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PHYSIOLOGY LOGIC EXERCISES to PHLEX your mental muscles Dr. Miriam Frommer Department of Physiology University of Sydney 1 INDEX OF CONTENTS What is PHLEX? Exercise categories Extra notes on additional material Concept maps with triggers Practical classes covered (order performed may vary) Model: Human cardiovascular system (BP) In vitro: Electrophysiology of nerve Skeletal muscle mechanics In vivo: Human nervous system -Senses Human nervous system -Reflexes Human respiratory system Human endocrine system Simulation: Gastrointestinal physiology Renal physiology Links to Answers (time delay before activation) Exercise category 4 Extra notes on additional material (some answers) Concept maps with triggers 2 Online resources (links to related topic areas) A. No Frills Generic Skills Guide General Introduction 9 Questions Exercises on Data Handling: Statistics&Graphs Exercises Preparatory to Writing Essays and Reports: Logical Thinking Exercises Testing Concepts: Force&Velocity, Resistance&Flow Answers to Exercises Appendix B “Data Analysis” Appendix D “Concept Mapping” Appendix E “Confusing Terms” B. No Frills Statistics Skills Guide General Introduction 5 Questions C. FLERT (Flexible Report Writing Template) Help with Report Writing Help with Report Content D. Post-tests Answers with explanations 3 What is PHLEX? PHLEX is a follow-up to the No Frills Generic Skills Guide (NFGS), focusing on the need to support students in the logical thinking necessary to substantiate and test the hypotheses they formulate before each practical class, as well as explain discrepancies between their predicted and observed results. It should help the development of the generic skill of clear communication, encompassing the knowledge, practical and thinking skills which are often very poorly developed in a number of science students. The 8 practicals covered are: electrophysiology of nerve; skeletal muscle mechanics; human nervous system (reflexes & senses), cardiovascular, respiratory, and endocrine systems; gastrointestinal and renal physiology. Since the NFGS Guide used the blood pressure practical as the example of how to approach experimental design, PHLEX will begin with this practical to illustrate how different types of exercises can be incorporated into students’ learning. However students should follow the order in which their practicals are carried out. There are 5 exercises for each practical; these should be done chronologically so that understanding is progressively built up and the links between the theoretical and the experimental aspects become consolidated. Some answers to these and other exercises will be provided online when sufficient time has elapsed after the relevant class for them to have been attempted. Exercise categories 1) Confusing terms glossary (of paired similar terms) 2) Misconception MCQs (and best explanations) with answers 3) Logical fallacies (and how to avoid them) 4) Sequences which make sense (as a concept map) 5) The three most important facts (for introduction/discussion of a report) Exercises 1) to 3) vary somewhat but 4) and 5) have a fairly standardized format. The topics covered in these are summarized in the table below. Specific topics per practical Practical Blood pressure Nerve Muscle Reflexes/Senses Respiration Endocrine Gastrointestinal Renal 4) Sequences Standing up Errors in description Isotonic force production Stretch reflex/ Pacinian corp. Rebreathing expired air Carbohydrate ingestion Protein ingestion Water ingestion 5) Facts/Relationships BP & postural change CAP & inter-electrode distance Active force & muscle length CAP & stimulus strength/2pt discrimin. Alveolar gases & ventilation Blood glucose & carbohydrate type Gastric secretion & drugs Urine osmolality & diuretics 4 Extra notes on additional material Once the questions in the 5 exercise categories have been attempted, it will have become apparent where there are gaps in a student’s knowledge and understanding. The most common of these are addressed in the extra notes provided, which focus especially on particularly difficult, more mathematical concepts and inter-relationships. Some of this material is in the form of questions and answers, and it is important to attempt to work out an answer first, before reading the one supplied. Extra notes are preceded each time by the following statement: When answering some of the questions on xxxx activity, the concepts below may be relevant. Specific topics per practical Practical Blood pressure Nerve Muscle Reflexes/Senses Respiration Endocrine Gastrointestinal Renal Additional material Systolic/diastolic pressures, ejection, TPR, HR, resistance & flow Myelin/size & resistance/capacitance, threshold, population code, CAP, CV/saltatory conduction, excitability/refractory periods Isometric/isotonic recording, muscle dimensions & force/velocity, in vivo vs in vitro stimulation EMG interpretation, pupillary reflex pathways & blocking drugs Gaseous equilibria, gas stores, breathing higher O2 concentrations Insulin-cation interactions, hormones & pregnancy maintenance Luminal