NSC 222 Human Physiology II Course Guide PDF

COURSE GUIDE NSC 222 HUMAN PHYSIOLOGY II Course Team Dr. R.O. Akomolafe, Dr. A. O. Ayoka, Dr. O. S. Akinsomisoye (Course Developers/Writers) Mr. O. S. Olukiran (Co-writer) Dr. O.O. Irinoye & Dr T.O. Oladogba (Course Editors)...

COURSE GUIDE NSC 222 HUMAN PHYSIOLOGY II Course Team Dr. R.O. Akomolafe, Dr. A. O. Ayoka, Dr. O. S. Akinsomisoye (Course Developers/Writers) Mr. O. S. Olukiran (Co-writer) Dr. O.O. Irinoye & Dr T.O. Oladogba (Course Editors) - Dr. Umar Zayyanu Usman, MBBS, MSc, PhD. (Reviewer) - Usmanu Danfodiyo University, Sokoto NATIONAL OPEN UNIVERSITY OF NIGERIA NSC 222 COURSE GUIDE © 2022 by NOUN Press National Open University of Nigeria Headquarters University Village Plot 91, Cadastral Zone Nnamdi Azikiwe Expressway Jabi, Abuja Lagos Office 14/16 Ahmadu Bello Way Victoria Island, Lagos e-mail: [email protected] URL: www.nou.edu.ng All rights reserved. No part of this book may be reproduced, in any form or by any means, without permission in writing from the publisher. Printed 2022 ISBN: 978-058-660-X ii NSC 222 COURSE GUIDE CONTENT PAGE General Introduction ……………………………………….. iv Course Aims ………………………………………………. iv Course Objectives ………………………………………….. iv Working through the Course ……………………….……… v Course Materials …………………………………………… v Study Units…………………………………………….……. v Reference Textbooks ………………………………………. vi Equipment and Software Needed to Access Course………... vi Number and Places of Meeting ……………………………... vi Discussion Forum …………………………………………… vii Course Evaluation…………………………………………… vii Grading Criteria ……………………………………………. vii Grading Scale ……………………………………………… viii Schedule of Assignments with Dates ……………………… vii Course Overview………………………………………….… vii How to Get the Most from this Course ……………………. vii iii NSC 222 COURSE GUIDE COURSE GUIDE GENERAL INTRODUCTION Welcome to the second year course in Human Physiology, NSC 222 – Human Physiology II. In the first level course, we learn that Physiology is about how the body does the work of helping us to attain a state of health. We also learnt that the nurse must be adequately grounded in Physiology to help her/him in determining the functionality of the various organs that also work within the systemic framework. The knowledge of normal function is also the basis of diagnosing disturbances of physiological processes. COURSE OBJECTIVES At the completion of this course, you will be able to: Apply the knowledge of respiratory and gastrointestinal physiology in analyzing health and nursing care needs and planning care of patients. WORKING THROUGH THIS COURSE The course will be delivered adopting the blended learning mode, 70% of online but interactive sessions and 30% of face-to-face during laboratory sessions. You are expected to register for this course online before you can have access to all the materials and have access to the class sessions online. You will have hard and soft copies of course materials, you will also have online interactive sessions, face-to-face sessions with instructors during practical sessions in the laboratory. The interactive online activities will be available to you on the course link on the Website of NOUN. There are activities and assignments online for every unit every week. You must visit the course sites weekly and do all assignments to meet deadlines and to contribute to the topical issues that would be raised for everyone’s contribution. You will be expected to read every module along with all assigned readings to prepare you to have meaningful contributions to all sessions and to complete all activities. You must attempt all the Self Assessment Questions (SAQ) at the end of every unit to help your understanding of the contents and to help you prepare for the in-course tests and the final examination. You will also be expected to keep a portfolio where you keep all your completed assignments. iv NSC 222 COURSE GUIDE COURSE MATERIALS Course Guide Course Text in Study Units Textbooks (Hard and electronic) and Book of Laboratory Practical Assignment File/Portfolio STUDY UNITS This course is made up of 5 modules comprising 13 units as listed below: Module 1 Gastrointestinal Physiology Unit 1 Organization and Secretions of the Gastrointestinal Tract Unit 2 Gastrointestinal Motility Unit 3 Gastrointestinal Hormones, Digestion and Absorption Module 2 Nervous System Unit 1 Overview of the Nervous System Unit 2 Integration of Central Nervous System with Other Systems Module 3 Endocrine System And Reproductive System Unit 1 The Endocrine System Unit 2 The Reproductive System Module 4 Urinary System Unit 1 Introduction to Urinary System Unit 2 The Structure of the Kidney Module 5 The Special Senses Unit 1 The Tongue and the Sense of Taste Unit 2 The Nose and the Sense of Smell Unit 3 The Ear and the Sense of Hearing Unit 4 The Eyes and the Sense of Vision v NSC 222 COURSE GUIDE REFERENCE TEXTBOOKS Berne, R.M., B. M., & Stanton, B. A. (2010). Berne & Levy physiology (6th Edt.). Philadelphia, PA: Mosby/Elsevier. Fox SI. (2012). Human Physiology. 12th edition, Mc Graw Hill, New York. Ganong WF. (2010). Review of Medical Physiology. 23rd edition, Mc Graw Hill, New York. Guyton AC, Hall JE. (2001). Textbook of Medical Physiology. Harcourt International Edition, 10th edition, W.B. Saunders, Philadelphia. Oyebola DO. (2002). Essential Physiology, Vol 1, Nihort Press. COURSE REQUIREMENTS AND EXPECTATIONS OF YOU Attendance of 95% of all interactive sessions, submission of all assignments to meet deadlines; participation in all CMA, attendance of all laboratory sessions with evidence as provided in the log book, submission of reports from all laboratory practical sessions and attendance of the final course examination. You are also expected to: 1. Be versatile in basic computer skills. 2. Participate in all laboratory practical up to 90% of the time. 3. Submit personal reports from laboratory practical sessions on schedule. 4. Log in to the class online discussion board at least once a week and contribute to ongoing discussions. 5. Contribute actively to group seminar presentations. EQUIPMENT AND SOFTWARE NEEDED TO ACCESS COURSE You will be expected to have the following tools: 1. A computer (laptop or desktop or a tablet) 2. Internet access, preferably broadband rather than dial-up access 3. MS Office software – Word PROCESSOR, Powerpoint, Spreadsheet 4. nBrowser – Preferably Internet Explorer, Moxilla Firefox 5. Adobe Acrobat Reader vi NSC 222 COURSE GUIDE NUMBER AND PLACES OF MEETING (ONLINE, FACE- TO-FACE, LABORATORY PRACTICALS) The details of these will be provided to you at the time of commencement of this course DISCUSSION FORUM There will be an online discussion forum and topics for discussion will be available for your contributions. It is mandatory that you participate in every discussion every week. Your participation links you, your face, your ideas and views to that of every member of the class and earns you some mark. COURSE EVALUATION There are two forms of evaluation of the progress you are making in this course. The first are the series of activities, assignments and end of unit, computer or tutor-marked assignments, and laboratory practical sessions and the report. These constitute the continuous assessment that all carry 30% of the total mark. The second is a written examination with multiple choice, short answers and essay questions that take 70% of the total mark that you will do on completion of the course. Students’ evaluation: The students will be assessed and evaluated based on the following criteria: In-Course Examination: In line with the university’s regulation, in- course examination will come up in the middle of the semester These would come in form of Computer Marked Assignment. This will be in addition to 1compulsory Tutor Marked Assignment (TMA’s) and three Computer marked Assignment that comes after every module….. o Laboratory practical: Attendance, the record of participation and other assignments will be graded and added to the other scores form other forms of examinations. Final Examination: The final written examination will come up at the end of the semester comprising essay and objective questions covering all the contents covered in the course. The final examination will amount to 60% of the total grade for the course. Learner-Facilitator evaluation of the course This will be done through group review, written assessment of learning (theory and laboratory practical) by you and the facilitators. vii NSC 222 COURSE GUIDE GRADING CRITERIA Grades will be based on the following Percentages Tutor Marked Individual Assignments 10% Computer marked Assignment 10% Group assignment 5% 30% Discussion Topic participation 5% Laboratory practical 10% End of Course examination 70% GRADING SCALE A = 70-100 B = 60 - 69 C = 50 - 59 F = < 49 SCHEDULE OF ASSIGNMENTS WITH DATES To be provided for each module by the facilitator in addition to the ones already spelt out in the course materials. SPECIFIC READING ASSIGNMENTS To be provided by each module COURSE OVERVIEW NSC 222 - Human Physiology (II) This course is in continuation of NSC 221, Human Physiology (I) where we covered the functional cell, cardio-vascular/cardio-pulmonary physiology. In this course we would cover respiratory and gastrointestinal physiology. Respiration is the commonly acknowledged sign of life. HOW TO GET THE MOST FROM THIS COURSE 1. Read and understand the context of this course by reading through this course guide paying attention to details. You must know the requirements before you will do well. 2. Develop a study plan for yourself. 3. Follow instructions about registration and master expectations in terms of reading, participation in discussion forum, end of unit and module assignments, laboratory practical and other directives given by the course coordinator, facilitators and tutors. viii NSC 222 COURSE GUIDE 4. Read your course texts and other reference textbooks. 5. Listen to audio files, watch the video clips and consult websites when given. 6. Participate actively in online discussion forum and make sure you are in touch with your study group and your course coordinator. 7. Submit your assignments as at when due. 8. Work ahead of the interactive sessions. 9. Work through your assignments when returned to you and do not wait until when examination is approaching before resolving any challenge you have with any unit or any topic. 10. Keep in touch with your study centre, the NOUN, School of Health Sciences websites as information will be provided continuously on these sites. 11. Be optimistic about doing well. ix  MAIN COURSE CONTENTS PAGE Module 1 Gastrointestinal Physiology…………………….. 1 Unit 1 Organization and Secretions of the Gastrointestinal Tract…………………………………..……………. 1 Unit 2 Gastrointestinal Motility………………………..….. 17 Unit 3 Gastrointestinal Hormones, Digestion and Absorption………………………………….……… 28 Module 2 Nervous and System……………………….…….. 41 Unit 1 Overview of the Nervous System………………….. 41 Unit 2 Integration of Central Nervous System with other Systems…………………………………………….. 47 Module 3 Endocrine System and Reproductive System…... 57 Unit 1 The Endocrine System…………………………..