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This 10 page Class Notes was uploaded by Jess Graff on Monday May 2, 2016. The Class Notes belongs to BMS 508 at University of New Hampshire taught by Mary Katherine Lockwood, PhD in Spring 2016. Since its upload, it has received 5 views. For similar materials see Human Anatomy and Physiology II in Biological Sciences at University of New Hampshire.
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Date Created: 05/02/16
BMS 508.03 4/1/2016 Chapter 24 (cont) Digestion & Metabolism (cont) The Digestive Tract • The Digestive Organs and the Peritoneum • Lined with serous membrane consisting of: • Superficial mesothelium covering a layer of areolar tissue • Serosa, or visceral peritoneum • Covers organs within peritoneal cavity • Parietal peritoneum • Lines inner surfaces of body wall • Peritoneal Fluid • Produced by serous membrane lining • Provides essential lubrication • Separates parietal and visceral surfaces • Allows sliding without friction or irritation • About 7 liters produced and absorbed daily, but very little in peritoneal cavity at one time • Ascites – excess peritoneal fluid causing abdominal swelling • Mesenteries • Double sheets of peritoneal membrane • Suspend portions of digestive tract within peritoneal cavity by sheets of serous membrane • That connect parietal peritoneum • With visceral peritoneum • Areolar tissue between mesothelial surfaces • Provides an access route to and from the digestive tract • For passage of blood vessels, nerves, and lymphatic vessels • Stabilize positions of attached organs • Prevent intestines from becoming entangled • Mesentery Development • During embryonic development • Digestive tract and accessory organs are suspended in peritoneal cavity by: • Dorsal mesentery • Ventral mesentery • Later disappears along most of digestive tract except at the lesser omentum and at the falciform ligament • The Lesser Omentum • Stabilizes position of stomach • Provides access route for blood vessels and other structures entering or leaving liver • The Falciform Ligament • Helps stabilize position of liver • Relative to diaphragm and abdominal wall • The Dorsal Mesentery • Enlarges to form an enormous pouch, called the greater omentum • Extends inferiorly between: • The body wall and the anterior surface of small intestine • Hangs like an apron • From lateral and inferior borders of stomach • Adipose Tissue in Greater Omentum • Conforms to shapes of surrounding organs • Pads and protects surfaces of abdomen • Provides insulation to reduce heat loss • Stores lipid energy reserves • The Mesentery Proper • A thick mesenterial sheet • Provides stability • Permits some independent movement • Suspends all but first 25 cm (10 in.) of small intestine • Is associated with initial portion of small intestine (duodenum) and pancreas • Fuses with posterior abdominal wall, locking structures in position • The Mesocolon • A mesentery associated with a portion of the large intestine • Transverse mesocolon supports transverse colon • Sigmoid mesocolon supports sigmoid colon • During development, mesocolon of ascending colon, descending colon, and the rectum: • Fuse to dorsal body wall • Lock regions in place • Histological Organization of the Digestive Tract • Four major layers of the digestive tract • Mucosa • Submucosa • Muscularis externa • Serosa • The Mucosa • Is the inner lining of digestive tract • Is a mucous membrane consisting of: • Epithelium, moistened by glandular secretions • Lamina propria of areolar tissue • The Digestive Epithelium • Mucosal epithelium is simple or stratified • Depending on location, function, and stresses • Oral cavity, pharynx, and esophagus • Mechanical stresses • Lined by stratified squamous epithelium • Stomach, small intestine, and most of large intestine • Absorption • Simple columnar epithelium with mucous cells • Enteroendocrine cells • Are scattered among columnar cells of digestive epithelium • Secrete hormones that: • Coordinate activities of the digestive tract and accessory glands • Lining of Digestive Tract • Folding increases surface area for absorption • Longitudinal folds, disappear as digestive tract fills • Permanent transverse folds (plicae circulares) • The Lamina Propria • Consists of a layer of areolar tissue that contains: • Blood vessels • Sensory nerve endings • Lymphatic vessels • Smooth muscle cells • Scattered areas of lymphatic tissue • Muscularis mucosae • Narrow band of smooth muscle and elastic fibers in lamina propria • Smooth muscle cells arranged in two concentric layers • Inner layer encircles lumen (circular muscle) • Outer layer contains muscle cells parallel to tract (longitudinal layer) • The Submucosa • Is a layer of dense irregular connective tissue • Surrounds muscularis mucosae • Has large blood vessels