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BIOL 204 ~ Topic Digestion Ferrara 8 Objective 1 Functions: • ingest food • alter food • absorption • eliminate waste Functions of the Digestive System Objective 2 The Structural Plan The digestive system consists of: •a tube, alimentary canal or gastrointestinal (GI) tract •accessory structures which secrete products into the GI tract Note: Salivary Glands: secrete saliva into the oral cavity Pancreas: secretes pancreatic juice into the duodenum Liver, Gall Bladder: secrete bile into the duodenum Objective 3 Terms For Physiology Ingestion: taking food into the body Propulsion: moving food within the digestive tract; most of this movement is accomplished by deglutition (swallowing) and peristalsis Peristalsis Mechanical Digestion: breaking down ingested material using grinding or crushing forces, such as mastication (chewing) and segmentation Segmentation Chemical Digestion: breaking down ingested material by hydrolysis specific enzymes are required Sucrase Sucrase + H2O Glucose + Fructose Lipase Lipid + H2O Glycerol + Fatty Acids Absorption: the transport of digested end products across the wall of the GI tract into the blood or lymph Defecation elimination of solid, unabsorbed waste products from the body Study suggestion For each portion of the GI tract list if there is any: • mechanical digestion • chemical digestion (what is digested, what enzymes) • absorption Something like this….. Objective 4 How Digestive Processes Are Regulated Various control mechanisms are used to regulate the type of chemical and mechanical activities that occur in the GI tract and the rate at which the processes proceed. The control mechanisms are sensitive to the volume and composition of lumenal contents. What is regulated: motility and secretions Receptors found in the wall of the GI tract respond to stimuli Mechanoreceptors: monitor the stretch of the wall of the GI tract Chemoreceptors: monitor the chemistry of the components; solute concentration (osmolarity), pH, presence of complex nutrients and end products of digestion Reflexes: may lead to stimulation or inhibition of the GI tract and its associated glands Short Reflexes: No CNS involvement; Controlled by intrinsic nerve plexuses (enteric nervous system) and hormones produced by enteroendocrine cells Long Reflexes: involves CNS Objective 5 Falciform Ligament: The Peritoneum binds the liver to the anterior abdominal wall and the diaphragm Falciform Ligament Greater Omentum: attaches to the greater curvature of the stomach Lesser Omentum: lies between the lesser curvature of the stomach and the liver Greater Omentum Lesser Omentum Mesentery: attaches the small intestine to the posterior abdominal wall Mesocolon: attaches the transverse colon to the posterior abdominal wall Mesentery Other terms: Retroperitoneal structures: lie posterior to the peritoneum; includes kidneys, pancreas, portions of the large intestine Intraperitoneal structures: lie within the peritoneum Infraperitonal structures: lie below the peritoneum; includes urinary bladder and uterus Retroperitoneal Infraperitoneal Objective 6 GI Tract Tunics Tunica Mucosa -innermost layer Sublayers: Epithelium Lamina propria Muscularis mucosae Functions: -lines the lumen -secretes mucus, enzymes -lymph nodes protect -muscle contracts and creates folds that increase surface area in some places Tunica Submucosa external to the mucosa Contains abundant blood vessels, collagenous and elastic fibers, lymph nodes and nerves (submucosal plexus) and in some places, glandular cells Functions to nourish the wall of the GI tract; the submucosal plexus regulates the muscular and glandular activity of the mucosa Submucosal plexus (at the arrow) regulates the muscular and glandular activity of the mucosa Tunica Muscularis external to the mucosa Consists of smooth muscle, usually arranged in two layers (outer, longitudinal; inner circular) The myenteric plexus is a group of nerve fibers that is located between the circular and the longitudinal layers Functions include segmentation and peristalsis; the circular muscle thickens in some places, forming valves called sphincters The myenteric plexus controls the motility of the GI tract Myenteric Plexus – between the circular and the longitudinal smooth muscle layers motility of the GI tract Tunica Serosa: outermost layer Below the diaphragm is equivalent to the visceral peritoneum (mesothelium covering loose areolar tissue) Above the diaphragm is called the tunica adventita and is made of dense, irregular connective tissue Function is protection Objective 7 The Oral (Buccal) Cavity Boundaries of the Oral Cavity: Anterior: lips (labia) Lateral: cheeks Inferior: tongue Superior: hard palate anteriorly and soft palate posteriorly Posterior: fauces (opening into the oropharynx) Subdivisions of