Digestive System

Digestive System

1. Layers of Digestive Tract

   • Mucosa(most superficial layer):
     • oral cavity: lined by stratified squamous
     • Stomach, small/large intestines: lined by simple columnar epithelium+goblet cells+enteroendocrine cells (which secrete hormones, break down foods and protects from acids)
     • Submucosa
       • dense irregular CT with blood vessels, lymphatic exocrine glands(secrete buffers and enzymes
         • submucosa plexus: outer layer of submucosa has sensory neurons, PSns ganglionic neurons, and sympathetic postgangionic fibers, innervate mucose and submucosa
       • Muscular Externa
         • smooth muscle cells with inner circular layer and outer longitudinal layer for mechanical processing and movement of materials
         • Serosa
           • (visceral peritoneum) serous membrane covering most digestive tract(covering the muscularies externae of the stomach, small/large intestines)

2. Oral Cavity

   • Oral mucosa lines oral cavity
     • stratified squamous, some keratinized cells in areas of abrasion such as the superior surface
     • occlusal
       • opposing surfaces of teeth
       • Periodontal ligament
         • collagen fibers extend from dentin of the root to the bone of the alveolus (bony socket where tooth sits)
         • Dentin
           • Mineralized matrix inside the tooth, you see enamel
           • Crown
             • exposed portion of tooth, you see enamel
             • Enamel
               • Calcium phosphate compound-hardest biological substance. Need calcium, phosphates, and Vitamin D during childhood to build enamel
               • Root
                 • Base of tooth, blood vessels and nerves enter the root canal through apical foramen
                 • Cementum
                   • Layer of bony substance that covers the dentin in the root region and anchors the tooth to the periodontal ligament

3. Swallowing (deglutition)

   • Buccal Phase
     • Bolus is pressed against the hard palate. The tongue retracts and pushes bolus into the oropharynx ( the soft palate lifts to isolate the nasopharynx). The buccal phase is all voluntary control.
     • Pharyngeal Phase
       • The swallowing reflex is initiated by stimuli to the palatal arches' receptors of the uvula (signals swallowing center in medulla oblongata). Larynx elevates and the epiglottis folds over the glottis so the bolus bypasses entry to the trachea
       • Esophageal Phase
         • Pharyngeal muscles contract and push bolus down the esophagus to the stomach

4. Four Regions of the Stomach are:

   • Cardia
     • smallest part of stomach, lots of mucous glands that coat the connection with the esophagus with alkaline mucous
     • Fundus
       • Superior to the junction of esophagus and stomach. The fundus contacts the diaphragm
       • Body
         • Largest region of stomach, mixing tank, gastric glands secrete most of the stomachs acids and enzymes
         • Pylorus
           • Curve of the J, empties into the duodenum. glands secrete mucus and hormones (gastrin)

5. Gastric Secretion Phases

   • Cephalic Phase
     • Stimuli ( smell of foods, thoughts of foods, sight of foods) trigger parasympathetic neurons. In response to stimulation the rate of gastric juice increases. this phase lasts for minutes
     • Gastric Phase
       • Stimuli (distention of stomac,) increase of pH in stomach, the presence of undigested material in the stomach) signal neural, hormonal, and local responses. This phase lasts for 3-4 hours
       • INTESTINAL PHASE
         • Stimuli is chyme arriving in the duodenum. Involves a neural response ( enterogastric reflex), and a hormonal response (secretion of CCK), gastric inhibitory peptide (GIP) and secretin).
           • results are inhibit gastric production , inhibit pepsinogen production, and reduce stomach contractions. This phase lasts for hours.

6. Subdivions of the small intestines

   • Duodenum
     • most proximal segment. Functions as a mixing bowl that mixes chyme and digestive secretions from the pancreas and liver, and neutralizes acids. Has few plicae and small villi
     • Stimulus of the chyme stimulates secretion of secretin which tells the pancreas to secrete pancreatic juice which alkalizes. CCK is released and increases production and secretion of pancreatic enzymes
     • Jejunum
       • Bulk of chemical digestion and nutrient absorption. Plicae and villi are more predominant. So if you cut out part of this segment you willl lose weight due to decreased absorption
       • Ileum
         • longest segment. ends at ileocecal valve which controls flow of materials from ileum to cecum. Distal ileum lack plicae(circular folds)

7. Pancreatic Enzymes

   • Carbohydrases ( ex: pancreatic-alpha-amylase)
     • Similar to salivary amylase, breaks down certain starches
     • Pancreatic Lipase
       • breaks down certain complex lipids, release fatty acids that can be absorbed
       • Nucleases
         • Break down of nucleic acids (DNA/ RNA) protects from some viral and bacterial species
         • Proteolytic Enzymes
           • Breakdown certain proteins. Majority of pancreatic enzymes are these which get activated in intestines
             • Proteases- enzymes that digest large protein complexes
             • Peptidases- enzymes that digest peptide chains in amino acids

8. Hepatitis Viruses

   • Hep A
     • Hep B
       • Hep C
         • Hep D
           • Hep E
             • Food Born
             • Rare in US only-acute infections lethal to pregnant women
           • Blood Born
           • in combo with hep B causes severe liver disease , liver cancer or failure
         • Blood Born
         • chronic cases can cause liver disease , liver cancer or failure, interferon and antiviral drugs slow the progression of the diease
       • Blood Born
         • Vaccine
       • chronic cases get cumulative liver damage, increase risk of liver cancer or fatal liver failure, interferon and antivarl drugs slow the progression of the disease
     • Food Born
       • Vaccine

9. Intestinal Hormones(endocrine)

   • Enterocrinin
     • Secretin
       • Cholecystokinin (CCK)
         • Gastric Inhibitory Peptide (GIP)
           • Vasoactive Intestinal Peptide (VIP)
             • Gastrin
               • made by G cells in duodenum as well as produced in the stomach. (+) by CN X, chyme in intestines or food in stomach. (+) production of acids, enzymes, and increases gastric motility
             • (+) by chyme in duodenum . (+) buffer secretion of intestines, inhibits acid production in the stomach. Dilates capillaries in stomach to increase absorption
           • (+) by fats and carbs entering duodenum. (+) lipid synthesis of adipose tissue and (+) glucose use in skeletal muscles. Increases release of insulin and inhibits gastric secretion and motility
         • (+) by chyme in duodenum (especially lipids and partial digest proteins) Decreases Hunger
       • (+) by chyme entering the duodenum. tells liver to secrete more buffers and secondary effect is the decrease of gastric motility and secretory rates of the stomach
     • Stimulated by chyme entering the duodenum. Stimulates duodenum submucosa glands to secrete alkaline mucus
   • Produced by duodenal enteroendocrine cells