stimuli for gastric secretion, saliva & pancreas secretions Salt & water transport, differential effects of diuretics Concept maps with triggers Concept maps provide a very powerful tool for summarizing relationships between physiological variables, and are employed throughout this course. Each practical has at least 2 concept map exercises associated with it, and each exercise contains a trigger to provide useful clues which help focus on the influence of particular variables. Note that though not all of these will have been under experimental control, they may nonetheless be important in determining the final status of the physiological variable. Suggested map solutions will be provided online; some overlap with additional material content, as is apparent by comparison with the previous table. Specific topics per practical Practical Blood pressure Nerve Muscle Reflexes/Senses Respiration Endocrine Gastrointestinal Renal Concept map 1 Posture, baroreceptors & BP Nerve characteristics & CAP All influences on force All influences on stretch reflex Receptive field Ventilation & alveolar gases Thyroid hormone & heat Protein & gastric secretion Influences on renal pressures Concept map 2 Bike riding & BP Nerve characteristics & CV All influences on velocity All influences on pupil reflex Temperature perception Inhaled gases & ventilation Insulin resistance & tolerance Acid chyme & neutralization Influences on renal salt & water 5 BLOOD PRESSURE PRACTICAL 1) Confusing terms glossary This exercise is an extension of Appendix E on Confusing Terms in the No Frills Generic Skills Guide (link). Read the introduction there, then scroll down to find the terms which are relevant to the blood pressure practical. Below are some from that list, but you should be able to add more from your lectures and textbooks. Have a look at Appendix D on Concept Mapping for ideas. Chronotropic – inotropic Constriction – contraction Negative feedback – long loop negative feedback Relaxation - dilation Resistance of arterioles - resistance of bronchioles Smooth muscle cells – cardiac muscle cells Starling’s Law – Starling’s hypothesis Vasodilator – bronchodilator Here are some additional ones: Diastole – diastolic Systole – systolic Choose 5 pairs and write down your own definitions of the two terms in each. This is absolutely critical to your understanding of these words, as you are required to draw out both the commonalities and the differences between them, and express these in a manner which is clear to your reader. Obviously, if you are confused and it’s not clear to you, that confusion will be conveyed to them. i) ii) iii) iv) v) Now see how well you do in choosing the correct answer for the definitions in the Confusing Terms exercise. 6 CONFUSING CARDIOVASCULAR TERMS The technical language of physiology can be very confusing, as many terms are specialized jargon not used in everyday speech, and derive from Latin and Greek roots. One way to test your understanding of these is to consider pairs of terms which, although superficially similar, are actually different. In this exercise you must choose from the list of statements about each pair the one which you believe is MOST correct. 1) Adrenaline – noradrenaline a) Adrenaline and noradrenaline are both sympathetic nervous system neurotransmitters, but adrenaline acts mainly on beta receptors and noradrenaline on alpha receptors b) Adrenaline is a hormone released by the adrenal medulla and noradrenaline is a neurotransmitter released by sympathetic nerve endings c) Adrenaline and noradrenaline are both released at pre-ganglionic sympathetic nerve endings in the adrenal medulla [Answer b] 2) Chronotropic - inotropic a) Chronotropic effects of sympathetic nerves are those which relate to the heart rate whereas inotropic effects are those which relate to contractility of the cardiac muscle b) Both chronotropic and inotropic refer to actions of the sympathetic nerves on cardiac muscle, chronotropic relating to the duration of the contraction and inotropic to its intensity c) The difference between chronotropic and inotropic is that the former describes stimulation of cardiac activity while the latter describes its inhibition [Answer a] 3) Conductance – conduction a) Conductance refers to action potentials spreading within one cardiac chamber whereas conduction refers to their spread between chambers b) Conductance and conduction have the same meaning and are used interchangeably c) Conductance is a property of membrane ion channels or cytoplasm and refers to the ease of passage of charged particles, while conduction is used to refer to the spread of a potential along an excitable cell [Answer c] 4) Constriction – contraction a) Constriction of smooth muscles is what follows as a consequence of their contraction b) Constriction and contraction are both the opposite of relaxation c) Constriction refers to the narrowing of a contained space and can result from the contraction of muscles surrounding that space 7 [Answer c] 5) Contractility – stroke volume a) Cardiac muscle contractility determines the peak isometric force achieved for a given