the Mouth: Vestibule: Oral Cavity : Proper area between the labia and the anterior surfaces of the teeth area between the posterior surfaces of the teeth and the fauces Structures Associated With The Soft Palate: Uvula: fingerlike projections extending down from the free edge of the palate Palatoglossal Arches: lateral arches that anchor the soft palate to the tongue Palatopharyngeal Arches: lateral arches that anchor the soft palate to the wall of the oropharynx; represent the boundary between the oral cavity and the oropharynx; contain the palatine tonsils Objective 8 Location: floor of the oral cavity Functions: • contains taste buds • contains mucus and serous glands • during mastication, grips food, mixes it with saliva and compacts the food into a bolus • used to articulate speech The Tongue Structure: interlacing bundles of skeletal muscle fibers covered by a mucus membrane it is anchored to the floor of the mouth by lingual frenulum Intrinsic Muscles of the Tongue: Originate and insert in the tongue Functions are to change the shape of the tongue (speech, deglutition) Extrinsic Muscles of the Tongue: Originate outside of the tongue on the mandible, hyoid bone, or temporal bone and insert on the tongue The function to move the tongue (protraction, retraction, elevation, depression, lateral movement) Papillae are elevations on the tongues dorsal surface: Filiform papillae: most numerous; are shaped like cones; scattered all over the surface of the tongue; used to provide friction Fungiform papillae are shaped like mushrooms and have a vascular core; they are scattered all over contain taste buds Circumvallate (vallate) papillae are the largest and most distinct; there are 10-12 of them located in a “V” formation at the back of the tongue; contain taste buds Filiform papillae Circumvallate papillae Fungiform papillae Taste Buds Objective 9 Salivary and Buccal Glands Saliva: secretion produced by extrinsic salivary glands and intrinsic salivary glands (buccal glands) Functions: 1. Cleanse the mouth 2. Solubilize chemicals in the food so that they can activate taste buds 3. Moistens food; makes it sticky so that it can form a bolus 4. Contains amylase an enzyme that digests carbohydrates Intrinsic Salivary Glands are located within the oral mucosa and distributed all over the mouth and the pharynx; continuously secrete a small amount of watery saliva Extrinsic Salivary Glands: Parotid Glands, Submandibular Gland, Sublingual Gland, Salivary Gland Tissue Objective 10 Saliva 1. Composition • 97- 99.5% water • Solutes: electrolytes (Na+, K+, Cl-, PO42-, HCO3-) amylase mucin lysozyme IgA wastes (urea, uric acid) serum albumen • pH ranges from 6.75 – 7.00 • Average daily volume 1000-1500 ml 2. Control of Salivation Think about food Note: parasympathetic increases secretion; sympathetic decreases Smell food Activate chemoreceptors or mechanoreceptors in the mouth See Food Presence of Irritants in the stomach and/or small intestine Salivatory Nuclei (Lower Pons, Upper Medulla) PNS Efferents in Facial (VII) and Glossopharyngeal (IX) nerves Increased Saliva Output Objective 11&12 Tooth Structure crown root neck enamel dentin cementum periodontal ligament root canal apical foramen Central incisors Lateral incisors Canines (Cuspids) Premolars (Bicuspids) Molars Create a dental formula for adult dentition: 2I, 1C, 2PM, 3 M (upper jaw) X2 2I, 1C, 2PM, 3 M (upper jaw) Objective 13 The Pharynx Location: posterior to the oral cavity Structure: mucosa: stratified squamous epithelium muscularis: inner longitudinal and outer circular muscle; skeletal muscle Objective 14 The Esophagus Location: in the mediastinum posterior to the trachea extends from the pharynx to the stomach Structure: mucosa is stratified squamous epithelium muscularis: upper third, skeletal muscle; mixture of skeletal and smooth muscle and lower third, smooth muscle Functions: move food to the stomach via peristalsis Objective 15 A. Digestive Processes Mouth/Pharynx/Esophagus Chemical Digestion ; Salivary amylase begins the digestion of carbohydrate: Salivary Amylase Starch, glycogen Dextrin Mastication (Chewing) • lips and cheeks help keep food between the teeth • tongue mixes food with the saliva • teeth grind and crush food into smaller pieces (helps prevent damage to the GI tract and increases the surface area for digestion) Deglutition (Swallowing) – 3 phases • Buccal • Pharyngeal • Esophageal 1. Oral (Buccal) Phase: Voluntary phase Tip of the tongue is raised against the hard palate Tongue contracts to put pressure on the bolus Bolus is forced into the pharynx Coordinated by the trigeminal nerve (V) 2. Pharyngeal/Esophageal Phase involuntary; coordinated by the swallowing centers in the upper medulla and pons and activated by the vagus nerve (X) Pharyngeal Phase Esophageal Phase bolus moves into the esophagus