Large Intestinal Motility

Approximately 7-10 L of ingested or secreted water enters the small intestine during a 24-hr period. Of this amount, about 600 mL reach the colon. The motility patterns of the large intestine are organized so that all but about 100 mL are absorbed. The remaining fecal material is then stored until it can be evacuated conveniently.

Structure and Innervation of the Large Intestine

Beginning from the ileocecal junction, the large intestine is anatomically divided into the cecum; the ascending, transverse, descending, and sigmoid colon; the rectum; and the anal canal (Fig. 11). The inner, circular layer of smooth muscle fibers is continuous from the cecum to the anal canal. Within the rectum and anal canal, the circular layer thickens to form the internal anal sphincter. The outer, longitudinal layer of smooth muscle fibers is discontinuous, consisting of three bands that run the entire length of the large

Transverse Haustra colon

Transverse Haustra colon

FIGURE 11 The anatomy of the large intestine.

intestine. These bands are called the teniae coli and fan out over the rectum to become relatively continuous. Layers of striated muscle distal to and overlapping the internal anal sphincter form the external anal sphincter.

The human large intestine is considerably thicker and shorter than the small intestine, and the teniae coli are easily visible. In addition, the colon is divided into segments, called haustra or haustrations, that give the appearance of a chain of small sacs. Haustra disappear during, and reappear after, contractions of a specific segment. They may also reform at other loci, so their locations are not fixed. Nevertheless, they probably have a structural basis as well as a functional one. Functionally, they may depend on the contractile state of the smooth muscle fibers; structurally, they may be influenced by areas of concentration of muscular tissue and by folds in the mucosa.

The myenteric plexus in the large intestine is concentrated underneath the teniae, that is, between the layers of longitudinal and circular muscle. The myenteric plexus receives input from local receptors and from both the parasympathetic and sympathetic systems. Parasympathetic fibers from the vagus innervate the cecum and ascending and transverse colons, and the descending and sigmoid colons and the rectum and anus are innervated by the pelvic nerves from the sacral region of the spinal cord. The pelvic nerves enter the colon near the rectosigmoid junction and project anteriorly and posteriorly along the myenteric plexus innervating it as well. Sympathetic innervation is supplied to the proximal colon via the superior mesen-teric ganglion, whereas the inferior mesenteric ganglion provides fibers to the distal colon. The rectum and anal canal receive sympathetic innervation from the hypogastric plexus. Within the autonomic system acetylcho-line, substance P, and tachykinins mediate contraction, whereas VIP and nitric oxide mediate relaxation. The voluntary external anal sphincter is innervated by the somatic pudendal nerves, where transmission is mediated by acetylcholine.

Motility of the Cecum and Proximal Colon

The pressure recorded by a sensor placed in the ileocecal sphincter is several millimeters of mercury greater than that in the ileum or colon. The pressure, however, is not constant. When the ileum is distended, the sphincter relaxes, allowing contents to flow into the colon. When the colon is distended the sphincter contracts, preventing reflux into the ileum. Thus, material moves intermittently from the ileum to the colon.

Most of the movements of the proximal colon are segmental, serving to mix contents back and forth, and

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