Digestion Histology and Upper GI Tract GI Functions Overview o Mouth mechanical and chemical breakdown deglutition swallowing o Pharynx and esophagus swallowing and avoiding airway o Stomach mechanical and chemical breakdown with some absorption o Small intestine chemical and mechanical breakdown with major absorption o Large intestine absorption of electrolytes and some vitamins o Rectum and anus defecation Layers of the GI Tract Mucosa o Epithelium Stratified squamous in mouth esophagus and anus Simple columnar in rest of GI tract Secretes enzymes and absorbs nutrients Specialized cells goblet secrete mucous onto cell surfaces Enteroendocrine cells secrete hormones controlling organ function o Lamina propria Thin layer of loose connective tissue Contains blood vessels and lymphatic tissue o Muscularis mucosae Thin layer of smooth muscle causes folds to form in mucosal layer Drives local movements to increase absorption by exposing ingested material to absorptive surfaces Layers of the GI Tract Submucosa o Loose connective tissue Contains blood vessels glands and lymphatic tissues o Submucosal plexus Meissner s Plexus Part of enteric nervous system Receives input from sympathetic thoracic and lumbar spinal cord and parasympathetic neurons vagus nerve and sacral spinal cord Regulates blood vessel diameter secretion from glands and neurosecretory neurons and local motility caused by muscularis mucosae o Layers of the GI Tract Muscularis Externa Skeletal muscle in mouth pharynx upper esophagus and anus is under voluntary control Allows control over deglutition and defecation Smooth muscle under involuntary control is located in Inner circular fibers and outer longitudinal fibers Allows motility for mixing and propulsion o Myenteric Plexus Auerbach s Plexus Part of the Enteric Nervous System ENS with some functions shared with submucosal plexus Provides parasympathetic and sympathetic innervation of circular and longitudinal smooth muscle layers Layers of GI Tract Serosa or Adventitia o Serosa for stomach and intestines A serous membrane also called the visceral peritoneum Secretes serous fluid Consists of areolar connective tissue covered with simple squamous epithelium o Adventitia for esophagus Consists of areolar connective tissue without the epithelium Peritoneum o Visceral peritoneum covers organs o Parietal peritoneum lines walls of body cavity Peritoneal cavity o Potential space containing serous fluid Digestion in the mouth o Mechanical Digestion mastication or chewing Breaks food into pieces Mixes with saliva to form a moist bolus o Chemical Digestion enzymatic Salivary amylase begins starch digestion in the mouth pH 6 5 7 0 but when the bolus hits the acidic gastric juices pH 2 5 digestion stops Lingual lipase is secreted by glands in tongue and begins breakdown of triglycerides lipids into fatty acids and glycerol Major glands parotid submandibular and sublingual glands All have ducts that empty into the oral cavity Sight smell sounds memory of food and tongue stimulation increase o Salivary Glands salivation o Saliva Fear and anxiety decrease salivation dry mouth Water HCO 3 enzymes Moistens food swallow Dissolves food taste HCO3 buffers acidic foods Protects mouth from infection with rinsing action Lysozyme helps destroy bacteria o Phases of swallowing deglutition Voluntary phase Oral cavity to oropharynx Food bolus pushed by the tongue into the oropharynx Involuntary phases Pharyngeal stage pharynx to esophagus o Being when sensory nerves in the pharynx signal the deglutition center in brainstem Breathing stops briefly Soft palate and uvula rise closing off nasopharynx Larynx rises Epiglottis covers the larynx Esophageal stage esophagus to stomach o Upper esophageal sphincter relaxes o Peristalsis pushes food down Circular fibers contract behind bolus Longitudinal fibers contract in front of bolus to shorten the distance of travel o Travel time 4 8 seconds for solids 1 second for liquids o Lower esophageal sphincter relaxes as food approaches Gastro Esophageal Reflux Disease GERD o If lower sphincter fails to open Distension of esophagus feels like chest pain or heart attack o If lower esophageal sphincter fails to close Stomach acids enter esophagus and cause heartburn GERD Worsened by a weak sphincter Particularly bad when laying down after large meal Smoking and alcohol make the sphincter relax worsening situation