… 57 Unit 2 The Reproductive System…………………………. 63 Module 4 Urinary System………………………………….. 68 Unit 1 Introduction to urinary System……………….……. 68 Unit 2 The Structure of the Kidney…………….………… 72 Module 5 The Special Senses……………………..…………. 78 Unit 1 The Tongue and the Sense of Taste………….…..… 78 Unit 2 The Nose and the Sense of Smell…………………. 84 Unit 3 The Ear and the Sense of Hearing…………………. 89 Unit 4 The Eyes and the Sense of Vision……...................... 96 NSC222 MODULE 1 MODULE 1 GASTROINTESTINAL PHYSIOLOGY Unit 1 Organization and Secretions of the Gastrointestinal Tract Unit 2 Gastrointestinal Motility Unit 3 Gastrointestinal Hormones, Digestion and Absorption UNIT 1 ORGANIZATION AND SECRETIONS OF THE GASTROINTESTINAL TRACT CONTENT 1.0 Introduction 2.0 Objectives 3.0 Main Contents 3.1 Organization of the gastrointestinal tract 3.2 The sphincters 3.3 Characteristics of sphincters 3.4 Functions of sphincters 4.0 Conclusion 5.0 Summary 6.0 Tutor- Marked Assignments 7.0 References/Further Reading 1.0 INTRODUCTION Beyond air exchange, the body depends on nutrients that should be taken, digested and absorbed to supply energy and other micronutrients needed. This module covers the organization of the gastrointestinal system, the secretions, the hormones and the processes of digestion and absorption. Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed and used as nutrients by the body. Most of the substances in the diet cannot be utilized as such. These substances must be broken into smaller particles so that they can be absorbed into the blood and distributed to various parts of the body for utilization. Digestive system is responsible for these functions. The gastrointestinal system performs digestive functions that provide the nutrients needed for energy and other organic functions. The GIT system is organized with different organs that also allow for the control release of contents through sphincters. In this unit, you will learn about the organization and the functions of some of the organs in the GIT. 1 NSC 222 HUMAN PHYSIOLOGY II 2.0 OBJECTIVE By the end of this unit, you will be able to: explain the Organization of the gastrointestinal tract GIT describe the sphincters in the GIT describe the salivary glands and the secretions. describe other secretions (pancreatic, intestinal, gall bladder) of the GIT and their functions. 3.0 MAIN CONTENT 3.1 Organization of the gastrointestinal tract The gastrointestinal system includes alimentary canal, extending from pharynx to anus and accessory organs like salivary gland, liver and pancreas. Figure 1 shows the entire alimentary tract. Figure 1: The Alimentary tract 2 NSC222 MODULE 1 Organizational structure The digestive tract includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus. The histology is similar in all parts except the esophagus and the anus where serous attachment is not present. Figure 2 shows a typical cross-section of the intestinal wall. The basic structures include: (i) serous coat (ii) muscular layer – longitudinal and circular layer (iii) sub-mucosa (iv) Mucosa Figure 2: Typical cross section of the gut Innervation The wall of gastrointestinal tract (GIT) has intrinsic nervous system (enteric nervous system) beginning from the esophagus extending down the anus. It is composed mainly of two plexus. 1. The outer one lies between the longitudinal and circular layer and is called Auerbach’s or Myenteric plexus. 2. submucosa plexus or Meissner’s plexus lies in the submucosa layer. The myenteric plexus control GIT movement. It is sensitive to stretch while the submucosal or meissner’s control secretion and local blood flow. It is sensitive to osmolar changes, pH changes and chemical composition of food. The enteric nervous system can function on its own, 3 NSC 222 HUMAN PHYSIOLOGY II independently of the extrinsic nerves supply to the gut. The extrinsic innervation can modify the activity of the enteric nervous system. The extrinsic supply comes from the autonomic nerves. The sympathetic fibres gives relaxation of smooth muscle and vasocontraction of smooth muscle, vasodilation and secretion of the digestive juice. 3.2 The sphincters The alimentary tract is divided into functional compartment by sphincter. They include: i. Upper esophageal sphincter ii. Lower esophageal sphincter iii. Pyloric iv. Sphincter of Oddi v. Ileocecal sphincter vi. Internal sphincter (smooth muscle/involuntary) vii. Outer sphincter (skeletal muscle/voluntary) Characteristics of sphincters i. They have high tension/pressure area. The tensions within the sphincter are very high. ii. The resting tone is greater than the other two adjacent segments which result in intraluminal high pressure zone that separate the two lumen to compartments. iii. It relaxes in response to appropriate stimulus, so that flow may occur from one compartment to the next. iv. It regulates or maintains aurocaudal (mouth to anus) flow of GIT contents Functions of sphincters i. The upper esophageal sphincter prevent air into the esophagus during respiration. ii. The lower esophageal sphincter prevents irritant gastric from refluxing into the esophagus iii. The pyloric sphincter separates the acidic environment of the stomach from the alkaline environment of the duodenum. iv. The ileocecal sphincter separates ileum from the cecum, disallowing the faeces in the cecum from refluxing back into the ileum. v. The sphincter of Oddi allows intermittent flow of pancreatic secretion and bile. vi. The internal sphincter has smooth muscle and it shows involuntary movement of defecation, while the outer anal sphincter has skeletal muscle and shows voluntary movement. 4 NSC222 MODULE 1 3.3 Salivary glands and Secretions The digestive secretion in the mouth comes from salivary gland. There are three major salivary glands. i. Parotid (behind the tongue) ii. Submaxillary (Submandibular) iii. Sublingual Figure 3: Formation and secretion of saliva by a submandibular salivary gland They are exocrine glands. Each has acini and duct cells. In the acini, two types of secreting cells have being found: i. Serous secretion containing an alpha-amylase; this is the enzyme for the digestion of starches. ii. A mucous secretion, containing mucin, which is important for lubrication purposes There is also lingual lipase, secreted by the Ebner’s glands. The parotid glands produce entirely the serous type of secretion; the submandibular glands secrete both the serous and mucous types and the sublingual produce only the mucous type of secretion. The submaxillary produces 10% secretion, sublingual produced 5% secretion and parotid produced 25%. Innervation The salivary glands are supplied by both sympathetic and parasympathetic nerves. The sympathetic action gives vasoconstriction which causes secretion to be viscous and scanty. 5 NSC 222 HUMAN PHYSIOLOGY II The parasympathetic effect causes vasodilation and causes copious secretion. The parasympathetic fiber of cranial nerve-7 supplies some submaxillary and sublingual and cranial nerve-9 supplies the parotid gland. Figure 4: Major salivary glands The parasympathetic is mediated via the release of acetylcholine that can be blocked by atropine. Composition of saliva i. The secretion of saliva per day ranges from 1000 ml- 1500ml. ii. It has a pH of 6.8 which ranges from 6.7- 7. iii. It contains 99.5% water and 0.5% solid The solid consists of organic and organic substances. The major inorganic constituent are: Na+, K+, Cl-, HCO3-, Ca++ and Mg2+. The organic substances include the enzyme- α- amylase, mucin, lysozymes, IgA, blood group antigens, urea, uric acid, etc. Duct cells function The saliva in the duct is isotonic while the saliva in the mouth is hypotonic. This is due to the change that takes place in the lumen of the ducts. The duct-epithelial cell shows active reabsorption of sodium ion in exchange for potassium ion. There is also reabsorption of Cl- in exchange for HCO3- since the duct cells are impermeable to water, the removal of sodium and chloride ions makes the saliva to be hypotonic. 6 NSC222 MODULE 1 Aldosterone which is a mineralocorticoid acts on salivary duct to cause sodium ion reabsorption in exchange for potassium ion. The concentration of Na+ and K+ in saliva depends on flow rate. At high flow rate, less time is allowed for transfer of ions and hence Na+ is more than K+ but the saliva remains isotonic at a high flow rate. Regulation of secretion Salivary secretion is regulated mainly by neural mechanisms. It shows both conditional and unconditional reflexes. Conditional reflex is established by learning, and the secretion can be seen from sight, smell and thought of food. Unconditional reflex comes from presence of food in the mouth. The secretion of saliva in both sides of reflexes is caused by activity of parasympathetic nerves (VII) and (IX) supplying the glands. Secretion is almost abolished (reduced) during sleep. There is a decrease in resting flow rate when there is dehydration, anxiety, fear and severe mental effort. Functions of the saliva i. The enzyme α- amylase acts on boiled starch and convert it to maltose. The beginning of carbohydrate digestion occurs in the mouth. ii. The mucin present in the saliva lubricate the food which helps in mastication iii. Saliva is necessary for swallowing iv. Helps in taste perception of food materials by dissolving them v. It facilitates speech. Speech is difficult in dry mouth vi. Lysozymes and IgA present in the saliva gives birth to bactericidal and immunity functions respectively. vii. It neutralizes the gastric acid that refluxes into the esophagus and release heart burn. 3.4 Gastric secretion Gastric juice is secreted by gastric glands present in the gastric mucosa of fundus and body of the stomach. There are 3 types of cells namely: i. Neck and surface epithelial cell that secrete mucous ii. The chief cell that secretes enzyme iii. Parietal or Oxyntic cells that secrete Hydrochloric acid (HCl) and intrinsic factors. 