PSIO 202 Digestive System Study Guide 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 Layers of the GI Tract Submucosa o Loose connective tissue 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 o Lamina propria function 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 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 o Salivary Glands 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 salivation Fear and anxiety decrease salivation dry mouth o Saliva 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 o SM modulates luminal pressure tension o SM tissue is present in sheets bundles or sheaths around tissues Stomach Smooth muscle SM Cardiovascular system Respiratory system Digestive system Urinary system Reproductive system Innervation and stimulation of SM o Primarily under control of ANS involuntary Diffuse branching of nerve fibers Transmitters include ACh and NE Membrane potential 50 to 60 mV o Smooth muscle cell types categorized based on excitation Single unitary visceral Multi unit Unitary SM cells o Walls of the digestive tract gall bladder urinary bladder o May not have direct contact with any motor neuron o Autorhythmic Adjacent cells connect via gap junctions transmission of APs from one fiber to another o Cell membranes adhere to one another transmission of force o Slow synchronized graded contraction o Entire sheet of muscle contracts Functional syncytium Structure of SM o Sarcoplasmic reticulum is poorly developed relative to skeletal muscle o Thick filaments of SM have actin gripping heads along their entire length o Thick and thin filaments are arranged diagonally within the cell contraction results in twisting motion Special features of SM o Capable of sustained contraction without fatigue and at very little energy o Maintains a low level of tension or tone even in the absence of action cost potentials o Stress relaxation response Stretch causes initial increase in tension tension decreases within 1 2 minutes allows SM to change length but maintain ability to contract important for storage organs Length Tension relationship o Smooth muscle operates over a wider range of resting lengths o Generally broader length tension relationship in smooth muscle compared to skeletal muscle Stomach Anatomy o Parts of the Stomach Cardia Fundus Body Pylorus Pyloric Sphincter o Stomach filling triggers secretions and motility o The stomach empties slowing into the duodenum as small squirts of chyme leave through the pyloric sphincter Mechanical digestion o Gentle mixing waves occur in the stomach to mix the bolus