Radioscintigraphy shows that the labeled material is moved relatively quickly into the transverse colon (Fig. 26.36),
where it is retained for about 24 hours. This suggests that the transverse colon is the primary location for the removal of water and electrolytes and the storage of solid feces in the large intestine.
A segmental pattern of motility programmed by the ENS accounts for the ultraslow forward movement of feces in the transverse colon. Ring-like contractions of the circular muscle divide the colon into pockets called haustra (Fig. 26.37). The motility pattern, called haustration, differs from segmental motility in the small intestine, in that the contracting segment and the receiving segments on either side remain in their respective states for longer periods. In addition, there is uniform repetition of the haustra along the colon. The contracting segments in some places appear to be fixed and are marked by a thickening of the circular muscle.
Haustrations are dynamic, in that they form and reform at different sites. The most common pattern in the fasting individual is for the contracting segment to propel the contents in both directions into receiving segments. This mechanism mixes and compresses the semiliquid feces in the haustral pockets and probably facilitates the absorption of water without any net forward propulsion.
Net forward propulsion occurs when sequential migration of the haustra occurs along the length of the bowel. The con-
» Colonic transit revealed by radioscintigra-phy. Successive scintigrams reveal that the longest dwell-time for intraluminal markers injected initially into the cecum is in the transverse colon. The image is faint after 48 hours, indicating that most of the marker has been excreted with the feces.
Haustra in the large intestine. This X-ray film shows haustral contractions in the ascending and the transverse colon. Between the haustral pockets are segments of contracted circular muscle. Ongoing activity of inhibitory motor neurons maintains the relaxed state of the circular muscle in the pockets. Inactivity of inhibitory motor neurons permits the contractions between the pockets.
tents of one haustral pocket are propelled into the next region, where a second pocket is formed, and from there to the next segment, where the same events occur. This pattern results in slow forward progression and is believed to be a mechanism for compacting the feces in storage.
Power propulsion is another programmed motor event in the transverse and the descending colon. This motor behavior fits the general pattern of neurally coordinated peristaltic propulsion and results in the mass movement of feces over long distances. Mass movements may be triggered by increased delivery of ileal chyme into the ascending colon following a meal. The increased incidence of mass movements and generalized increase in segmental movements following a meal is called the gastrocolic reflex. Irritant laxatives, such as castor oil, act to initiate the motor program for power propulsion in the colon. The presence of threatening agents in the colonic lumen, such as parasites, enterotoxins, and food antigens, can also initiate power propulsion.
Mass movement of feces (power propulsion) in the healthy bowel usually starts in the middle of the transverse colon and is preceded by relaxation of the circular muscle and the downstream disappearance of haustral contractions. A large portion of the colon may be emptied as the contents are propelled at rates up to 5 cm/min as far as the rectosigmoid region. Haustration returns after the passage of the power contractions.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.