7 NSC 222 HUMAN PHYSIOLOGY II Composition of gastric secretion The volume of secretion per day is about 2 liters and the pH varies between 1.8 – 2.0. The inorganic constituents include Na+, Cl-, PO4-, K+ and SO4-. The organic substances present in the secretion are digestive enzyme (pepsinogen, renin, lipase), mucin and intrinsic factors. Function of HCl (Hydrochloric acid) The concentrated Hcl in the gastric juice is necessary to activate pepsinogen to pepsin, the extreme acidity is bactericidal. The acid pH in the upper part of duodenum facilitates ion absorption. Function of gastric juice The beginning of protein digestion takes place in the stomach. Pepsin acts on protein and convert it to peptone. The enzyme is secreted by chief cell as active pepsinogen. Gastric renin: Is a milk curdling enzyme which is absent in human but present in cows. Gastric lipase: Is a weak fat-splitting enzyme. Intrinsic factor: Is secreted by the parietal cell of the fundus. It is required for the absorption of vitamin B12 (cyanocobalamin or extrinsic factor). The absorption of vitamin B12 occurs in the terminal ileum. Mucus: Is secreted by surface epithelial cell and neck cell of the gland. The surface epithelial cell also secrete bicarbonates, the mucus and the bicarbonate form gel in the lining of the gastric mucosa. This gel protects the mucosa from the action of the acid. There is a pH gradient from the lumen to the mucosal wall of the stomach. The pH in the mucosal is 7 and that of the lumen is 2. The presence of HCO3- and mucus form the acid mucosa barrier. Certain condition like chronic stress, alcohol and aspirin tends to arose the acid mucosa barrier. Hydrochloric acid secretion Hydrochloric acid (HCl) is the gastric juice secreted from the parietal cells. The secretion of Hcl is an active process and it is transported against the electrochemical gradient. The concentration of Hcl in gastric juice is 150 mEq/L whereas it is 00004 mEq/L in plasma. The source of H+ is from the dissociation of H2CO3. The H2CO3 is formed from hydration of CO2 in the presence of carbonic anhydrase enzyme. Carbonic acid dissociates to H+ and HCO3-. The 8 NSC222 MODULE 1 hydrogen ion formed is exchanged with potassium ion and the transport carrier is H+- K+ ATPase. The energy released from the breakdown of ATP is utilized for the active transport of H+. Chloride ion is also actively transported from the cell into the lumen and coupled with H+ to form Hcl. The active transport of Hcl is followed by the passive transport of water into the lumen. The secretion of H+ leaves bicarbonate ion within the cell. The HCO3- combines with Na+ and enters the blood as NaHCO3. It is known during digestion, alkaline level in the blood and urine rise and it is called Postprandial alkaline tide. Agents causing secretion of Hydrochloric acid i. Gastrin is released from pyloric antrum ii. Acetylcholine is secreted from the vagal ending iii. Histamine comes from enterochromaffin cell lining the mucosa When vagus is activated it releases acetylcholine. This stimulates the release of GRP (Gastric Releasing Peptide) followed by secretion of gastrin. Vagal stimulation also causes the release of histamine. All these cause release of Hcl. When there is peptic ulcer, treatment could be induced by inhibiting acid secretion, and this could be achieved from the following i. Inhibiting H2 receptors by cimetidine ii. Blocking hydrogen ion – potassium ion ATPase by omeprazole. Regulation of gastric secretion Gastric juice secretion is regulated by both neural and hormonal mechanisms. Neural regulation is mediated via the vagus. Acetylcholine is released by vagal ending. The acetylcholine activates phospholipase C which in turn raises intracellular Ca2+. The intracellular Ca2+ promotes the secretion of gastric juice. Hormonal regulation is by gastrin which is secreted from the pyloric antrum. 3.4.1 Phases of gastric secretion There are 3 phases of gastric secretion: (1) Cephalic phase (2) Gastric phase (3) Intestinal phase Cephalic phase Conditional reflexes like sight, smell and thought of food causes secretion of gastric juice. The presence of food in the mouth also causes secretion in the stomach. The cephalic phase occurs by the activity of the vagus. Shaming-feeding experiments in animals like a dog gives an example of cephalic secretion. The quantity of juice secreted is small. The cephalic 9 NSC 222 HUMAN PHYSIOLOGY II phase accounts for about 10% of the total secretion associated with a meal. Gastric phase The gastric phase accounts for about 80% of the total secretion of gastric juice. This phase is initiated by the presence of food in the stomach. The food stretches the stomach wall and this distension stimulates the gastric glands via both the extrinsic (vagal) and intrinsic (nerve plexuses) reflex pathways to produce gastric juice. Distension of the pyloric antrum also results in the release of gastrin into the blood by an intrinsic reflex. In addition, some substances in the food, known as secretagogues, elicit release of gastrin by the intrinsic reflex. Such substances include meat extracts, protein digestion products, alcohol, bile acids and caffeine. During this phase, maximum secretion occurs. Intestinal phase The arrival of food and the product of digestion in the intestine also stimulates gastric secretion. The quantity produced is very small. However, the presence of food in the duodenum inhibits secretion of gastric juice. This inhibition is mediated through enterogastric reflex. The presence of acid and fat in the duodenum causes the release of secretin and cholecystokinin. Also, there is the release of VIP, GIP- all of which are enterogastrones. i.e. they cause inhibition of gastric secretion. 3.3 Pancreatic secretion The pancreas has both exocrine and endocrine functions. The digestive enzymes are secreted from exocrine pancreas. The exocrine pancreas consists of acini and ducts. Composition About 1500 ml of pancreatic juice are secreted daily. Pancreatic juice is a watery alkaline fluid; isotonic with plasma and rich in digestive enzymes. The alkalinity is due to HCO3- secreted from the duct epithelial cells. Other inorganic substances are Na+, K+, Cl- and SO4-. The organic constituents includes: peptidase, amylase, lipase, nuclease. Regulation of pancreatic secretion The regulation is mainly from hormones. The 2 types of hormone regulating 2 types of secretion. i. Secretin is produced by the duodenal mucosa. Presence of acid chyme in the duodenum is the main stimulus resulting in secretion of watery fluid which in bicarbonate that helps to neutralize the acid pH. 10 NSC222 MODULE 1 ii. Cholecystokinin (CCK) causes secretion of the pancreatic juice which in digestive enzymes. It also acts on the gut bladder. Stimulus is product of food digestion entering the duodenum. 3.5 Bile Bile is secreted by the liver cells (liver lobules) and stored in the gallbladder. Secretion of bile The volume secreted per day is 500-700 ml, and secretion occurs when chyme enters the duodenum. PH is 7.6- 7.8. \ Composition of bile Bile released from the gall bladder into the duodenum has the following composition. Water - 92% Bile Salts - 6% Bilirubin - 0.3% Cholesterol – 0.3% Fatty acids - 0.3% Lecithin – 0.3% Other lipids – 0.2% Na+ - 130mEq/L K+ - 12mEq/L Ca2+ - 23mEq/L Cl- - 25mEq/L HCO3- - 10mEq/L BILE SALTS They are secretory products of the liver. They are formed from cholesterol which gives rise to primary bile acid which becomes conjugated into Na+ and K+ to form taurocholate and glucocholate. Formation of bile salts Bile salts are formed from bile acids. There are two primary bile acids in human, namely cholic acid and chenodeoxycholic acid, which are formed in liver and enter the intestine through bile. Due to the bacterial action in the intestine, the primary bile acids are converted into secondary bile acids: 11 NSC 222 HUMAN PHYSIOLOGY II Cholic acid → deoxycholic acid Chenodeoxycholic acid → lithocholic acid Secondary bile acids from intestine are transported back to liver through enterohepatic circulation. In liver, the secondary bile acids are conjugated with glycine (amino acid) or taurin (derivative of an amino acid) and form conjugated bile acids, namely glycocholic acid and taurocholic acids. These bile acids combine with sodium or potassium ions to form the salts, sodium or potassium glycocholate and sodium or potassium taurocholate Functions of bile salts Bile salts are required for digestion and absorption of fats in the intestine. The functions of bile salts are: 1. Emulsification of Fats 2. Absorption of Fats 3. Cholagogue Action 4. Choleretic Action 5. Laxative Action 6. revention of Gallstone Formation Bile pigments Bile pigments are the excretory products in bile. Bilirubin and biliverdin are the two bile pigments and bilirubin is the major bile pigment in human beings. Formation of bile salts Bile pigments are formed during the breakdown of hemoglobin, which is released from the destroyed RBCs in the reticuloendothelial system.The breakdown of haemoglobin gives rise to bilirubin and biliverdin which are bile pigment. They are excretory products of the liver. The golden yellow of the bile is due to the presence of the pigment. Figure 5: Formation and circulation of bile pigments 12 NSC222 MODULE 1 Phospholipids Lecithin is the chief phospholipids present in the bile. It is insoluble in water. It becomes solubilized in micelles. Cholesterol Cholesterol in bile is solubilized in micelles. The presence of bile salt keeps cholesterol in solution and prevents its precipitation to form stones. Functions of the bile Digestion and absorption of fats and fat soluble vitamins (ADEK) depends on presence of bile. The fat is lipid soluble while the digestive enzyme lipase is enzyme soluble. To facilitate the action of lipase on lipids, bile shows the following effects: i. The bile produces emulsification of fat. By this, large molecules are broken down into smaller ones. ii. They show hydrotropic effects. This action of bile enables lipase enzyme to digest the fat. iii. Bile reduces surface tension. This effect facilitates the lipase enzyme action. i. The micelles that is formed after digestion promotes absorption. The micelles consist of digested glycerides combined with bile. FUNCTIONS OF BILE Most of the functions of bile are due to the bile salts. 1. digestive function 2. absorptive functions 3. excretory functions 4. laxative action 5. antiseptic action 6. choleretic action 7. maintenance of ph in 8. prevention of gallstone formation 9. lubrication function 10. cholagogue action ENTEROHEPATIC CIRCULATION The bile salts that are secreted into the duodenum are reabsorbed and recirculated. About 90% of the bile salt that enter the small intestine are absorbed from the terminal ileum, and enter the liver through the portal 13 NSC 222 HUMAN PHYSIOLOGY II circulation. From the liver, it is recirculated into the duodenum; this forms enterohepatic circulation. Figure 6. Enterohepatic circulation of bile acids The bile salts in circulating pool are only 3-6gm but the quantity required for digestion is 4—8 gram. Normal digestion and absorption of fat can take place by recirculation. The total circulating pool circulate twice during digestion of each meal. The rate of synthesis depends on the rate of returns to the liver. About 0.2 gm/day of bile is lost in faeces. Any condition that affects enterohepatic circulation and decrease bile pool and causes mal absorption of bile and fat soluble vitamins may result in steatorrhea. Regulation of bile Bile secretion is regulated by hormones. There are 2 mechanisms: 14 NSC222 MODULE 1 i. Hormone secretin produced from duodenum when acid chyme enters acts on the biliary duct cells to increase secretion of water and electrolytes which helps to neutralize the acid chyme, and this action is known as hydrocholeretic effects. a. The bile salt in the bile causes stimulation of the liver to secrete more bile. This action is called choleretic effect. ii. Hormone cholecystokinin (CCK) also acts on gall bladder smooth muscle causing contraction and expulsion of bile. This action of CCK on gall bladder is called chologugue effect. Gall bladder Gall bladder stores, releases and makes the bile into concentration by active reabsorption of Na+ and HCO3-, passively by the reabsorption of water. Gall stones There are 2 types of gall stone- cholesterol stone and pigment stone. Normally, cholesterol and lecithin are found in solution by the formation of micelles. When there is alteration in concentration, cholesterol crystallizes to form stones. Cholesterol stones are radiolucent. Pigment stones are formed due to infection or obstruction of the biliary tree. The conjugation of bilirubin makes it insoluble resulting in precipitation. Pigment stones are radiopaque. Intestinal secretion The small intestine consists of duodenum, jejunum, and ileum. The small intestinal secretion is mainly water, mucus and electrolytes. It is alkaline in nature and ranges between 10001500ml/day. Digestive enzymes are not secreted into the lumen, they are present in the apical surface of the villi. At the base of the villi are glands called Crypts of Lieberkuhn which contains cells that secrete mucus. The intestinal gland of the duodenum are called Brunner’s gland which also secrete mucus. The mucus secreted by the Brunner’s gland provides protection to the mucosa lining against mechanical damage and also lubricate the mucosa lining. Regulation of intestinal secretion Regulation is neural and hormonal. Vagal stimulation during digestion causes secretion of intestinal hormones. Vasoactive intestinal peptide (VIP) hormones increase secretion of intestinal glands. The most important is the local enteric nervous system. 