Digestive System (GIS) Cephalic & Gastric Phase PDF

DIGESTIVE SYSTEM GASTROINTESTINAL SYTEM (GIS) CEPHALIC & GASTRIC PHASE 1. Explain secretory changes that occur on different segments of the gastrointes9nal tract during cephalic, gastric, and intes9nal phases. 2. Explain the cephalic phase s9mulus such as thinking about the consump9on of food, olfact...

DIGESTIVE SYSTEM GASTROINTESTINAL SYTEM (GIS) CEPHALIC & GASTRIC PHASE 1. Explain secretory changes that occur on different segments of the gastrointes9nal tract during cephalic, gastric, and intes9nal phases. 2. Explain the cephalic phase s9mulus such as thinking about the consump9on of food, olfactory, visual inputs and auditory inputs effects on secre9on from the glandular cells. 3. Explain how the brain centers (limbic system, hypothalamus, cortex) influence secre9on and the mo9lity of the GI tract 4. Explain the structures of the salivary glands, and their secretory elements; primary and secondary steps of secre9on 5. Explain why the saliva is hypotonic comparing to plasma. 6. Define the regula9on of saliva under both parasympathe9c and sympathe9c s9mula9on. 7. Give example for the effect of the impaired salivary secre9on; Sjögren's syndrome. 8. Define working mechanisms of upper and lower esophageal sphincters. 2 9. Explain swallowing mechanism and the peristaltic movement of esophagus (primary and secondary peristalsis). 10. Give examples with the paralysis of the swallowing mechanism; under anesthesia, damage to cranial nerves or muscular dystrophies. 3 ACCESORY ORGANS DIGESTIVE TRACT (ALIMENTARY) CANAL 4 Digestion in the mouth (chewing-mastication) § teeth § tongue (hypoglossal nerve – CN XII) § chewing muscles (trigeminal nerve – CN V) Chewing has both voluntary and involuntary components. The involuntary component: Pressure of food in mouth elicits reflexive chewing actions coordinated by medulla (lateral medullary and pontine reticular formation) Voluntary chewing can override involuntary or reflex chewing at any time. CHEWING (1) mixes food with saliva, lubricating it to facilitate swallowing (formation of bolus) (2) reduces the size of food particles, which facilitates swallowing (3) mixes ingested carbohydrates with salivary amylase to begin carbohydrate digestion. Salivary Glands: A salivary gland has two types of secretory cells: Serous cells produce a watery fluid that includes the digestive enzyme salivary amylase (ptyalin). Mucous cells secrete a thick liquid called mucus, which binds food particles and lubricates the food during swallowing. When a person sees, smells, tastes, or even thinks about food, parasympathetic nerve impulses elicit the secretion of watery saliva. Parotid glands secrete watery fluid that is rich in salivary amylase. Submandibular glands secrete a more viscous fluid than the parotid glands. Sublingual glands: secretory cells are primarily the mucous type, making their secretions thick 7 SALIVA Main chemical features 800-1500 ml/day pH : 6-7 High [HCO3- ] High [K+] α-Amylase and lingual lipase Stimulus Parasympathetic discharges on salivary glands àprominent increase in secretion, by stimulation of glandular cells and vasodilation in the tissue. Sympathetic discharges à slight increase with higher mucin content. Physiology 5th Edition, by Linda S. Costanzo 9 The acinar cells (at the acinus end of the duct) secrete the initial saliva, which is isotonic and has approximately the same electrolyte composition as plasma. Then ductal cells modify the initial saliva. absorption of Na+ and Cl- and secretion of K + and HCO3-. Physiology 5th Edi7on, by Linda S. Costanzo When the myoepithelial cells are stimulated they contract to eject saliva into the mouth. 10 Relationship between the composition of saliva and the salivary flow rate Physiology 5th Edition, by Linda S. Costanzo At high flow rates, the ductal cells have less time to modify the saliva 11 The acinar cells secrete: Alpha-amylase: begins the initial digestion of starch (only 5% of starch is hydrolized) in mouth and becomes deactivated in stomach Starch α - amylase Maltose Small polymers of glucose Lingual lipase: optimal pH 4-5. Lingual lipase is more important for infants and patients with pancreatic deficiency Mucin glycoproteins Kallikrein (an enzyme that cleaves high-molecular-weight kininogen into bradykinin, a vasodilator) IgA, lysozyme The most potent stimuli acidictasting substances e.g. citric acid Facial nerve Glossopharyngeal nerve Physiology 5th Edition, by Linda S. Costanzo Saliva secretion is stimulated by both parasympathetic and sympathetic nervous systems (although parasympathetic control is dominant). 13 Functions of Saliva Supply an aqueous environment and also mucus that coats the surface of the bolus and thus aids in swallowing. It starts chemical digestion of starch. The aqueous environment permits diffusion of taste molecules to specific receptors on the tongue Salivary secretions also reduce microbial contamination of the oral cavity (antibacterial compounds: thiocyanate ions, antibodies, proteolytic enzymes). Swallowing Mechanism: Swallowing has three stages. 1. The bolus of food or liquid is “voluntarily” rolled posteriorly into the oropharynx by pressure of the tongue upward and backward. 2. Food s@mulates sensory receptors around the pharyngeal opening à swallowing reflex: 1. The soD palate (including the uvula) raises, preven@ng food from entering the nasal cavity. 2. EpigloIs closes off the top of the larynx à food is less likely to enter the trachea. The swallowing reflex momentarily inhibits breathing. 3. The tongue blocks off the mouth. 4. Muscles in the laryngopharynx relax, opening the esophagus. 5. A peristaltic wave begins in the pharyngeal muscles and forces food into the esophagus. 3. Peristalsis transports the food in the esophagus to the stomach. 15 Swallowing (deglutition) center is located in the medulla and lower pons 1.Voluntary Stage 2.Pharyngeal Stage (