pre-load and after-load, and stroke volume is the volume ejected by each ventricular contraction b) Contractility is an intrinsic property of cardiac muscle whereas stroke volume varies from beat to beat c) Pre-load and after-load changes affect both contractility and stroke volume, since these both depend on the length of the cardiac muscle fibres and the pressure against which they eject [Answer a] 6) Diastole – diastolic pressure a) Diastole is the phase of the cardiac cycle during which the ventricles are passively filled by the venous return, while diastolic pressure is the lowest pressure reached in the arteries b) Diastole is the phase of the cardiac cycle during which the heart relaxes and filling occurs, while diastolic pressure is the lowest pressure reached in the arteries c) Diastole is the phase of the cardiac cycle during which the heart relaxes and filling occurs, while diastolic pressure is the lowest pressure reached in the ventricles [Answer b] 7) Diastolic pressure - end-diastolic pressure a) Diastolic pressure is the pressure in the ventricles at the beginning of diastole and end-diastolic pressure is the pressure in the ventricles at the end of diastole b) Diastolic pressure is the lowest pressure reached in the arteries, while end-diastolic pressure is the lowest pressure reached in the ventricles c) Diastolic pressure is the lowest pressure reached in the arteries, while end-diastolic pressure is the pressure in the ventricles at the end of diastole [Answer c] 8) End-diastolic volume – stroke volume a) End-diastolic volume is the volume of the ventricles at the end of diastole whereas stroke volume is the volume ejected by each ventricular contraction and is equal to end-diastolic volume minus end-systolic volume b) End-diastolic volume is the volume of the ventricles at the end of diastole whereas stroke volume is the volume ejected by each ventricular contraction and is equal to end-systolic volume minus end-diastolic volume c) End-diastolic volume is the volume of blood in the aorta at the end of diastole while stroke volume is the volume which is added to this by each ventricular contraction [Answer a] 8 9) Flow- flux a) Flow refers to the movement of blood, whereas flux refers to the movement of substances dissolved in blood b) Flow refers to movement of a substance while flux refers to the net transfer of a substance from one bodily compartment to another c) Flow describes movement due to passive forces, whereas flux can be due to either passive or active forces [Answer b] 10) Pulse rate – pulse pressure a) Pulse rate and pulse pressure are different names for the same parameter, the number of times per minute the heart beats and exerts a pressure which can be felt as a pulse in arteries near the body surface b) Pulse rate is the same as heart rate whereas pulse pressure is the same as arterial pressure c) Pulse rate is the number of heartbeats per minute (same as heart rate) while pulse pressure is the difference between the value of systolic and diastolic pressure [Answer c] 11) Systole- systolic pressure a) Systole is the phase of the cardiac cycle during which all the valves are closed, while systolic pressure is the highest pressure reached in the ventricles b) Systole is the phase of the cardiac cycle during which the heart contracts, while systolic pressure is the highest pressure reached in the ventricles c) Systole is the phase of the cardiac cycle during which the heart contracts, while systolic pressure is the highest pressure reached in the arteries [Answer c] 12) Starling’s Law – Starling’s hypothesis a) Starling’s Law says that the heart must beat more forcibly whenever the stroke volume is increased, while Starling’s hypothesis says that fluid will be lost from capillaries if the hydrostatic forces exceed the osmotic ones b) Starling’s Law refers to the increased contractility due to sympathetic nerve stimulation, while Starling’s hypothesis refers to the increased blood pressure resulting from this c) Starling’s Law refers to the relationship between end-diastolic volume and stroke volume, whereas Starling’s hypothesis refers to the balance of hydrostatic and osmotic forces causing fluid movement across the capillary wall [Answer c] 9 2) Misconception MCQs This exercise is somewhat similar to the Post-test questions, but focused on teasing out the most common misconceptions which trip students up even before they begin an experiment, and prevent them from “seeing the wood for the trees”. Because practical classes are run according to a set protocol laid down in the notes, it is all too easy to gloss over the connections between the background theory and the aims of the experiment. When students write practical reports, the introduction, which is supposed to set out the reasoning behind what they will do in the laboratory, often fails to cover this adequately, again suggesting a failure to come to grips with key concepts and their inter-relatedness. Essential connections become hidden in vague generalities which often are quite irrelevant to the actual procedures followed and hence the outcomes obtained. The first