The colon is relatively poorly supplied with cells that release bioactive peptides and other regulatory factors. Exceptions are enterochromaffin cells, which release 5-HT, and cells that synthesize peptide YY, so named because its sequence contains two adjacent tyrosine residues. Peptide YY is synthesized by enteroendocrine cells localized in the terminal ileum and colon and is released in response to lipid in the lumen. It decreases gastric emptying and intestinal propulsive motility. Peptide YY also reduces Cl− and thus fluid secretion by intestinal epithelial cells. Thus peptide YY has been characterized as an “ileal brake” in that it is released if nutrients, especially fat, are not absorbed by the time the meal reaches the terminal ileum and proximal part of the colon. By reducing propulsion of the intestinal contents, in part by limiting their fluidity and distention-induced motility, peptide YY provides more time for the meal to be retained in the small intestine where its constituent nutrients can be digested and absorbed.
Patterns of Colonic Motility
To appreciate colonic motility the functional anatomy of the colonic musculature will be reviewed first, followed by a discussion of the regulation of colonic motility.
Functional Anatomy of the Colonic Musculature
As in other segments of the intestine, the colon consists of functional layers, with a columnar epithelium most closely apposed to the lumen, which is then underlaid by the lamina propria, serosa, and muscle layers. Similarly the colonic mucosa is surrounded by continuous layers of circular muscle that can occlude the lumen. Indeed, at intervals •Fig. 31.1 Majoranatomicsubdivisionsofthecolon.Sigmoid colon Cecum Rectum Ileocecal valve Terminal ileum •Fig. 31.2 Radiographshowingaprominenthaustralpatterninthecolonofanormalindividual.(FromKeatsTE.An Atlas of Normal Roentgen Variants. 2nded.StLouis:Mosby–YearBook;1979.) the circular muscle contracts to divide the colon into segments called haustra. These haustra are readily appreciated if the colon is viewed at laparotomy or by x-ray imaging as shown in
Fig. 31.2 . The arrangement of the majority of the longitudinal muscle fibers, however, is distinct from •Fig. 31.3 Anatomyoftherectumandanalcanal.
that in the small intestine. Three nonoverlapping bands of longitudinal muscle known as the taeniae coli extend along the length of the colon.
Although the circular and longitudinal muscle layers of the colon are electrically coupled, this process is less efficient than in the small intestine. Thus propulsive motility in the colon is less effective than in the small intestine. Activity of the enteric nervous system also provides for the segmenting contractions that form the haustra. Contents can be moved back and forward between haustra, which is a means of retarding passage of the colonic contents and maximizing their contact time with the epithelium. In contrast, when rapid propulsion is called for, the contractions forming the haustra relax and the contour of the colon is smoothened.
The colon terminates in the rectum, which is joined to the colon at an acute angle (the rectosigmoid junction ) (Fig.