15 NSC 222 HUMAN PHYSIOLOGY II 4.0 CONCLUSION The gastrointestinal system performs digestive functions that provide the nutrients needed for energy and other organic functions. The GIT system is organized with different organs that also allow for control release of contents through sphincters. 5.0 SUMMARY In this unit, you have learnt about the organization of the gastrointestinal tract and the various sphincters with different characteristics and functions. You have also learnt that the salivary glands of different types produce secretions that also perform digestive functions. Gastric, pancreatic, gall bladder secretions also perform different functions. 6.0 TUTOR-MARKED ASSIGNMENT Activity: - See Laboratory manual and the experiments on gastrointestinal system. Please answer the following questions: 1. Describe the organization of the gastrointestinal tract 2. Describe the sphincters in the GIT 3. Explain what gastric secretion is and list its major composition 4. Describe the three phases of gastric secretion 5. Discuss the pancreatic secretion and its major composition 6. Describe the composition and functions of the bile. 7.0 REFERENCES/FURTHER READING Berne, R.M., B. M., & Stanton, B. A. (2010). Berne & Levy physiology (6th Edt.). Philadelphia, PA: Mosby/Elsevier. Fox, S.I. (2012). Human Physiology. 12th edition, Mc Graw Hill, New York. Ganong W.F. (2010). Review of Medical Physiology. 23rd edition, Mc Graw Hill, New York. Guyton, A.C& Hall J.E. (2001). Textbook of Medical Physiology. Harcourt International Edition, 10th edition, W.B. Saunders, Philadelphia. Oyebola, D.O. (2002). Essential Physiology, Vol 1, Nihort Press. 16 NSC222 MODULE 1 UNIT 2 GASTROINTESTINAL MOTILITY CONTENT 1.0 Introduction 2.0 Objectives 3.0 Main Contents 3.1 Gastrointestinal motility 3.2 Chewing 3.3 Deglutition (Swallowing) 3.4 Gastric motility 3.5 Motility of the small intestine 3.6 Defeacation 4.0 Conclusion 5.0 Summary 6.0 Tutor Marked Assignments 7.0 References and other resources 1.0 INTRODUCTION The physical activity of processing the food items from the point of entry to the point of eliminating the waist takes a course in gastrointestinal motility. This unit traces these actions from the mouth to the anus. 2.0 OBJECTIVE By the end of this unit, you will be able to: explain the term, Gastrointestinal motility describe the act and use of Mastication (Chewing) describe the process of Swallowing (Deglutition) list the functions of the stomach describe the act of Gastro-intestinal motility describe the process of Defeacation. 3.0 MAIN CONTENT 3.1 Gastrointestinal motility The main function of the alimentary tract is to ingest, digest and absorb food materials. For this to be carried out, appropriate mixing must be provided. The mixing/propulsion are different at each compartmental stage of the tract. It requires both neural and hormonal mechanisms. i. Mastication (Chewing) ii. Swallowing (Deglutition) iii. Gastric motility iv. Motility of the small intestine v. Motility of the colon 17 NSC 222 HUMAN PHYSIOLOGY II vi. Defeacation (passage of faeces) 3.2 Chewing Chewing is the process by which food brought into the mouth is broken down into smaller pieces by the teeth. In the process, the food is mixed with saliva. Chewing makes it easier to swallow the food and contributes to its enjoyment by homogenizing and mixing the food with saliva, thus releasing taste-producing substances. Chewing can be carried out voluntarily, but is more frequently a reflex activity. By subdividing the food into smaller particles, chewing makes it possible for the food to mix more readily with digestive secretions of the stomach and duodenum. The tongue and the cheek muscles are used to keep the food mass between the teeth during mastication. 3.3 Deglutition (Swallowing) It refers to the act of swallowing. It consists of oral, pharyngeal and esophageal stages. The 1st stage is voluntary, the 2nd and 3rd stages are involuntary and reflex in nature. Figure 1: Stages of deglutition. A. Preparatory stage; B. Oral stage; C. Pharyngeal stage; D. Esophageal stage Oral stage It is voluntary. The food is masticated by mixing with saliva. The solid food is converted to a soft bolus and positioned in the dorsum of the tongue. In this stage, the bolus passes through the oral cavity toward the pharynx assisted by the tongue pressing against the hard palate. 18 NSC222 MODULE 1 Pharyngeal stage The presence of food at the entry of the pharynx stimulates receptors in the tonsil and epiglottis which initiate the reflex. This stage is involuntary: the efferent and afferent impulses are carried by cranial nerves V, IX, X and XII. The center is the swallowing center located in the medulla and the lower pons. This reflex allows the bolus to enter the esophagus and not the trachea. The entry of the bolus into the nasopharynx is prevented by: i. The soft palate raises and presses against the posterior pharyngeal wall. ii. There is an upward and forward movement of the larynx which causes the glottis approximately with the epiglottis and seal with the larynx. The vocal cord also approximately inhibiting speech. iii. There is ceasation of respiration at this stage. The bolus is therefore directed at the esophagus. Esophageal stage The bolus enters the esophagus from the pharynx through the upper esophageal sphincter and enters the stomach through the lower esophageal sphincter. Entry of food bolus causes distension and relaxation of upper esophageal sphincter. This is as a result of inhibition of the vagus nerve. This distension initiate a wave of peristalsis which spread along the length of esophagus pushing the bolus forward. These are: primary peristalsis wave travelling at 3-4cm/sec (frequency). The force of gravity helps the wave of movement. Liquid travels faster than solid to reach the stomach. When the bolus reaches the lower esophageal sphincter, it relaxes and allows the bolus to enter the stomach, after which it closes to prevent regurgitation into the esophagus; this action is regulated by the myenteric plexus which secretes VIP or NO (nitric oxide). If the primary peristalsis does not completely empty the esophagus, one or more secondary peristalsis arises from distal part of esophagus. Disorders of swallowing 1. Dysphagia - difficulty in swallowing 2. Achalasia – difficulty in emptying the food from the esophagus to stomach due to absence of peristalsis in the lower 3rd or failure of cardiac sphincter to relax. 3.4 Gastric motility Functions of the stomach i. Storage organ: stomach can store large quantity of food, as it shows receptive relaxation. 19 NSC 222 HUMAN PHYSIOLOGY II ii. Beginning of protein digestion by pepsin occurs in the stomach. iii. It makes food into chyme by propulsive, mixing and retropulsive movement. iv. The acid in the stomach has bactericidal effect. The gastric secretion has intrinsic factor which is necessary for absorption of vitamin B12. v. The acid in the stomach convert cane sugar to fructose and glucose. The acid in chyme in the duodenum facilitates iron absorption. Figure 2: Physiologic anatomy of the stomach Movement in the stomach Receptive relaxation The stomach shows receptive relaxation, accommodating large volume of food. The receptors for this are present in the wall of pharynx and the stomach. The food is stored in the fundus and the body. The afferent and efferent are the vagus. It causes the plexus to secrete VIP. Vagotomy does not completely abolish reflective relaxation, it only decreases it. Digestive peristalsis (Mixing of food) The distal part of the stomach shows peristalsis. The distension of the wall of the distal part (antrum) stimulates intrinsic plexus. The smooth muscle first shows slow waves which is not propagatory, and it’s called basal electrical rhythm (BER). The membrane potential reaches the threshold level of pharynx. The entry of Na+ and Ca2+ causes depolarization, resulting in spike potential (action potential) which is 20 NSC222 MODULE 1 propagatory and forms peristalsis. Peristalsis consists of wave of contraction followed by relaxation frequency of which is 3/min. When the food reaches the pyloric sphincter, it retropul back to the antrum because the pyloric sphincter is closed. The propulsion, mixing and retropulsion in the pyloric breaks down the food to chyme and helps through mixing by the gastric juice. Each time the peristalsis arises at the sphincter, only 2-3 ml is empty to the duodenum. Gastric emptying Gastric emptying occurs mainly due to gastroduodenal plexus cycle caused by digestive peristalsis of the stomach. The transit time for the gastric empty of the food is 3-4 hrs. As peristalsis reaches the sphincter, the chyme is retropuls allowing only small quantity (2-3 ml) to emptied into the duodenum and closes back. Factors influencing emptying Liquid leaves the stomach earlier than solid. Carbohydrate empty faster than protein and fat is emptied last. Increase in acid pH inhibit emptying. Isotonic leaves earlier than hypo or hypertonic fluids. Enterogastric reflex Presence of fats, acid or hyper osmolar solution in the duodenum inhibit gastric emptying. Other duodenal factors include: i. The degree of distension of duodenum ii. Presence of any degree of irritant in the duodenal mucosa iii. Degree of acidity in the duodenal chyme iv. Degree of osmolarity of the chyme The inhibition is mediated by neural and hormonal mechanisms. The neural involves the inhibition of the vagus while hormonal mechanism includes release of secretin, VIP, CCK and GIP; these hormones inhibit gastric motility. 