells of Cajal, which
act as pacemakers and produce spontaneous electrical slow waves with a
frequency of 12 per minute in the duodenum and 10 per minute in the ileum. A
contraction is achieved when a slow wave occurs at the same time that an
excitatory neurotransmitter is released from a motor neuron of the enteric
nervous system.
Motility
studies (manometry) and transit studies are used to investigate small bowel
motor physiology. Manometry studies are conducted using a catheter that
measures intraluminal pressure induced by smooth muscle contractions. Transit
studies include hydrogen breath testing, small bowel scintigraphy, and wireless
motility capsule testing. The wireless motility capsule measures transit time,
pressure, pH, and temperature from the mouth to the anus. It correlates well
with scintigraphy. Patients can resume normal daily activities while data are
being collected by the capsule. It also eliminates radiation exposure and
provides a complete transit profile of the gastrointestinal tract.
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| GRADIENT AND ILEOCECAL SPHINCTER |
The junction of the small intestine with the colon is sometimes referred to as the ileocecal valve partly because of its structural appearance in some anatomic specimens and partly because the end of the ileum, being wedged into the wall of the colon, seems to function, in some individuals, in the manner of a flutter valve. The ileocecal junction functions as a true sphincter, meaning that it regulates the flow of material from the ileum to the cecum, as well as preventing its retrograde passage. Thus the contact of the intestinal contents with the terminal ileal mucosa is prolonged, favoring maximal intestinal absorption. The sphincter opens when a peristaltic wave, passing along the terminal ileum, builds up enough pressure to overcome the resistance of the sphincter. The cecum at first manifests receptive relaxation. Increasing pressure in the cecum, either by over-distention or by a peristaltic contraction, causes a reflex contraction of the sphincter, p eventing overfilling of the cecum and cecoileal reflux.