bolus moves into the esophagus the soft palate elevates, and larynx rises (close off openings) peristalsis: longitudinal/circular muscle contraction relaxation of the upper esophageal sphincter relaxation of the lower esophageal sphincter and cardiac sphincter pharyngeal muscles contract on bolus How is the mouth blocked? tongue How is the opening to the nasopharynx blocked? Soft palate How is the glottis blocked? epiglottis Objective 16 Location: The Stomach left abdominopelvic cavity, largely in the left hypochondriac region and epigastric region Gross Structure: volume ranges from 50 ml to 4L it is about 25 cm long Stomach Regions: Cardiac, fundus, body, pyloris Stomach Curvatures: Greater (lateral), lesser (medial) Stomach Sphincters: Cardiac (gastroesophageal), pyloric Rugae: macroscopic folds in an empty stomach Objective 17 Stomach Histology The stomach has four tunics: A. Tunica Mucosa: Gastric pits: invaginations which contain specialized secretory cells (gastric glands) Mucous Neck Cells: secrete a thin layer of protective mucus Parietal Cells: secrete HCL (activates pepsinogen and helps prevent bacterial infection) and intrinsic factor Mechanism Used to Make HCl: CO2 HCO3 Cl- CO2 + H2O carbonic anhydrase H2CO3 HCO3- H+ H+ K+ K+ Cl- Chief Cells: secrete pepsinogen, an inactive precursor of pepsin; pepsin initiates protein digestion: HCl Pepsinogen Pepsin Pepsin Protein Peptides Enteroendocrine Cells: secrete gastrin, serotonin, histamine and somatostatin Tunica Submucosa: typical Tunica Muscularis: three layered – outer longitudinal, middle circular, inner oblique Tunica Serosa:visceral peritoneum Objective 18 Secretory Activity of the Stomach Gastric Juice: •average daily volume = 2 to 3 liters •includes water, HCl, mucus, intrinsic factor and enzymes renin: converts milk protein (casein) into a solid curd in infants lipase: digests lipid; not very active in adults pepsin: initiates the digestion of proteins (note protein digestion begins in stomach) What is the role of HCl? Regulation of Gastric Secretion: Gastric secretion is controlled by hormones and neural pathways using both short and long reflexes Stimulus is either volume or composition of chyme Response is either motility (peristalsis and segmentation) or secretion of enzymes or juices Autonomic innervation of the stomach is both sympathetic and parasympathetic Phases of gastric secretion: 1. Cephalic Phase: occurs before food reaches the stomach optic nerve (?) olfactory receptors taste buds cerebral cortex see food smell food taste food think about food hypothalamus vagal nuclei of the medulla vagus nerve (X) increased secretion by gastric glands Does this ring a bell? 2. The Gastric Phase: occurs after the bolus reaches the stomach 3. The intestinal Phase: occurs when chyme reaches the small intestine Objective 19 Stomach Mechanical Activity of the - the three layered tunica muscularis causes directional movement (peristalsis) and mixes chyme with gastric juice 1. Stomach Filling: - as food is moved toward the stomach during deglutition, the stomach smooth muscle relaxes (receptive relaxation); this is triggered by the swallowing center of the medulla and is accomplished by vagal efferents swallowing center vagus (X) stomach relaxes - when the bolus reaches the stomach, the stomach wall stretches; the pressure of the food on the stomach wall activates a local reflex in which the neurons use the neurotransmitter NO; this causes relaxation of the stomach smooth muscle (receptive relaxation) NO causes relaxation bolus stretches the stomach 2. Contractile Activity - Mixing - peristalsis is weak in the fundus and the cardiac region and stronger in the pyloris - peristaltic waves are maintained by pacemaker cells in the longitudinal smooth muscle layer of the greater curvature - each wave of peristalsis closes the pyloric sphincter - Contractile Activity - Emptying - peristaltic waves get stronger with time; when the pressure in the stomach is very high, it can exceed the ability of the pyloric sphincter to remain closed and chyme enters the duodenum 4. Control of Gastric Emptying - the duodenum and the stomach work together to control stomach emptying - liquids pass faster than solids and carbohydrates pass faster than proteins which pass faster than lipids - gastric factors: - - stretching of the stomach wall activates stretch receptors which induce local and vagovagal reflexes; these reflexes increase gastric activity - low acidity and the presence of peptides initiates the release of gastrin by G cells; gastrin increases stomach motility intestinal factors: - both neural mechanisms (the enterogastric reflex) and hormonal factors (enterogastrones) are used by the small intestine to control the rate of gastric emptying What are the hormones and what do they do, i.e. stimulate or inhibit gastric emptying?