Figure 1626

Diagram of immunoglobulin A (IgA) secretion and transport. Immunoglobulin A (IgA) is secreted by plasma cells into the lamina propria. Here, it dimerizes and then binds to a transmembrane Fc receptor on the membrane of the enterocyte. The extracellular portion of the membrane receptor will remain with the IgA dimer and will later become the secretory component of the IgA. The IgA-receptor complex enters the cell by endocytosis and is carried to the apical surface within the endocytotic vesicles (a process called transcytosis). The vesicle fuses with the apical plasma membrane, releasing the IgA-receptor complex as secretory IgA (slgA). The IgA monomers and dimers, the Fc receptors, and the endocytotic vesicles are greatly exaggerated in size for clarity. The actual sizes of the vesicles involved approximate those shown in the adjacent enterocytes.

ate ions. This highly alkaline secretion probably serves to protect the proximal small intestine by neutralizing the acid-containing chyme delivered to it. It also brings the intestinal contents close to the optimal pH for the pancreatic enzymes that are also delivered to the duodenum.

Muscularis Externa

The muscularis externa consists of an inner layer of circularly arranged smooth muscle cells and an outer layer of longitudinally arranged smooth muscle cells. The main components of the myenteric plexus (Auerbach's plexus) are located between these two muscle layers (Fig. 16.28). Two kinds of muscular contraction occur in the small intestine. Local contractions displace intestinal contents both proximally and distally, this type of contraction is called segmentation. These contractions primarily involve the circular muscle layer. They serve to circulate the chyme locally, mixing it with digestive juices and moving it into contact with the mucosa for absorption. Peristal

muscularis mucosae/

_.unner s glands i? submucosa

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