14. digestive system

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Transcript of 14. digestive system

  • 1. The Digestive System Histology

2. Introduction The digestive system consists of a muscular digestive (tube) tract and accessory organs - digestive tract: oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine - accessory organs: teeth, tongue, and glandular organs (salivary glands, lever, pancreas) which secrete into ducts Food enters the digestive tract, along the way secretions of the glandular organs assist in preparing organic and inorganic nutrients for absorption 3. Functions of the Digestive System Ingestion occurs when foods and liquids enter the digestive tract via the mouth Mechanical processing squashing with the tongue, tearing and crushing with the teeth - swirling, mixing, churning, and propulsive motions provide mechanical processing after swallowing Digestion chemical and enzymatic breakdown of carbohydrates, lipids, and proteins - small organic molecules can be absorbed by the digestive epithelium 4. Secretion digestion involves the action of acids, enzymes, and buffers - produced by the lining but mostly by the accessory organs (pancreas) Absorption movement of organic molecules, electrolytes, vitamins, and water across the digestive epithelium - into the interstitial fluid of the digestive tract Excretion into the digestive tract, primarily by the accessory glands (especially the liver) Compaction progressive dehydration of indigestible materials and organic wastes (feces) - defecation is the elimination of feces from the body 5. Lining of the Digestive Tract Plays a defensive role by protecting surrounding tissues against: 1. the corrosive effects of digestive acids and enzymes 2. mechanical stresses, such as abrasion 3. pathogens that are swallowed with food or that reside within the digestive tract 6. Components of the Digestive SystemFig 25.1 7. Histological Organization of the Digestive Tract Major layers of the digestive tract include the: Mucosa Submucosa Muscularis externa Serosa 8. The Mucosa A mucous membrane consisting of a layer of loose CT covered by an epithelium moistened by glandular secretions - plicae (folds) increase surface area for absorption - lamina propria, underlying layer of areolar tissue (BVs, sensory nerve endings, lymphatic vessels, smooth muscle fibers - muscularis mucosae, smooth muscle fibers arranged in 2 thin concentric layers: - the circular layer, inner layer encircles the lumen and - the longitudinal layer, outer layer whose muscle fibers lie parallel to the long axis of the tract 9. Histology of the Mucosa Organ MouthEpithelium Nonkeratinized Stratified SquamousPharynxNonkeratinized Stratified SquamousEsophagusNonkeratinized Stratified SquamousStomach Small Intestine Large Intestine AnusSimple Columnar Simple Columnar Simple Columnar Nonkeratinized Stratified 10. Histology of the Mucosa OrganFolds of the epitheliumEsophagusnoneStomachL: Rugae, S: gastric pitsSmall Intestine L: Plicae circulares, Villi S: Crypts of Lieberkuhn, microvilli Large Intestine L: Haustra S: Intestinal glands 11. Figure 25.2a Copyright 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 12. The Submucosa A layer of dense, irregular CT surrounds the muscularis mucosae - contains large BVs and lymphatics - some regions contain exocrine glands that secrete buffers and enzymes into the lumen - in the outer margin the submucosal plexus, a network of nerve fibers and scattered neuron cell bodies innervates the mucosa - include the sensory neurons, parasympathetic ganglia, and sympathetic postganglionic fibers 13. Histology of the Submucosa OrganSpecialized structuresEsophagusSubmucosal mucous glandsStomachNoneDuodenumBrunners glandsIleumPeyers PatchesLarge Intestine None 14. The Muscularis Externa Dominated by smooth muscle fibers surrounds the submucosa - fibers are arranged in circular (inner) and longitudinal (outer) layers - essential in mechanical processing and in propulsion of materials along the digestive tract - movements coordinated by the myenteric (Auerbach) plexus, a network of parasympathetic ganglia and sympathetic postganglionic fibers (sandwiched between the muscle layers) - muscularis externa forms sphincters (valves) that help prevent materials from moving along the tract at the wrong time or direction 15. Histology of the Muscularis OrganSmooth muscle layersEsophagus2, circular and longitudinalStomach3, oblique, circular, and longitudinalSmall Intestine 2, circular and longitudinal Large Intestine 2, circular and longitudinal 16. The Serosa A serous membrane covers the muscularis externa along most regions of the digestive tract within the peritoneal cavity - no serosa surrounds the muscularis externa of the oral cavity, pharynx, esophagus, and rectum - instead adventitia, a fibrous sheath of collagen fibers attaches the digestive tract to adjacent structures Serosa or visceral