exercise is a simple one requiring you to choose the correct option which best completes the statement in the stem. Q1 Vasoconstriction of arterioles immediately results in: a) a fall in diastolic pressure because there is a drop in venous return to the heart b) a rise in diastolic pressure because flow-off of blood from the arteries is slowed down c) a larger stroke volume because the systolic pressure rises in the aorta d) a higher heart rate because the heart has to increase its cardiac output Q2 When heart rate increases following postural change: this must be due to baroreceptors sensing a fall in arterial pressure stroke volume must have decreased to keep cardiac output constant it indicates that sympathetic outflow has risen arterial blood pressure will increase proportionally a) b) c) d) [Answers: 1b 2c] The second exercise requires you to first choose the best option then choose the best explanation for why the other options should not be chosen. Concepts are listed to help you focus on the relevant physiology. Concepts: heart rate, diastolic pressure, venous return, muscle pump, cardiac output, end-diastolic volume, sympathetic, stroke volume, arteriolar muscle contraction, total peripheral resistance, vasodilator metabolites, systolic pressure, mean arterial pressure, venous pooling, baroreceptor reflex 10 The two experimental situations below are each followed by 5 statements explaining relevant physiological relationships. First choose the correct statement which BEST deals with the concepts involved. You will then be required to choose the reasons giving the best explanation for rejecting the other explanatory statements. Experimental situation 1) When a person exercises vigorously on a bike, their heart rate increases while their diastolic pressure decreases. A. As there is an increased venous return due to muscle pump activity, the increase in cardiac output means that heart rate must increase, but the decreased filling time means that end-diastolic volume is less, hence the drop in diastolic pressure. B. Bike riding is a stressful activity resulting in increased sympathetic outflow to the cardiovascular system, hence heart rate and stroke volume both increase, resulting in a drop in pressure during diastole. C. During bike riding there is increased sympathetic stimulation of heart rate and arteriolar muscle contraction, but the total peripheral resistance decreases because of the release of vasodilator metabolites, resulting in a decreased diastolic pressure. D. As the heart rate increases during bike riding, this causes a rise in systolic pressure which must be offset by a drop in diastolic pressure, otherwise mean arterial pressure will rise too high. E. The increase in heart rate during bike riding is accompanied by a decrease in stroke volume in order to keep cardiac output constant, and this lowers diastolic pressure. Experimental situation 2) Although gravity influences pressures in the cardiovascular system, changing from a lying to a standing position causes very little change in mean arterial pressure (MAP) despite a rise in heart rate. Remember: MAP = HR x SV x TPR, so a nearly constant MAP with an increased HR implies that either SV or TPR or both must have decreased A. Since gravity makes it harder for blood to flow back to the heart on standing, the body manages to maintain cardiac output by increasing both heart rate and total peripheral resistance. B. Changing position from lying to standing results in greater skeletal muscle activity, producing a change in total peripheral resistance which stabilizes MAP. C. The fact that heart rate rises but MAP does not means that total peripheral resistance must have fallen, since cardiac output has risen. D. Venous pooling on standing is accompanied by pooling in the arteries and arterioles in the legs as well, because the extra column of blood increases the hydrostatic pressure in the feet compared to at the level of the heart. E. The explanation for an almost constant MAP with postural changes is that venous return is not restored despite the increased sympathetic outflow caused by the baroreceptor reflex, so that stroke volume remains below normal. 11 The correct statement which best deals with the concepts in the previous exercise is indicated by an asterisk. For each of the other statements, 2 or 3 possible reasons are provided for why certain elements are NOT correct. Choose the reason(s) which you think provide the BEST explanations. Experimental situation 1) When a person exercises vigorously on a bike, their heart rate increases while their diastolic pressure decreases. A. As there is an increased venous return due to muscle pump activity, the increase in cardiac output means that heart rate must increase, but the decreased filling time means that end-diastolic volume is less, hence the drop in diastolic pressure. This is wrong because: a) the increased heart rate does not occur because of an increased cardiac output due to increased venous return, as heart rate is regulated by the autonomic nervous system b) both the increase in heart rate and the decrease in total peripheral resistance