and lymphatic vessels • May contain exocrine glands • Secrete buffers and enzymes into digestive tract • Submucosal Plexus • Also called Meissner’s plexus • Innervates the mucosa and submucosa • Contains: • Sensory neurons • Parasympathetic ganglionic neurons • Sympathetic postganglionic fibers • The Muscularis Externa • Is dominated by smooth muscle cells • Are arranged in: • Inner circular layer • Outer longitudinal layer • Involved in: • Mechanical processing • Movement of materials along digestive tract • Movements coordinated by enteric nervous system (ENS) • Sensory neurons • Interneurons • Motor neurons • ENS • Innervated primarily by parasympathetic division of ANS • Sympathetic postganglionic fibers • The mucosa • The myenteric plexus (Auerbach’s plexus) • The Serosa • Serous membrane covering muscularis externa • Except in oral cavity, pharynx, esophagus, and rectum • Where adventitia, a dense sheath of collagen fibers, firmly attaches the digestive tract to adjacent structures • The Movement of Digestive Materials • By muscular layers of digestive tract • Consist of visceral smooth muscle tissue • Along digestive tract • Has rhythmic cycles of activity • Controlled by pacesetter cells • Cells undergo spontaneous depolarization • Triggering wave of contraction through entire muscular sheet • Pacesetter Cells • Located in muscularis mucosae and muscularis externa • Surrounding lumen of digestive tract • Peristalsis • Consists of waves of muscular contractions • Moves a bolus along the length of the digestive tract • Peristaltic Motion • Circular muscles contract behind bolus • While circular muscles ahead of bolus relax • Longitudinal muscles ahead of bolus contract • Shortening adjacent segments • Wave of contraction in circular muscles • Forces bolus forward • Segmentation • Cycles of contraction • Churn and fragment the bolus • Mix contents with intestinal secretions • Does not follow a set pattern • Does not push materials in any one direction • Control of Digestive Functions • Local Factors • Prostaglandins, histamine, and other chemicals released into interstitial fluid • May affect adjacent cells within small segment of digestive tract • Coordinate response to changing conditions • For example, variations in local pH, chemical, or physical stimuli • Affect only a portion of tract • Neural Mechanisms • Control • Movement of materials along digestive tract • Secretory functions • Motor neurons • Control smooth muscle contraction and glandular secretion • Located in myenteric plexus • Neural Mechanisms • Short reflexes • Are responsible for local reflexes • Control small segments of digestive tract • Operate entirely outside of CNS control • Sensory neurons • Motor neurons • Interneurons • Long reflexes • Higher level control of digestive and glandular activities • Control large-scale peristaltic waves • Involve interneurons and motor neurons in CNS • May involve parasympathetic motor fibers that synapse in the myenteric plexus • Glossopharyngeal, vagus, or pelvic nerves • Hormonal Mechanisms • At least 18 peptide hormones that affect: • Most aspects of digestive function • Activities of other systems • Are produced by enteroendocrine cells in digestive tract • Reach target organs after distribution in bloodstream The Oral Cavity • Functions of the Oral Cavity 1. Sensory analysis • Of material before swallowing 2. Mechanical processing • Through actions of teeth, tongue, and palatal surfaces 3. Lubrication • Mixing with mucus and salivary gland secretions 4. Limited digestion • Of carbohydrates and lipids • Oral Mucosa 1. Lining of oral cavity 2. Has stratified squamous epithelium 3. Of cheeks, lips, and inferior surface of tongue • Is relatively thin, nonkeratinized, and delicate 4. Inferior to tongue is thin and vascular enough to rapidly absorb lipid- soluble drugs 5. Cheeks are supported by pads of fat and the buccinator muscles • Labia 1. Also called lips 2. Anteriorly, the mucosa of each cheek is continuous with that of the lips • Vestibule 1. Space between the cheeks (or lips) and the teeth • Gingivae (Gums) 1. Ridges of oral mucosa 2. Surround base of each tooth on alveolar processes of maxillary bones and mandible • The Tongue 1. Manipulates materials inside mouth 2. Functions of the tongue • Mechanical processing by compression, abrasion, and distortion • Manipulation to assist in chewing and to prepare material for swallowing • Sensory analysis by touch, temperature, and taste receptors • Secretion of mucins and the enzyme lingual lipase • Salivary Glands 1. Three pairs secrete into oral cavity • Parotid salivary glands • Sublingual salivary glands • Submandibular salivary glands 2. Each pair has distinctive cellular organization • And produces saliva with different properties
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