Muscularis externa

Four major tissue layers, from the lumen outward, form the large intestine the mucosa, submucosa, muscularis externa, and serosa (Fig. 88-2). Complete replacement of surface epithelial cells occurs approximately weekly, with the total number of epithelial cells remaining constant in normal colonic tissue. As patients age, abnormal cells accumulate on the surface epithelium and protrude into the stream of fecal matter their contact with fecal mutagens can lead to further cell mutations and eventual adenoma formation.4... 

Muscularis externa. The outer layer of the wall is the muscularis externa. In most regions of the tract, it consists of two layers of muscle an inner circular layer and an outer longitudinal layer. Contraction of the circular layer narrows the lumen of the tube. Contraction of the longitudinal layer causes the tube to shorten. 

The ability of such a protein to access the muscularis externa and thereby influence peristalsis in vivo has to be called into question, but a role for regulation of local spasm via inhibition of the muscularis mucosa is feasible. Similarly, a role for parasite AChEs in the latter phenomenon cannot yet be discounted. 

Figure 8.1 (A) Cross-sectional view of the organization of the small intestine, illustrating the serosa, the longitudinal and circular muscle layers (=muscularis externa), the submucosa, and the intestinal mucosa. The intestinal mucosa consists of four layers, the inner surface cell monolayer of enterocytes, the basal membrane, the lamina propria (connective tissue, blood capillaries), and the muscularis mucosae, (B) Schematic representation of an enterocyte (small intestinal epithehal cell) (according to Tso and Crissinger [151], with permission).

Rectum Mucosa, submucosa, muscularis externa, serosa Transports waste... 

Muscularis Externa Inner Circular and Outer Longitudinal Layers... 

The next layer is the muscularis externa, which contains circular muscle fibers on the top layers and longitudinal muscle fibers on the outer layers. Finally, the serosa is found which is continuous with the peritoneum. Food movement through the stomach into the intestines occurs by contraction of the muscle fibers in the musclularis. 

In contrast, in the small intestine (Figure 3.15), the mucosa is composed of surface projections or villi with a core of lamina propria and muscle fibers. In the lamina propria itself are found intestinal glands, fine connective tissues, reticular cells, and lymphatic tissue. The submucosa is filled with glands in the duodenum whereas in the muscularis externa an inner circular layer and an outer longitudinal layer of muscle are present. Parasym-... 

These external neural influences on intestinal motility are common targets for prokinetic drugs, but events within the bowel can have important effects on intestinal motility and cause the bowel to be refractory to traditional prokinetic therapy. Release of cytokines from activated inflammatory cells is probably an important feature of ileus in many cases. Ileus secondary to reperfusion injury is an anticipated response in horses with small intestinal obstruction. However, even apparently mild intestinal injury can initiate cellular responses that lead to impaired motility. Mild intestinal insult by gentle surgical manipulation activated adhesion molecules on leukocytes and increased the expression of P-selectin and intercellular adhesion molecule 1 on endothelial cells within the vasculature of the muscularis layer of the intestine (Kalff et al 1999). Surgical manipulation of the rodent small intestine resulted in substantial extravasation of leukocytes into the intestinal muscularis, consisting mainly of polymorphonuclear neutrophils, monocytes and mast cells and lasting for days. This cellular inflammatory response within the intestinal muscularis externa was associated with a marked decrease in jejunal circular muscle activity (Kalff et al 1998). 

Kalff, J.C., Turler, A., Schwarz, N.T, Schraut, W.H., Lee, K.K.W., Tweardy, D.J., Billiar, T.R., Simmons, R.L., and Bauer, A.J. (2003) Intra-abdominal activation of a local inflammatory response within the human muscularis externa during laparotomy, Ann. Surg. 237 301-315. 

The wall of the duodenum is, like the gastric wall, divided into the mucosa, submucosa, muscularis externa and serosa. The main function of the duodenum is the digestion of nutrients (Ross and Romrell 1989). 

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