which occur during bike riding contribute to the drop in diastolic pressure c) a drop in diastolic pressure is not causally related to a drop in end-diastolic volume, and a rise in heart rate does not cause a decreased filling time until extremely high rates are achieved B. Bike riding is a stressful activity resulting in increased sympathetic outflow to the cardiovascular system, hence heart rate and stroke volume both increase, resulting in a drop in pressure during diastole. This is wrong because: a) increased heart rate and stroke volume will always increase arterial blood pressure, both systolic and diastolic b) increased heart rate and stroke volume may increase arterial blood pressure, both systolic and diastolic, but this depends on concurrent changes in total peripheral resistance c) diastole refers to the heart, being the period during the cardiac cycle when the muscle is relaxed, and cardiac pressures during this period are not normally related to heart rate and stroke volume *C. During bike riding there is increased sympathetic stimulation of heart rate and arteriolar muscle contraction, but the total peripheral resistance decreases because of the release of vasodilator metabolites, resulting in a decreased diastolic pressure. This is correct. D. As the heart rate increases during bike riding, this causes a rise in systolic pressure which must be offset by a drop in diastolic pressure, otherwise mean arterial pressure will rise too high. This is wrong because: a) even if systolic pressure were to rise due to heart rate increases during exercise, the way this would affect diastolic pressure would be to cause it to rise as well b) the factor which prevents mean arterial pressure from rising too high is not a rule saying it must not do so, but a mechanism involving metabolic autoregulation of peripheral resistance 12 E. The increase in heart rate during bike riding is accompanied by a decrease in stroke volume in order to keep cardiac output constant, and this lowers diastolic pressure. This is wrong because: a) the product of heart and stroke volume, namely cardiac output, is not kept constant by any feedback control mechanism operating during exercise b) even if stroke volume were to fall, diastolic pressure would not necessarily also decrease c) during bike riding there is a trade-off between the increased cardiac activity and the metabolically induced vasodilatation, so that systolic pressure rises slightly while diastolic pressure falls, and mean pressure shows only a small increase Correct explanations for Experimental situation 1 [A a and c, B b and c, D a and b, E a, b and c] Experimental situation 2) Although gravity influences pressures in the cardiovascular system, changing from a lying to a standing position causes very little change in mean arterial pressure (MAP) despite a rise in heart rate. A. Since gravity makes it harder for blood to flow back to the heart on standing, the body manages to maintain cardiac output by increasing both heart rate and total peripheral resistance. This is wrong because: a) gravity does not make it harder for blood to return to the heart as there is a greater pressure gradient from the veins to the right atrium due to venous pooling b) increasing total peripheral resistance does not maintain cardiac output c) increasing heart rate may not maintain cardiac output B. Changing position from lying to standing results in greater skeletal muscle activity, producing a change in total peripheral resistance whch stabilises MAP. This is wrong because: a) although there may be an increase in skeletal muscle activity when upright, it is the smooth muscle of the arterioles that is responsible for changes in total peripheral resistance b) if arteriolar smooth muscle has constricted more when upright due to increased sympathetic tone, this could not explain little change occurring in MAP since it would result in a larger rise C. The fact that heart rate rises but MAP does not means that total peripheral resistance must have fallen, since cardiac output has risen. This is wrong because: a) the fact that heart rate rises but MAP does not, implies that cardiac filling has been compromised by the shorter time for diastole b) cardiac output may not have risen, even if heart rate has, so a fall in total peripheral resistance may not be the reason for the almost constant MAP c) since heart rate has risen, sympathetic tone has increased, so total peripheral resistance has probably also risen, implying a fall in stroke volume 13 D. Venous pooling on standing is accompanied by pooling in the arteries and arterioles in the legs as well, because the extra column of blood increases the hydrostatic pressure in the feet compared to at the level of the heart. This is wrong because: a) pooling occurs in the veins because they are distensible, but not in the arteries and arterioles because they have smooth muscle in their wall, making them more rigid b) hydrostatic pressure increase occurs in both the venous and arterial sides of the circulation but the pressure gradient between them remains constant, so flow into the veins does not