peritoneum is continuous with the parietal peritoneum that lines the inner surfaces of the body wall 17. Histology of the Serosa OrganSerosaEsophagusAdventitia due to the fact that the esophagus is not in a cavityStomachVisceral PeritoneumSmall Intestine Visceral Peritoneum Large Intestine Visceral Peritoneum AnusAdventitia 18. Histology of the Digestive System Basic Histological Layers 1. Mucosa a. Epithelium b. Lamina Propria c. Muscularis Mucosae 2. Submucosa a. Submucosal plexus Plexus of Meissner 3. Muscularis a. Myenteric plexus Plexus of Auerbach 4. Serosa 19. Histological Structure of the Digestive TractFigure 25.2bCopyright 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 20. Muscularis Layers and the Movement of Digestive Materials Visceral smooth muscle tissue digestive tract a single smooth muscle cell: 5-10um in diameter and 30200um in length - surrounded by CT (no tendons or aponeuroses) - contractile proteins not organized into sacromeres - muscle cells are nonstriated, involuntary muscle - contractions are as strong as skeletal or cardiac muscle - muscle cells are arranged in sheets - adjacent cells are electrically connected by gap junction - 1 cell contracts spreads like a wave throughout the tissue stimulus: activation of a motor neuron, local response to chemicals, hormones, O2 & CO2 levels, stretching, irritation 21. Contractile filaments of smooth muscle cells are not rigidly organized - plasticity, tolerate extreme stretching, a stretched cell adapts to its new length and retains ability to contract on demand - pacemaker cells in the muscularis mucosae and muscularis externa undergo spontaneous depolarization - which trigger contractions leading to 2 types of movement: peristalsis and segmentation 22. Peristalsis Muscularis externa via peristalsis propels materials from 1 region of the tract to another Waves of contractions move a bolus of food 1. circular muscles contract behind the digestive contents 2. followed by longitudinal muscle contraction that shorten adjacent segments 3. wave of contraction in circular muscle layer forces bolus forward Copyright 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig 25.3a 23. Segmentation Contractions in most areas of the small intestine and some regions of the large intestine churn and fragment the digestive materials The contents are mixed with intestinal secretions No net movement is producedFigure 25.3 Copyright 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 24. Peristalsis and Segmentation May be triggered by pacesetter cells, hormones, chemicals, and physical stimulation Peristaltic waves can also be initiated by afferent and efferent fibers - glossopharyngeal, vagus or pelvic nerves Afferent fibers synapse within the myenteric plexus produce localized myenteric reflexes - short reflexes do not involve the CNS - enteric nervous system, neural network that coordinates short reflexes (as many neurons and NTs as the SC) - long reflexes, involve interneurons and motor neurons in the CNS control large-scale peristaltic waves 25. Mesenteries Most regions of the digestive tract are suspended by sheets of serous membrane - connect the parietal with the visceral peritoneum Mesenteries - fused, double sheets of peritoneal membrane - stabilize positions of attached organs - prevent entanglement of intestines - areolar CT between the mesothelial surfaces provide a route for BVs, nerves, and lymphatics, to and from the digestive tract - during development, the digestive tract and accessory organs are suspended by dorsal and ventral mesenteries 26. Lesser omentum - remnant of the embryonic ventral mesentery - persists only on ventral surface of the stomach, between the stomach and liver - and the falciform ligament, that lies between the liver and the anterior abdominal wall and diaphragm Greater omentum - dorsal mesentry becomes enlarged to form a pouch - loose CT within the mesentery contains a thick layer of adipose tissue, the lipids are an important energy reserve - provides insulation that reduces heat loss - contains numerous lymph nodes to help protect the body from pathogens that evaded the defenses of the digestive tract 27. Copyright 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 25.4c 28. Mesentery proper all but the first 25cm of the small intestine is suspended by this thick mesenterial sheet - provides stability but permits some independent movement Mesocolon attached to the large intestine Transverse mesocolon suspends the middle portion of the large intestine (transverse colon) Sigmoid mesocolon suspends the sigmoid colon that leads to the rectum and anus 29. Copyright 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 25.4d 30. The Oral Cavity Digestive tract path of food from the mouth to the anus The oral cavity includes: - the tongue, salivary glands, teeth, mastication Functions include: 1) analysis of material before swallowing 2) mech