![]() Phase I is characterized by motor quiescence, phase II by seemingly disorganized pressure waves occurring at submaximal rates, and phase III by sustained pressure waves occurring at maximal rates. The fasting pattern or interdigestive motor cycle (IDMC) consists of three phases. This segmenting pattern is hypothesized to assist in mixing intraluminal contents and in facilitating their contact with the absorptive mucosal surface. Rhythmic segmentations or pressure waves traveling only short distances also are observed. The fed or postprandial pattern begins within 10 to 20 minutes of meal ingestion and abates 4 to 6 hours afterward. These two reflexes are present even in the absence of any extrinsic innervation to the small intestine and contribute to peristalsis when they are propagated in a coordinated fashion along the length of the intestine. These patterns include ascending excitation and descending inhibition in which muscular contraction occurs proximal to a stimulus, such as the presence of a bolus of ingested food, and muscular relaxation occurs distal to the stimulus ( Fig. Several distinctive patterns of muscularis propria activity have been observed to occur in the small intestine. Alternative induction mechanisms, such as antigen presentation within mesenteric lymph nodes, are also likely to exist. These lymphocytes then migrate into the systemic circulation via the thoracic duct and ultimately accumulate in the intestinal mucosa at effector sites. APCs interact with and prime naïve lymphocytes, which then exit through the draining lymphatics to enter the mesenteric lymph nodes where they undergo differentiation. Dendritic cells, in addition, may sample luminal antigens directly through their dendrite-like processes that extend through epithelial tight junctions. Using transepithelial vesicular transport, M cells transfer microbes to underlying professional antigen-presenting cells (APCs), such as dendritic cells. These cells possess an apical membrane with microfolds rather than microvilli, which is characteristic of most intestinal epithelial cells. Overlying Peyer’s patches are a specialized epithelium containing microfold (M) cells. ![]() Peyer’s patches are macroscopic aggregates of B-cell follicles and intervening T-cell areas found in the lamina propria of the small intestine, primarily the distal ileum. 10 Inductive sites include Peyer’s patches, mesenteric lymph nodes, and smaller isolated lymphoid follicles scattered throughout the small intestine ( Fig. The GALT is conceptually divided into inductive and effector sites. Absorbed amino acids and peptides then enter the portal venous circulation. Epithelial absorption occurs for both single amino acids and di- or tripeptides via specific membrane-bound transporters. Additional digestion occurs through the actions of peptidases that exist in the enterocyte brush border and cytoplasm. The final products of intraluminal protein digestion consist of neutral and basic amino acids and peptides two to six amino acids in length ( Fig. In response to the presence of bile acids, enterokinase is liberated from the intestinal brush border membrane to catalyze the conversion of trypsinogen to active trypsin trypsin in turn activates itself and other proteases. This is in contrast to pancreatic amylase and lipase, which are secreted in their active forms. These enzymes are secreted as inactive proenzymes. ![]() Digestion continues in the duodenum with the actions of a variety of pancreatic peptidases. This is not, however, an essential step, since surgical patients who are acholorhydric or who have lost part or all their stomach are still able to successfully digest proteins. ![]() Protein digestion begins in the stomach with action of pepsins. In addition to dietary proteins, approximately one half of the protein load that enters the small intestine is derived from endogenous sources including salivary and gastrointestinal secretions and desquamated intestinal epithelial cells. Ten to fifteen percent of energy consumption in the average Western diet consists of proteins.
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