change c) venous pooling results in a smaller pressure gradient for returning blood from the veins to the heart *E. The explanation for an almost constant MAP with postural changes is that venous return is not restored despite the increased sympathetic outflow caused by the baroreceptor reflex, so that stroke volume remains below normal. This is correct. Correct explanations for Experimental situation 2 [A b and c, B a and b, C b and c, D a,b and c] 3) Logical fallacies (and how to avoid them) Following are statements written by students which either contain errors of fact or logic or are insufficient in terms of the physiological mechanisms involved. Explain as concisely as possible what you think is wrong with each and how they could best be corrected. a) To calculate mean arterial pressure from measurements made of systolic and diastolic pressure, the correct formula is: MAP = Diastolic Pressure + 1/3 Pulse Rate Explanation: Pulse rate is number of heartbeats per minute and is not a pressure. MAP is equal to diastolic pressure plus one-third of the difference between diastolic and systolic pressure, or pulse pressure. Extra useful information: The reason that it is one-third and not one half of the pulse pressure( i.e. that MAP is not halfway between diastolic and systolic pressure) is that the pressure versus time curve is not sinusoidal but has a steep rise and a slower fall as a result of the heart spending longer in diastole than in systole. So the mean pressure is nearer to diastolic than it is to systolic. b) As the muscles need more oxygen during dynamic exercise, CO and HR must increase. Explanation: This statement is not sufficient to demonstrate an understanding of the physiological mechanisms involved in the response of the heart to dynamic exercise. There must be some processes which operate to increase heart rate, and since cardiac 14 output is the product of heart rate and stroke volume, it (CO) will also increase provided that stroke volume is not decreased when heart rate increases . The principal mechanism for achieving these changes is the sympathetic nervous system outflow to the heart, which causes positive chronotropic and inotropic effects i.e. increases both rate and contractility. This then increases stroke volume and cardiac output. Extra useful information: As a consequence of both the increased cardiac output and the locally produced metabolites causing vasodilatation, there is an increased blood flow to the exercising muscles (receive a greater proportion of the greater total flow). c) When HR rises, ventricular filling is compromised, so SV falls to keep CO constant. Explanation: The first part of this statement is fine – when heart rate rises (albeit to very high values) ventricular filling can be compromised because there is such a short time before the next ejection. As a result of the decreased end-diastolic volume stroke volume falls (Starling’s Law). So what’s wrong? The above sequence of inter-related mechanisms has just explained the fall in SV, which does not happen IN ORDER to keep cardiac output constant. CO may or may not remain constant, depending on the net trade-off between an increased HR and a decreased SV, since CO = HR x SV. One recurring error which students make when discussing variables representing physiological parameters in an equation, is to ignore the number of these determining, or contributing to, the value of a particular parameter, and to assume that change in just one of them will produce a proportional change in the other parameter. d) When HR rises, MAP must also rise. Explanation: Given that MAP = HR x SV x TPR, it is NOT correct to say that, just because HR has increased, MAP will also increase, since SV and TPR may have also changed, with each either increasing or decreasing, and this may not necessarily result in an overall increase in the product of the 3 terms on the RHS of the equation. The reverse is also NOT correct i.e. because MAP has increased, this does not imply that HR must have increased, as one of the other 2 terms could be responsible for the overall increase in their product. Logical errors like this are one of the main reasons why students fail to gain marks in exam questions. Answers are expected to be mathematically correct and not just include a series of physiological relationships which may or may not be relevant. 15 4) Sequences which make sense Rearrange the following 20 concepts relating to postural change into a numbered chronological sequence which shows how one leads to the next (as in a concept map). 1. Standing up Decreased systolic pressure Distension of veins Decreased stroke volume Decreased central venous volume Decreased venous return Decreased cardiac output Decreased end diastolic volume Decreased pulse pressure Decreased atrial natriuretic peptide secretion Decreased MAP Pooling of blood Increased heart rate Increased hydrostatic pressure below the heart Increased sympathetic activity Increased adrenaline secretion Increased renin and angiotensin secretion Increased antidiuretic hormone secretion Increased peripheral resistance Increased contractility 1. Standing up 16 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about blood pressure variation with posture. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 17 When answering some of the questions on cardiovascular activity, the concepts below may be relevant. a) Systolic pressure is the highest pressure reached in the arteries. It is increased by a more forceful ejection and a larger volume of ejected blood per unit time, which in turn are the result of increases in stretch of cardiac muscle (i.e. increases in end diastolic volume) and/or increases in contractility (due to sympathetico-adrenal activity). It also depends on the distensibility of the arteries i.e. their ability to accommodate an increase in volume. Determinants of systolic BP Systolic BP = highest pressure in artery Force/velocity of L ventricular contraction Volume of blood ejected/stroke volume Distensibility of artery Determinants of Ejection Force and volume of ejection Pre-load/stretch of cardiac muscle fibres i.e. end-diastolic volume Sympathetic stimulation/adrenaline i.e. contractility After-load/aortic pressure i.e. systemic arterial pressure 18 Diastolic pressure is the lowest pressure reached in the arteries, just before the next stroke volume begins to be ejected. A rise in heart rate and a rise in peripheral resistance both increase diastolic pressure, because the former produces the next ejection before the pressure has had time to drop as low, and the latter slows down the flow-off into the periphery, so that the next ejection occurs while there is a higher pressure. Determinants of diastolic BP Diastolic BP = lowest pressure in artery Previous systolic pressure reached Time between ejections ie. Heart rate Ease of flow-off into arterioles i.e. TPR b) Total peripheral resistance is given by the formulae: TPR = 8ηl/∏ r4 where η = blood viscosity and l = length of vessel, and TPR = MAP/CO where MAP = mean arterial pressure & CO = cardiac output Determinants of TPR Decrease in Diameter Of Arterioles Increase in TPR Increase in Blood Viscosity Increase in Length of Blood Vessels 19 Determinants of HR Stimulation of Sympathetic Nerves to Pacemaker (SA node) Increase in Heart Rate Inhibition of Parasympathetic (vagal) nerves to Pacemaker (SA node) Increase in Circulating Adrenaline Concentration Extra note on gravity Gravity increases all pressures below the heart and decreases those above – this applies equally to arteries, arterioles, capillaries, veins and venules. Those below the heart will tend to distend, those above will tend to collapse. This is because there is the added pressure of the column of blood extending from the heart to the vessels below, and the lesser pressure of the points above the heart equal to the column of blood between them and the heart (think of a U-tube and comparison of hydrostatic pressures if there are points higher up). However the arteries and arterioles are thick-walled enough to withstand these pressures without either distending or collapsing, whereas the veins and venules are too thin-walled. They therefore become distended if pressure is sufficiently high below the heart, which reduces the volume of blood returning to the heart because it pools there. At levels beginning a few centimetres above the heart, similar vessels have a distending pressure which becomes increasingly sub-atmospheric the further up you go (the level of the heart is considered to be atmospheric), resulting in collapse. However the vessels in the skull are attached to tissues which prevent their collapse. When a person stands up, all the vessels at any particular levels are subjected to the same change in hydrostatic pressure, hence the DIFFERENCE in pressure which drives the blood around the circulation, from the arterial to the venous side, remains constant. It is only when some of that pressure gradient is lost, due to pooling in distensible vessels, that the driving pressure decreases. One way of thinking of this is in terms of electrical equivalents, where the extra venous capacity is like a capacitor which stores charge (blood) and reduces current (flow). Hence venous compliance is much greater than arterial compliance. Resistance and flow The following mathematical relationships are relevant to understanding the theory of practicals on the cardiovascular, respiratory and renal systems. (For the nervous system the electrical relationships apply directly.) 