C h a p t e r 2 8
Structure and Function of the Gastrointestinal System
685
occur because of inflammation or are secondary to the
effects of toxins or unabsorbed materials. Delayed pas-
sage of chyme in the small intestine also can be a prob-
lem. Transient interruption of intestinal motility often
occurs after GI surgery. Intubation with suction often is
required to remove the accumulating intestinal contents
and gases until activity is resumed.
Colonic Motility and Defecation
The storage function of the colon dictates that move-
ments in this section of the gut are different from those
in the small intestine. Movements in the colon are of
two types. First are the segmental mixing movements,
called
haustrations
, so named because they occur within
sacculations called
haustra.
These movements produce a
local digging-type action, which ensures that all portions
of the fecal mass are exposed to the intestinal surface.
Second are the propulsive mass movements, in which a
large segment of the colon (
≥
20 cm) contracts as a unit,
moving the fecal contents forward as a unit. Mass move-
ments last approximately 30 seconds, followed by a 2- to
3-minute period of relaxation, after which another con-
traction occurs. A series of mass movements lasts only for
10 to 30 minutes and may occur only several times a day.
Defecation normally is initiated by the mass movements.
Defecation (discharge of feces from the rectum) is con-
trolled by the action of two sphincters, the internal and
external anal sphincters. The internal sphincter is a several-
centimeters-long, circular thickening of smooth muscle that
lies inside the anus. The external sphincter, which is com-
posed of striated voluntary muscle, surrounds the internal
sphincter. The external sphincter is controlledbynervefibers
in the pudendal nerve, which is part of the somatic nervous
system and therefore under voluntary control. Defecation
is controlled by defecation reflexes. One of these reflexes is
the intrinsic myenteric reflex mediated by the local enteric
nervous system. It is initiated by distention of the rectal
wall, with initiation of reflex peristaltic waves that spread
through the descending colon, sigmoid colon, and rectum.
A second defecation reflex, the parasympathetic reflex, is
integrated at the level of the sacral cord. When the nerve
endings in the rectum are stimulated, signals are transmit-
ted first to the sacral cord and then reflexively back to the
descending colon, sigmoid colon, rectum, and anus by the
pelvic nerves. These impulses greatly increase peristaltic
movements as well as relax the internal sphincter.
To prevent involuntary defecation from occurring,
the external anal sphincter is under the conscious con-
trol of the cortex. As afferent impulses arrive at the
sacral cord, signaling the presence of a distended rectum,
messages are transmitted to the cortex. If defecation is
inappropriate, the cortex initiates impulses that con-
strict the external sphincter and inhibit efferent para-
sympathetic activity. Normally, the afferent impulses in
this reflex loop fatigue easily, and the urge to defecate
soon ceases. At a more convenient time, contraction of
the abdominal muscles compresses the contents in the
large bowel, reinitiating afferent impulses to the cord.
A
B
FIGURE 28-6.
Two types of
small intestine movements:
(A)
Mixing segmentation
waves in which slow
contractions of the circular
muscle layer occlude the
lumen and drive the contents
forward and backward; and
(B)
propulsive peristaltic
movements in which
segmental contractions
followed by sequential
relaxation moves the contents
forward.
SUMMARY CONCEPTS
■■
Smooth muscle of the digestive tract propels food
and fluids along its length with each part of the GI
tract undergoing movements consistent with its
digestive, absorptive, and storage functions.
■■
The smooth muscle in the wall of the GI tract
undergoes cyclic self-propagating changes in
membrane potentials, called slow waves, that
maintain motility. Although the activity of GI
smooth muscle is self-propagating, its rate and
strength of contractions are regulated by input
from the enteric and autonomic nervous systems.
■■
The enteric nervous system, which lies entirely
within the gut, controls motility, intestinal
(continued)