3.5 Motility of the small intestine The movement that are seen in small intestine are as follows: 1. Segmental contraction 2. Peristalsis contraction 3. Pendular contraction 4. Villi movement 21 NSC 222 HUMAN PHYSIOLOGY II Segmental contraction The frequently occurring movement in the small intestine is the segmental contraction. Slow waves develop in the circular smooth muscle of the wall due to stimulation of the plexus. When the slow wave reaches threshold, segmental contraction that is propagatory develops. The frequency of the slow wave is highest in the duodenum (12/min) and lowest in the ileum (8/min). This facilitates the bolus to be propelled aborally. The segmental contraction involves ring-like regular constriction along the length of the segment of the intestine. The constricted part latter relaxes and the relaxed part constrict. This process is repeated over and over again resulting in the bolus moving back and forth within the lumen. Functions of segmental peristalsis are as follows: i. Bolus mixes well with digestive enzymes and facilitate completion of digestion ii. The segmental contraction causes exposure of digested food to the villi surface for absorption iii. The occurrence of segmental contraction in the proximal segment and inhibition in the distal segment facilitate propulsion of bolus toward the segment colon. Peristalsis contraction Sometimes the longitudinal muscle contraction gives pendular contraction which facilitates mixing of bolus with digestive enzymes. Villi movement There is contraction of the smooth muscle of the villi that results into forward and backward movement. The hormone- villi kinin stimulates the movement which facilitates absorption of digested food. Gastroileal reflex Distension of the stomach by food causes relaxation of ileocecal sphincter and allows emptying of the ileal content into the caecum. Distension of the ileum will also cause the relaxation of the sphincter and emptying. On the other hand, the distension of the caecum will result in contraction of the caeca and prevent reflux of the caecal content into the ileum. This is facilitated by the ileocecal valve. The activity of the ileocecal sphincter is controlled by the myenteric plexus. Functions of the small intestine i. It contains pancreatic bile and intestinal secretion 22 NSC222 MODULE 1 ii. Completion of digestion and absorption of digested food occurs in the small intestine iii. Presence of villi facilitates absorption of digested food iv. The duodenal mucosa secretes gastrointestinal hormones like secretin, CCK, VIP, motilin, etc. v. Payer’s patches in the ileum are lymphoid organs which help in immunity. Migrating motor complex It is an interdigesting peristalsis occurring in the stomach and small intestine. It occurs in between meal at every 70-90 mins interval. Each peristalsis last 10mins. It is developed due to activity of intrinsic myenteric system. It helps in sweeping the content of the stomach and small intestine towards colon toward interdigestive period. Motilin is the hormone responsible. The large intestine It includes the caecum, ascending colon, transverse colon and descending or sigmoid colon, rectum and anus. The mucosa secretes mucus but there are no villi, hence no absorption of food, no digestive enzymes. However, it has important functions: i. Absorption of water and electrolytes ii. Formation of faeces iii. Secretion of mucus to lubricate the faeces iv. The bacteria fluoride synthesis vit B and vit K. Motility of the large intestine i. The large intestine shows haustral shutting (segmental contraction) ii. Mass peristalsis (contraction) iii. Peristalsis Haustral shutting It is similar to segmental contraction of the small intestine. In the large intestine, the longitudinal muscle forming 3 bands taenia coli. The enteric plexus below this is greater and the region adjacent has a thin wall. It gives rise to sac-like pouches along the length of segment of the colon that are called haustrau. The back and forth movement causes the chyme to be exposed for absorption of water and electrolytes. Out of 1,500ml of the mixed chyme 23 NSC 222 HUMAN PHYSIOLOGY II that enters the colon per day, all will be absorbed leaving 50-100ml in the faeces. The haustral shutting also facilitate propulsive movement of the faeces to the distal colon. Transit time in the colon is very slow (5-10cm/hr). Mass peristalsis It occurs in a large segment of the colon, the contraction is powerful enough to cause the colon to be in contracted state for a long period. It is stimulated by myenteric plexus. The main function is to sweep the faeces along the segment of the colon. The mass contraction occurs 3-5 times per day and usually lead to defeacation. Figure 3: The human colon 3.6 Defeacation Defeacation is the act of passing faeces. It is a complex behavior involving both reflex and voluntary actions. The rectum is normally empty. When faeces are pushed into the rectum by mass movements, the urge to defeacate is felt. The anal sphincters however prevent escape of faeces unless the individual is prepared for defeacation. There are two anal sphincters, the internal and the external sphincters. The internal sphincters consist of a circular smooth muscle that is in the anal wall, while the external sphincter consists of striated voluntary muscle that 24 NSC222 MODULE 1 surrounds the internal sphincter and also extends distal to it. The internal sphincter is supplied by parasympathetic nerves and the external sphincter, which is under voluntary control, is supplied by somatic nerves. Act of defecation Act of defecation is preceded by voluntary efforts like assuming an appropriate posture, voluntary relaxation of external sphincter and the compression of abdominal contents by voluntary contraction of abdominal muscles. Usually, the rectum is empty. During the development of mass movement, the feces is pushed into rectum and the defecation reflex is initiated. The process of defecation involves the contraction of rectum and relaxation of internal and external anal sphincters. Internal anal sphincter is made up of smooth muscle and it is innervated by parasympathetic nerve fibers via pelvic nerve. External anal sphincter is composed of skeletal muscle and it is controlled by somatic nerve fibers, which pass through pudendal nerve. Pudendal nerve always keeps the external sphincter constricted and the sphincter can relax only when the pudendal nerve is inhibited. The process of defeacation can be subdivided into main components: i. The part under the defeacation reflexes ii. The part under voluntary control The defeacation reflexes There are two types of defeacation reflexes i. The intrinsic defeacation reflex ii. The parasympathetic defeacation reflex The intrinsic defeacation reflex is mediated via the myenteric plexuses. When faeces enter the rectum, distension of the rectal wall initiate peristaltic waves via a local reflex circuit and these peristaltic waves spread to the descending colon, sigmoid colon and rectum forcing faeces towards the anus. As the peristaltic waves approach the anus, the internal and external relaxes. If the external sphincter is also relaxed, defeacation will occur. The peristaltic waves produced by the intrinsic defeacation reflex is usually weak and may not be effective in causing defeacation. This weak contraction is often reinforced by contractions mediated by the parasympathetic defeacation reflex, which involves parasympathetic nerves in the sacral segment of the spinal cord. These parasympathetic impulses augment the ineffectual weak movements produced by the intrinsic defeacation reflex so that they become powerful and effective in emptying the bowel. 25 NSC 222 HUMAN PHYSIOLOGY II In spite of the two reflexes above, defeacation can only occur if the circumstance is socially acceptable for the act. The ability not to defeacate in some circumstances is due to the fact that the conscious mind takes over voluntary control of the external sphincter. Relaxation of the internal sphincter and forward movement of the faeces towards the anus normally cause an instantaneous contraction of the external sphincter. Impulses from the cerebral cortex which pass through the somatic nerves to the external sphincter will either inhibit the sphincter to allow defeacation to occur or further contract it if the circumstance is not conducive to defeacation. When the circumstances are right for defeacation to occur, the defeacation reflex is followed by relaxation of the external anal sphincter. Intra-abdominal pressure is elevated to aid in the expulsion of faeces. Evacuation is normally preceded by a deep breath, so that the diaphragm descends towards the abdominal cavity. The glottis is closed and contraction of the respiratory muscles on full lungs raises both the intrathoracic and intra-abdominal pressure. Contraction of the muscles of the wall of the abdomen causes a further increase in intra-abdominal pressure. The additional pressure generated by this bearing down effort as well as the strong contractions of the defeacation reflex helps to force faeces out of the anus through the relaxed sphincters. Figure 4: Afferent and efferent pathways of the parasympathetic mechanism for enhancing the defeacation reflex 26 NSC222 MODULE 1 4.0 CONCLUSION The main function of the alimentary tract is to ingest, digest and absorb food materials. For this to be carried out, appropriate mixing must be provided. The mixing/propulsion are different at each compartmental stage of the tract. It requires both neural and hormonal mechanisms. 5.0 SUMMARY In this unit, you have learnt about gastrointestinal motility, the acts of mastication, swallowing and movement of the bulk through the tract. 6.0 TUTOR-MARKED ASSIGNMENT Activity: See the Laboratory manual and directives from the Facilitator Please answer the following questions: Describe the act and use of Mastication (Chewing) Describe the process of Swallowing (Deglutition) List the functions of the stomach Describe the act of Gastro-intestinal motility Describe the process of Defeacation. 7.0 REFERENCES/FURTHER READING Berne, R.M., B. M., & Stanton, B. A. (2010). Berne & Levy physiology (6th Edt.). Philadelphia, PA: Mosby/Elsevier. Fox, S.I. (2012). Human Physiology. 12th edition, Mc Graw Hill, New York. Ganong W.F. (2010). Review of Medical Physiology. 23rd edition, Mc Graw Hill, New York. Guyton, A.C& Hall J.E. (2001). Textbook of Medical Physiology. Harcourt International Edition, 10th edition, W.B. Saunders, Philadelphia. Oyebola, D.O. (2002). Essential Physiology, Vol 1, Nihort Press. 