Pt2

• Alimentary canal


  Performs the full range of digestive functions as it moves food along its length

  
• Accessory organs


  Support digestion in different ways

  
• Functions of the digestive system


  • Ingest the food
  • Break food down into small molecules that can cross plasma membranes
  • Absorb these nutrient molecules
  • Eliminate nondigestible wastes

  
• Layers of the GI tract


  • Mucosa
  • Submucosa
  • Muscularis
  • Serosa
• Mucosa


  The inner lining of the digestive canal

  
• Components of the mucosa


  • Epithelium
  • Lamina propria
  • Muscularis mucosae
• Epithelium


  Nonkeratinized stratified squamous epithelium, simple columnar epithelium, exocrine cells, enteroendocrine cells
• Lamina propria


  Layer of areolar connective tissue that supports the epithelium and binds it to the muscularis mucosae
• Muscularis mucosae


  Smooth muscle fiber that increases the surface area for digestion and absorption
• Submucosa


  Thick connective tissue layer that binds mucosa to the muscularis and contains blood, lymphatic vessels, glands, lymphatic tissue
• Muscularis


  Skeletal muscle for voluntary swallowing and smooth muscle for involuntary digestive canal motility
• Serosa


  Also known as the visceral peritoneum, composed of areolar connective tissue and simple squamous epithelium
• Neural innervation


  Controlled by the central nervous system and regulated by the enteric nervous system and autonomic nervous system
• Enteric nervous system


  Known as the "brain of the gut", can operate independently, consists of 100 million neurons from the esophagus to the anus

  
• Enteric nervous system components


  • Plexus of Auerbach/Myenteric Plexus
  • Plexus of Meissner/Submucosal Plexus
  • Enteric sensory neurons
  • Enteric motor neurons
  • Enteric interneurons

  
• Autonomic nervous system


  Regulates the enteric nervous system, parasympathetic nerves increase digestive canal secretion and motility, sympathetic nerves decrease digestive canal secretion and motility

  
• Digestive canal reflex pathway


  1. Stimuli from digestive tract

  2. Sensory neurons carry action potentials via vagus nerve to central nervous system

  3. Effect: increased salivation, pancreatic secretions, altered digestive canal secretion and motility

  
• Hunger


  Stretch receptors in GI tract inhibit appetite when distended, ghrelin released when blood sugar is low causes sensation of hunger, high concentrations of ghrelin, peptide, and leptin cause sensation of being full
• Peritoneum


  • Largest serous membrane in the body, consists of parietal peritoneum and visceral peritoneum, contains peritoneal cavity with lubricating serous fluid, retroperitoneal organs
• Major peritoneal folds


  • Greater omentum
  • Falciform ligament
  • Lesser omentum
  • Mesentery
  • Mesocolon
• Ingestion


  Active involuntary process of taking foods and liquids into the mouth before they can be acted on
• Propulsion


  Alternating contractions and relaxations of smooth muscle in the walls of the GI tract to propel food from one organ to the next
• Digestion


  1. Cephalic phase

  2. Gastric phase

  3. Intestinal phase

  
• Cephalic phase


  Taste, smell, or thought of food in the mouth stimulate the medulla oblongata, vagus nerves carry parasympathetic action potentials to the stomach to activate enteric plexus neurons and stimulate secretion

  
• Gastric phase


  Distension of the stomach stimulates mechanoreceptors, activating a parasympathetic reflex that increases stomach secretions, local reflexes also increase secretions, gastrin and histamine further stimulate secretion

  
• Intestinal phase


  Chyme in the duodenum with low pH or containing lipids inhibits gastric secretions by stimulating chemoreceptors, local reflexes, and secretion of secretin and cholecystokinin
• Digestive enzymes


  • Amylase
  • Proteases
  • Lipases
  • Nucleases
  • Peptidases
• Amylase


  Breaks down carbohydrates into simpler sugars, found in saliva and pancreatic juice
• Proteases


  Break down proteins into amino acids, examples are pepsin, trypsin, chymotrypsin, carboxypeptidase
• Lipases


  Break down fats into fatty acids and glycerol, found in saliva, gastric, and pancreatic juices
• Nucleases


  Break down nucleic acids into nucleotides in the small intestine, include ribonuclease and deoxyribonuclease
• Peptidases


  Break down peptides into individual amino acids in the small intestine, include aminopeptidase and dipeptidase
• Gastrin


  Promotes secretion of gastric juice, increases gastric motility, promotes growth of gastric mucosa, constricts lower esophageal sphincter and relaxes pyloric sphincter
• Secretin


  Inhibits secretion of gastric juice, stimulates secretion of pancreatic juice and bile rich in bicarbonate, promotes normal growth and maintenance of the pancreas, enhances effects of CCK
• Cholecystokinin (CCK)


  Increases enzyme flow in the pancreas, stimulates secretion of pancreatic juice, causes ejection of bile from the gallbladder, induces satiety, inhibits gastric emptying

  
• Villi


  Fingerlike projections of the mucosa that increase the surface area of the epithelium available for absorption and digestion

  
• Plasma membrane


  The outer membrane of a cell that separates the cell from its external environment
• Hormones in digestion


  • Gastrin
  • Secretin
  • Cholecystokinin (CCK)
• Gastrin


  Helps promote the secretion of gastric juice in the stomach, increases gastric motility, promotes the growth of gastric mucosa, constricts the lower esophageal sphincter and relaxes the pyloric sphincter
• Secretin


  Inhibits the secretion of gastric juice and stomach motility, stimulates secretion of pancreatic juice and bile rich in bicarbonate ions, promotes normal growth and maintenance of the pancreas, enhances the effects of CCK