27 NSC 222 HUMAN PHYSIOLOGY II UNIT 3 GASTROINTESTINAL HORMONES, DIGESTION AND ABSORPTION CONTENT 1.0 Introduction 2.0 Objectives 3.0 Main Contents 3.1 Gastrointestinal hormones 3.2 Digestion 3.3 Absorption 4.0 Conclusion 5.0 Summary 6.0 Tutor Marked Assignments 7.0 References and other resources 1.0 INTRODUCTION Gastrointestinal hormones are chemical agents secreted by the endocrine glands released into general circulation and acts on the alimentary tract. Gastrointestinal (GI) hormones are the hormones secreted in GI tract. These hormones are polypeptides in nature and belong to the family of local hormones. Major function of these hormones is to regulate the secretory activities and motility of the GI tract. They include the following: (a) gastrin (b) secretin (c) cholecystokinin- pancreozymin (CCK-P2) (d) gastric-inhibitory peptide (GIP) (e) vasoactive intestinal peptide (VIP) (f) motilin (g) gastric releasing peptide (GRP). Importantly, hormones help with digestion. The food which the body needs can be classified into carbohydrate, proteins and fats. This cannot be absorbed in their natural forms through the gastrointestinal tract. Before they can be made available for body use, they must be broken down into smaller molecules which can be absorbed. This process of breaking down is known as digestion. It is the end product of digestion that is now absorbed. The basic process in the digestion of all food types is called hydrolysis. This unit will improve your knowledge on gastric hormones, digestion and absrbtion. 2.0 OBJECTIVE By the end of this unit, you will be able to: describe the gastric hormones explain the process of digestion in human discuss the process of digestion of proteins and fats discuss the absorption processes of carbohydrate, fat, proteins and vitamins. 28 NSC222 MODULE 1 3.0 Main Content 3.1 Gastric Hormones 3.1.1 Gastrin Gastrin is a peptide with 34 amino acid residues. It is secreted mainly by the G cells of pyloric glands of stomach. It is also secreted by TG cells in stomach, duodenum and jejunum. In fetus, the islets of Langerhans also secrete this hormone There are three forms of gastrin which are being isolated. We have G-17-17 amino acid residue G-34-34 amino acid residue G-14-14 amino acid residue G-17 is the most biological active form, the half-life between G-14 and G-17 is 2-3mins in circulation while G-34 has 15mins as its half-life. Action of gastrin i. It stimulates gastric acid and pepsin secretion (more than 1000 times more potent than histamine) ii. It stimulates growth of gastric mucosa iii. It stimulates insulin and glucagon secretion after a protein meal but not after carbohydrate iv. It stimulates the contraction of cardiac sphincter v. It stimulates gastric motility Regulation of gastrin i. Distension of the stomach. ii. Presence of protein and product of protein breakdown. iii. Stimulation of parasympathetic supply, pyloric action and acetylcholine secretion. iv. Blood borne factor like Ca2+ and epinephrine. v. Presence of acid in the stomach has a negative feedback control which inhibits gastrin secretion. vi. Substances like secretin, GIP, VIP, glucagon, and calcitonin inhibits gastrin secretion. 3.1.2 CCK – P2 (cholecystokinin – pancreozymin) Cholecystokinin is made up of 39 amino acid residues. Previously it was thought that there were two separate hormones, namely pancreozymin and cholecystokinin. It was thought that pancreozymin stimulated the secretion of pancreatic juice with large amount of enzymes and the cholecystokinin stimulated the contraction of gallbladder. But now it is 29 NSC 222 HUMAN PHYSIOLOGY II established that the same hormone has actions on both pancreas and gallbladder. So, it is named as cholecystokinin-pancreozymin (CCK-PZ) or cholecystokinin (CCK). Cholecystokinin is secreted by I cells in mucosa of duodenum and jejunum. A small quantity of the hormone is secreted in the ileum also. It is found in the nerves in many parts of the body, including distal ileum, colon, and brain. It is secreted in the mucosa of the upper intestine (duodenum). Porcine CCK exists in various form, e.g. 58, 39, 33, 12, 8 and 4. Half –life is 5 mins. Action of CCK- P2 i. It stimulates secretion of pancreatic juice, rich in enzyme or rich in digestive juice. ii. It augment action of secretin in producing secretion of an alkaline pancreatic juice iii. It inhibits gastric emptying iv. It exerts a trophic effect on the pancreas stimulating the growth of the exocrine cells v. It increases the secretion of interleukins vi. It enhances motility of small intestine and colon vii. It stimulates the contraction of the gall bladder and the relaxation of the sphincter of Oddi viii. Along with secretin, it augments contraction of pyloric sphincter thus preventing the reflux of duodenal content into the stomach ix. It stimulates the glucagon secretion 3.1.3 Secretin Secretin is a peptide hormone with 27 amino acid residues. Historical importance of secretin is that, it was the first ever hormone discovered. It was discovered in 1902 by Bayliss and Starling. It is secreted by the S cells of duodenum, jejunum and ileum. Secretin is first produced in an inactive form called prosecretin. It is converted into secretin by the acidity of chyme. Secretin is stimulated by the entry of the product of food digestion into the duodenum, e.g. amino acid, protein and fat. There is a positive feedback regulation of secretin by presence of food. Action of Secretin i. It increases the secretion of HCO3- by pancreatic duct and biliary tract ii. It promotes the secretion of watery alkali pancreatic juice iii. It augment the action of CCK in producing pancreatic juice rich in digestive enzymes iv. It decreases gastric acid secretion v. It causes contraction of the pyloric sphincter vi. It increases insulin secretion Regulation of secretin 30 NSC222 MODULE 1 i. Secretion of secretin is stimulated by acid chyme bathing the duodenum ii. Alkali secretion helps to neutralized the acidity iii. Products of protein digestion is also a stimulus. 3.1.4 Gastric inhibitory peptide i. Gastric inhibitory peptides is produced in the K-cell in the mucosal of duodenum and jejunum. It consists of 43 amino acids. In large doses, it causes inhibition of gastric secretion and motility. In smaller doses, it does not show this action. ii. It stimulates insulin secretion and forms one of the important B- cells stimulating hormones. Regulation of gastric inhibitory peptide 1. Secretion of GIP is caused by present of glucose and fats in the duodenum. 3.1.5 Vasoactive intestinal peptide (VIP) Vasoactive intestinal polypeptide (VIP) contains 28 amino acid residues. This polypeptide is secreted in the stomach and small intestine. A small amount of this hormone is also secreted in large intestine Action of vasoactive intestinal peptide i. It stimulates intestinal secretion of electrolytes and water ii. It relaxes intestinal smooth muscles including the sphincter iii. It causes dilatation of peripheral blood vessels iv. It inhibits gastric acid secretion v. It potentiate action of acetylcholine of salivary gland. Enterogastrones: They are hormones which causes inhibition of gastric secretion and motility. They are secreted in the duodenum in response to the presence of acid chyme in the lumen. They include secretin, GIP and VIP. 3.1.6 Motilin Motilin is built by 22 amino acid residues. It is secreted by Mo cells, which are present in stomach and intestine. It is also believed to be secreted by enterochromoffin cells of intestine. Motilin is secreted when the chyme from stomach enters the duodenum. The concentration in the blood undergoes cyclic fluctuation during fasting. The peak blood level corresponds to the beginning of the activity of myoelectric motor complex (mmc). 31 NSC 222 HUMAN PHYSIOLOGY II Action of motilin i. It stimulates gastric acid secretion ii. It causes contraction of the gall bladder to increase bile concentration. iii. It prepares the intestine for the next meal. Other hormones include somatostatin, glucagon, gastric releasing peptide (GRP), neurotensin and substance P. 3.2 Digestion Digestion of carbohydrate Almost all carbohydrate of the diet are large polysaccharide or disaccharide, which are combinations of monosaccharides bound together by the process of condensation. Hydrogen ion (H+) has been removed from one of the monosaccharide while a hydroxyl ion (OH-) has been removed from the next one. The two monosaccharides combines with each other at this site of removal and the H+ and OH- combine to form water (H2O). The dietary carbohydrate is mainly; Amylopectin, which consists of chains of glucose molecules joined by 1,4 - α linkages, with some branches linked by 1,6 – α linkages. For digestion of carbohydrate to take place the specific ion returns the H and OH ions to the polysaccharide and thereby separate the monosaccharides from each other and this process is called hydrolysis. R1 R11 + H2O R1H + R11OH Three major sources of carbohydrate exists in the normal human diet and these are : sucrose; the disaccharide known as cane sugar, lactose; the disaccharide in milk and starches, large polysaccharides present in almost all non-animals foods, particularly grains and tubers. When food is chewed, it is mixed with saliva, which contains the enzymes ptyalin and this is an α-amylase from the parotid gland. This enzyme hydrolyzes starch into the disaccharide maltose and isomaltose. An isomaltose is small polymers of glucose containing 3 to 9 glucose molecules. Because the food remains in the mouth for only a short time, about 3 to 5% of all the starches eaten becomes hydrolyzed. Even though the food does not remain in the mouth long enough for ptyalin to complete the breakdown of starches into maltose, its actions can continue as long as an hour after the food has entered the stomach i.e. until the content of the fundus mix with the stomach secretions. Then, the activities of the α- amylase is blocked by the acid of gastric secretions. Before food properly mix with this gastric secretions, about 30-40% of the starches would have been hydrolyzed into maltose. 32 NSC222 MODULE 1 Pancreatic secretions contain a large quantity of α-amylase and immediately after the chyme empties from the stomach into the duodenum and mixes with pancreatic juice, starches that have not already been split and digested amyloses are converted before they pass the jejunum. The epithelial cells contain four enzymes: lactase, sucrase, maltase, and isomaltose, which are capable of splitting disaccharides: lactose, sucrose, maltose and isomaltose into their constituents monosaccharides. The enzymes are located in the brush border of the cells lining the lumen of the intestine. Lactose splits into a molecule of glucose and a molecule of galactose. Sucrose splits into a molecule of glucose and fructose. Maltose splits into 2 molecules of glucose and isomaltose into several molecules of glucose. Since the ordinary diet contains far more starches than sucrose and lactose. Glucose represents about 80% of the final product of carbohydrate, while galactose and fructose represent about 10% each. Figure 5. The digestion of carbohydrates 33 NSC 222 HUMAN PHYSIOLOGY II Digestion of fat Lipid digestion begins in the stomach. Gastric lipase is released in large quantities from gastric chief cells; it adsorbs to the surface of fat droplets dispersed in the gastric contents and hydrolyzes component triglycerides to diglycerides and free fatty acids. However, little lipid assimilation can take place in the stomach because of the acidic pH of the lumen, which results in protonation of the free fatty acids released by gastric lipase. Lipolysis is also incomplete in the stomach because gastric lipase, despite its optimum catalytic activity at acidic pH, is not capable of hydrolyzing the second position of the triglyceride ester, which means that the molecule cannot be fully broken down into components that can be absorbed into the body. There is also little if any breakdown of cholesterol esters or the esters of fat-soluble vitamins. Indeed, gastric lipolysis is dispensable in healthy individuals because of the marked excess of pancreatic enzymes. The majority of lipolysis takes place in the small intestine in health. Pancreatic juice contains three important lipolytic enzymes that are optimized for activity at neutral pH. The first of these is pancreatic lipase. This enzyme differs from the stomach enzyme in that it is capable of hydrolyzing both the 1 and 2 positions of triglyceride to yield a large quantity of free fatty acids and monoglycerides. At neutral pH, the head groups of the free fatty acids are charged, and thus these molecules migrate to the surface of the oil droplets. Lipase also displays an apparent paradox in that it is inhibited by bile acids, which also form part of the small intestinal contents. Bile acids adsorb to the surface of the oil droplets and would thereby cause lipase to dissociate. However, lipase activity is sustained by an important cofactor, colipase, which is also supplied in pancreatic juice. Colipase is a bridging molecule that binds both to bile acids and to lipase; it anchors lipase to the oil droplet even in the presence of bile acids. Pancreatic juice also contains two additional enzymes that are important in fat digestion. The first of these is phospholipase A2, which hydrolyzes phospholipids such as those present in cell membranes. Predictably, this enzyme would be quite toxic in the absence of dietary substrates, and thus it is secreted as an inactive pro-form that is activated only when it reaches the small intestine. Furthermore, pancreatic juice contains a relatively nonspecific, so-called cholesterol esterase that can break down not only esters of cholesterol, as its name implies, but also esters of fat-soluble vitamins and even triglycerides. Interestingly, this enzyme requires bile acids for activity (contrast with lipase, discussed earlier), and it is related to an enzyme produced in breast milk that plays an important role in lipolysis in neonates. 34 NSC222 MODULE 1 Figure 6: Schematic diagram of lipid metabolism As lipolysis proceeds, the products are abstracted from the lipid droplet, first into a lamellar, or membrane, phase and subsequently into mixed micelles composed of lipolytic products, as well as bile acids. The amphipathic (meaning that they have both a hydrophobic and hydrophilic face) bile acids serve to shield the hydrophobic regions of lipolytic products from water while presenting their own hydrophilic faces to the aqueous environment (Fig. 29-15). Micelles are truly in solution and thus markedly increase the solubility of lipid in the intestinal contents. This increases the rate at which molecules such as fatty acids can diffuse to the absorptive epithelial surface. Nevertheless, given the very large surface area of the small intestine and the appreciable solubility of the products of triglyceride hydrolysis, micelles are not essential for the absorption of triglyceride. Thus, patients who have insufficient output of bile acids (caused, for example, by a gallstone that obstructs bile output) do not normally show fat malabsorption. On the other hand, cholesterol and the fat-soluble vitamins are almost totally insoluble in water and accordingly 35 NSC 222 HUMAN PHYSIOLOGY II require micelles to be absorbed even after they have been digested. Thus, if luminal bile acid concentrations fall below the critical micellar concentration, patients can become deficient in fat-soluble vitamins. Digestion of proteins Dietary proteins are derived entirely from meat and vegetables. Proteins are formed from long chains of amino acids bind together by peptide linkages. Digestion of proteins, unlike carbohydrate start in the stomach. Figure 7: Schematic diagram of protein metabolism Pepsin is capable of digesting essentially all the different types of proteins in diet. One of its important feature is to digest collagen and albuminoid that is affected little by other digestive enzyme. Collagen is a major constituent of intercellular connective tissue of meat. Pepsin, however only begins the process of protein digestion by providing as much as 10- 30% of the total protein digestion. Most protein digestion occurs in the small intestine under the influence of a proteolytic enzymes of the pancreatic secretion. When protein leaves the stomach, they are in form of proteases, peptones and large polypeptides. On entering the small intestine, they are trapped by the pancreatic enzymes, trypsin, chymotrypsin and carboxypolypeptidase. Both trypsin and chymotrypsin transmit protein molecules into small polypeptides. Carboxypolypeptidase then cleaves individual amino acids from the carboxyl ends of the polypeptides. The brush border of small intestine 36 NSC222 MODULE 1 contains several enzymes for hydrolyzing the final linkages of remaining dipeptides and other small polypeptides. These enzymes are; aminopolypeptidase and dipeptidase. 3.2 Absorption The end product of the digestion of different types of food ingested or secreted electrolytes, vitamins and large quantity of water secreted in various digestive juices must be moved from the lumen of the gut across the epithelium to the interstitial fluid. The process of transport from gut lumen into body’s interstitial fluid is called absorption. The main absorptive portion of the gut is the small intestine with a large surface area. The surface area is achieved by the following: i. The mucosal infolding, called the valvulae corniventis. ii. Billions of small villi projecting about 1mm from the surface of the mucosa iii. A brush border consisting of about 600 villi per cell The combination of the three above increase the absorptive area of the mucosal about 600 folds. The total area of small intestine is 250m2. Absorption occurs basically by active transport and diffusion. Absorption of carbohydrate Glucose, other hexoses and pentoses are rapidly absorbed across the wall of small intestine. These sugars are absorbed before the content of the small intestine reach the terminal part of the ileum. The transport is an active process, and this can be demonstrated by several important experimental observation and they are: i. Transport can be blocked by metabolic inhibitors such as IAA, cyanite and phlorhizin ii. Transport is selective, the order of preferences for transporting difference monosaccharides and their relative rate of transport in comparison with glucose are; galcatose-1.1, glucose-1.0, fructose-0.4, mannose-1.2, xylose-0.15, arabinose-0.1. iii. There is competition between certain sugars for the respective carrier system Glucose and galactose in small intestine enter the cells by secondary active transport with sodium ion. Transport of other hexoses are affected by the amount of Na ion in interstitial lumen. Glucose and Na share the same co-transporter or symport called Na-dependent glucose transporter (SGLT). There are two members of this family; SGLT 1 and SGLT 2. Since the intracellular concentration of Na ions is low in interstitial cell, Na diffuses into the cell along its concentration gradient. Glucose moves 37 NSC 222 HUMAN PHYSIOLOGY II with the Na and is released in the cell. The Na ion is transported into the lateral intracellular spaces and the glucose from the inside of the cell by another transporter; the SGLT 2 into the interstitium and finally into the capillaries. Absorption of glucose is called a secondary active transport because the energy for transport is provided indirectly by the active transport of Na out of the cell. Fructose, however, utilizes a different mechanism its absorption is independent of Na ions or transport of glucose and galactose. It is transported by facilitated diffusion from the intestinal lumen into the cells of the intestine by a transporter GLUT 5 and out of the cell into the interstitium by another transporter GLUT 2. Some fructose is converted to glucose in the mucosal cells. Pentoses are absorbed by simple diffusion. All monosaccharides are absorbed into the portal blood draining the small intestine. Absorption of lipids (fats) The digestive end product of fat dissolve in the lipid portion of bile acid micelles. These micelles are soluble in the chyme. In this form, the monoglycerides and fatty acids are transported to the surfaces of the brush border microvilli penetrating the recesses of the moving agitating microvilli. Here, the monoglycerides and the fatty acids diffuse through the epithelial membrane, because they are soluble in the membrane. The bile acid micelles is now left to diffuse back into the chyme, absorbing more monoglycerides and fatty acids. Undigested triglycerides and diglycerides are both highly soluble in the lipid membrane of epithelial cells. However, only small quantity of these are normally absorbed, because the bile micelles will not ferry them to the epithelial membrane. On entering the epithelial cells, fatty acids and monoglycerides are taken up by the small endoplasmic reticulum and are recombined to form new triglycerides. Once form the triglycerides aggregate within the endoplasmic reticulum into globules along with absorbed cholesterol and phospholipids globules are called chylomicrons. These globules diffuses to the side of the epithelial cells and is extrude by process of cellular exocytosis into the space between the cells. From the site of the epithelial cells, chylomicrons find their ways into the central lactus of the villi, and from here they are propelled along with the lymph by the lymphatic pump upward through the thoracic duct to be emptied into the great vein of the neck. Between 80 and 90% of all fat absorbed from the gut get into the interstitium in this manner. 38 NSC222 MODULE 1 Absorption of proteins Amino acids (and some small peptides) are absorbed mainly in the duodenum and upper jejunum into the portal blood. D- amino acids are absorbed passively, whereas L-amino acids are absorbed actively by a Na-linked carrier mechanism. There are at least four specific mechanisms by which amino acids are absorbed: one for neutral amino acids, one for basic amino acids, one for acidic amino acids and one for the imino acids (proline, sarcosine). Dipeptides and tripeptides may be absorbed from the lumen and are later hydrolyzed within the epithelial cells. In neonates, antibodies and other proteins contained in colostrum may be absorbed in their intact form by pinocytosis. Absorption of Water Only a small amount of water move across the gastric mucosa but water moves in both directions across the mucosa of the small and large intestines in response to osmotic gradient. Absorption of ions/electrolytes Na+ is actively absorbed throughout the small and large intestines. Active transport of Na+ is important in the secondary transport of glucose and some amino acid. Chloride ions are rapidly absorbed mainly by passive diffusion in the upper part of the small intestines. They move along with the absorbed Na+ to balance the electrical gradient caused by Na+ ion absorption. K+ ions are absorbed across the gastrointestinal mucosa by diffusion. Absorption of Vitamins Water soluble vitamins are rapidly absorbed while the absorption of fat soluble vitamins is dependent of fat absorption. Most vitamins are absorbed in the upper small intestine but vitamin B12 is absorbed in the ileum. 4.0 CONCLUSION Six different hormones secreted in different parts of the gastrointestinal tract are discussed with information on different functions performed to prepare the different organs to support digestion and absorption in different parts of the tract. 39 NSC 222 HUMAN PHYSIOLOGY II 5.0 SUMMARY In this unit, you have learnt about the following hormones, Gastrin, Ccholecystokinin – pancreozymin (CCK – P2), Secretin, Gastric inhibitory peptide, Vasoactive intestinal peptide (VIP) and Motilin. You have also learnt about digestion of the various smaller molecules in different areas of the gastro-intestinal tract. Absorption through active, passive transport and diffusion in different areas of the gastro-intestinal tract. 6.0 TUTOR-MARKED ASSIGNMENT 1. Describe the forms and actions of the following hormones: a) Gastrin b) CCK – P2 (cholecystokinin – pancreozymin) c) Secretin d) Gastric inhibitory peptide e) Vasoactive intestinal peptide (VIP) f) Motilin 7.0 REFERENCES/FURTHER READING Berne, R.M., B. M., & Stanton, B. A. (2010). Berne & Levy physiology (6th Edt.). Philadelphia, PA: Mosby/Elsevier. Fox, S.I. (2012). Human Physiology. 12th edition, Mc Graw Hill, New York. Ganong W.F. (2010). Review of Medical Physiology. 23rd edition, Mc Graw Hill, New York. Guyton, A.C& Hall J.E. (2001). Textbook of Medical Physiology. Harcourt International Edition, 10th edition, W.B. Saunders, Philadelphia. Oyebola, D.O. (2002). Essential Physiology, Vol 1, Nihort Press. 40 NSC222 MODULE 2 MODULE 2 NERVOUS SYSTEM Unit 1 Overview of the Nervous System Unit 2 Integration of Central Nervous System with other Systems UNIT 1 OVERVIEW OF THE NERVOUS SYSTEM CONTENTS 1.0 Introduction 2.0 Objectives 3.0 Main Content 3.1 Division of the Nervous System 3.2 Anatomy of the Nervous System 4.0 Conclusion 5.0 Summary 6.0 Tutor-Marked Assignment 7.0 References/Further Readings 1.0 INTRODUCTION The nervous system consists of the brain, spinal cord, sensory organs, and all of the nerves that connect these organs with the rest of the body. Together, these organs are responsible for the control of the body and communication among its parts. The brain and spinal cord form the control Centre known as the central nervous system (CNS), where information is evaluated and decisions made. The sensory nerves and sense organs of the peripheral nervous system (PNS) monitor when you sit, stand, or walk by controlling muscular activities. Your body temperature remains stable on a cold winter day or in a warm kitchen Because your rate of heat generation and heat loss are closely regulated. 2.0 OBJECTIVES By the end of this unit, you will be able to: explain the activities of the nervous system describe a general overview of the nervous system describe the anatomical divisions of the nervous system and their functions describe the structure of the brain. 41 NSC 222 HUMAN PHYSIOLOGY II 3.0 MAIN CONTENT The nervous system, which accounts for a mere 3 percent of the total Body weight, is the most complex organ system. It is vital not only to life but also to our appreciation of life. This unit details with the Structure and function of neural tissue and introduces principles of neurophysiology that are vital to an understanding of the nervous system’s capabilities and limitations. 3.1 Division of the Nervous System Central Nervous System The brain and spinal cord together form the central nervous system, or CNS. The CNS acts as the control center of the body by providing its processing, memory, and regulation systems. The CNS takes in all of the conscious and subconscious sensory information from the body’s sensory receptors to stay aware of the body’s internal and external conditions. Using this sensory information, it makes decisions about both conscious and subconscious actions to take to maintain the body’s homeostasis and ensure its survival. The CNS is also responsible for the higher functions of the nervous system such as language, creativity, expression, emotions, and personality. The brain is the seat of consciousness and determines who we are as individuals. Peripheral Nervous System The peripheral nervous system (PNS) includes all of the parts of the nervous system outside of the brain and spinal cord. These parts include all of the cranial and spinal nerves, ganglia, and sensory receptors. Somatic Nervous System The somatic nervous system (SNS) is a division of the PNS that includes all of the voluntary efferent neurons. The SNS is the only consciously controlled part of the PNS and is responsible for stimulating skeletal muscles in the body. Autonomic Nervous System The autonomic nervous system (ANS) is a division of the PNS that includes all of the involuntary efferent neurons. The ANS controls subconscious effectors such as visceral muscle tissue, cardiac muscle tissue, and glandular tissue. There are 2 divisions of the autonomic nervous system in the body: the sympathetic and parasympathetic divisions. 42 NSC222 MODULE 2 Sympathetic. The sympathetic division forms the body’s “fight or flight” response to stress, danger, excitement, exercise, emotions, and embarrassment. The sympathetic division increases respiration and heart rate, releases adrenaline and other stress hormones, and decreases digestion to cope with these situations. Parasympathetic. The parasympathetic division forms the body’s “rest and digest” response when the body is relaxed, resting, or feeding. The parasympathetic works to undo the work of the sympathetic division after a stressful situation. Among other functions, the parasympathetic division works to decrease respiration and heart rate, increase digestion, and permit the elimination of wastes. Enteric Nervous System The enteric nervous system (ENS) is the division of the ANS that is responsible for regulating digestion and the function of the digestive organs. The ENS receives signals from the central nervous system through both the sympathetic and parasympathetic divisions of the autonomic nervous system to help regulate its functions. However, the ENS mostly works independently of the CNS and continues to function without any outside input. For this reason, the ENS is often called the “brain of the gut” or the body’s “second brain.” The ENS is an immense system—almost as many neurons exist in the ENS as in the spinal cord. Action Potentials Neurons function through the generation and propagation of electrochemical signals known as action potentials (APs). An AP is created by the movement of sodium and potassium ions through the membrane of neurons. Resting Potential: At rest, neurons maintain a concentration of sodium ions outside of the cell and potassium ions inside of the cell. This concentration is maintained by the sodium-potassium pump of the cell membrane which pumps 3 sodium ions out of the cell for every 2 potassium ions that are pumped into the cell. The ion concentration results in a resting electrical potential of - 70 millivolts (mV), which means that the inside of the cell has a negative charge compared to its surroundings. Threshold Potential: If a stimulus permits enough positive ions to enter a region of the cell to cause it to reach -55 mV, that region of the cell will open its voltage-gated sodium channels and allow sodium ions to diffuse into the cell. -55 mV is the threshold potential for neurons as this is the “trigger” voltage that they must reach to cross the threshold into forming an action potential. 43 NSC 222 HUMAN PHYSIOLOGY II Depolarization: Sodium carries a positive charge that causes the cell to become depolarized (positively charged) compared to its normal negative charge. The voltage for depolarization of all neurons is +30 mV. The depolarization of the cell is the AP that is transmitted by the neuron as a nerve signal. The positive ions spread into neighbouring regions of the cell, initiating a new AP in those regions as they reach -55 mV. The AP continues to spread down the cell membrane of the neuron until it reaches the end of an axon. Repolarization: After the depolarization voltage of +30 mV is reached, voltage-gated potassium ion channels open, allowing positive potassium ions to diffuse out of the cell. The loss of potassium along with the pumping of sodium ions back out of the cell through the sodium- potassium pump restores the cell to the -55 mV resting potential. At this point the neuron is ready to start a new action potential. Synapses A synapse is the junction between a neuron and another cell. Synapses may form between 2 neurons or between a neuron and an effector cell. There are two types of synapses found in the body: chemical synapses and electrical synapses. Chemical synapses: At the end of a neuron’s axon is an enlarged region of the axon known as the axon terminal. The axon terminal is separated from the next cell by a small gap known a the synaptic cleft. When an AP reaches the axon terminal, it Opens voltage-gated calcium ion channels. Calcium ions cause vesicles containing chemicals known as neurotransmitters (NT) to release their contents by exocytosis into the synaptic cleft. The NT molecules cross the synaptic cleft and bind to receptor molecules on the cell, forming a synapse with the neuron. These receptor molecules open ion channels that may either stimulate the receptor cell to form a new action potential or may inhibit the cell from forming an action potential when stimulated by another neuron. Electrical synapses: Electrical synapses are formed when 2 neurons are connected be small holes called gap junctions. The gap junctions allow electric current to pass from one neuron to the other, so that an AP in one cell is passed directly on to the other cell through the synapse. 3.3 Anatomy of the Nervous Tissue The majority of the nervous system is tissue made up of two classes of cells: neurons and neuroglia. 44 NSC222 MODULE 2 Neurons. Neurons, also known as nerve cells, communicate within the body by transmitting electrochemical signals. Neurons are different from other cells in the body due to the many long cellular processes that extend from their cell body. The cell body is the roughly round part of a neuron that contains the nucleus, mitochondria, and most of the cellular organelles. Small tree-like structures called dendrites extend from the cell body to pick up stimuli from the environment, other neurons, or sensory receptor cells. Long transmitting processes called axons extend from the cell body to send signals to other neurons or effector cells in the body. There are 3 basic classes of neurons: afferent neurons, efferent neurons, and interneurons. 1. Afferent neurons 2. Efferent neurons 3. Interneurons Neuroglia Neuroglia, also known as glial cells, act as the “helper” cells of the nervous system. Each neuron in the body is surrounded by 6 to 60 neu