20 Blood is pumped out of the heart and distributed to the different organs of the body along a series of parallel blood vessels. This process can usefully be modelled by an electrically equivalent circuit, in which: the pressure P generated by the heart contracting is represented by a battery of emf = V total blood flow is represented by the current I total resistance to blood flow is represented by R, which is made up of a number of individual organ resistances, R1, R2, etc. Let us assume for simplicity that initially there are only 2 organs of identical resistance in the circuit. You should draw a simple diagram to illustrate it. 1. What are the mathematical relationships between V, I and R, and between R1 and R2 and R? P/V = I x R 1/R = 1/R1 + 1/R2 = 2/R2 R = R2/2 i.e. total resistance is half either individual resistance For a given V, I or current is divided equally between the resistances 2. Describe what happens to both total blood flow and its distribution between organs 1 and 2 when: a) resistance R2 doubles (due to narrowing of the vessels) 1/R = 1/R1 + 1/2R2 = 1/R2 + 1/2R2 = 3/2R2 R = 2R2/3 i.e. total resistance is now increased to two-thirds the original resistances i.e. it has increased by one-sixth For a given V, flow I will be decreased by one-sixth, and will be distributed between the two resistances in a ratio of 2:1 i.e. the original resistance will get 2/3 of 5/6 (5/9) of the original flow, and the doubled resistance will get 1/3 of 5/6 (5/18) of the original flow b) resistance R3 equal to R1 is added in parallel to R2 (due to opening up of vessels). 1/R = 1/R1 + 1/R2 + 1/R3 = 3/R2 R = R2/3 i.e. total resistance is now one-third either individual resistance, while flow I has trebled for a given V and is again divided equally between the resistances, with each receiving one-third of the total flow. 3. What changes in the circuit could make blood flow to a particular organ cease? Blood flow will cease when the resistance of that organ becomes infinitely large by comparison with the resistance of other organs in parallel i.e. when there are alternate pathways of lower resistance. This is equivalent to making R2 in a) very large, so that 1/R2 becomes very small, and R approaches R1. Notice though that any parallel resistance lowers the total R, and the smaller it is, the more the total resistance decreases. This is what happens when the blood vessels in an organ vasodilate. 21 Concept Maps for Blood Pressure Prac Map the cardiovascular variables which influence the MAP when posture is changed. Trigger: Did standing up suddenly from a crouching position produce a different cardiovascular response from when measurements were made after 2 minutes in the standing position? How do these outcomes demonstrate the differences between passive and active responses? Map the cardiovascular variables which influence the MAP when riding a bike. Trigger: Given that that most important functional outcome of a bike riding exercise is increased blood flow to the active muscles, how does the body regulate the “cardio” and the “vascular” functions so as to achieve this? 22 NERVE PRACTICAL 1) Confusing terms glossary This exercise is an extension of Appendix E on Confusing Terms in the No Frills Generic Skills Guide. Read the introduction there, then scroll down to find the terms which are relevant to the electrophysiology of nerve practical. Below are some from that list, but you should be able to add more from your lectures and textbooks. Refractoriness – refraction Here are some additional ones: Action potential – threshold potential Capacitance - conductance Conductance – conduction Electronic conduction – saltatory conduction Electrical potential – electrochemical equilibrium Nernst potential – resting potential Pacemaker potential – threshold potential Write down your own definitions of 5 of these pairs of terms, then have a look at the Confusing Terms list. CONFUSING NERVE TERMS 1) Action potential – compound action potential An action potential is recorded from a single excitable cell with one electrode being intracellular, whereas a compound action potential is recorded from a number of cells with both electrodes being extracellular. 2) Capacitance – conductance Capacitance refers to the ability of the membrane to build up and hold charge, while conductance is a measure of the ability of a current to pass across the resistant membrane. 23 3) Conductance – conduction The difference between these terms is that conductance is used to refer to ions moving through membrane channels, whereas conduction is used to refer to currents moving along excitable cells. 4) Electrotonic conduction – saltatory conduction Electrotonic conduction occurs when the membrane potential changes (depolarisations or hyperpolarisations) spread passively, depending only on the membrane capacitance and the membrane and axoplasmic conductances. Saltatory conduction occurs when the action potential jumps from one node of Ranvier to the next due to the passive electrotonic spread of the depolarisation from node to node. 5) Electrical potential – electrochemical equilibrium Electrical potentials are the consequence of charge separation, so that when there is no change in the charge stored on the membrane capacitance the potential remains constant, whereas when ions flow as current through membrane channels and alter the charge separation a change in potential occurs, referred to as a depolarisation or a hyperpolarisation. Electrochemical equilibrium for a particular ion occurs when the membrane potential is equal to its Nernst potential, since this is when the electrical force acting on the ion is exactly balanced by the chemical force due to its concentration gradient. 6) Nernst potential - resting potential Nernst potential for a particular ion is that potential where the electrical and chemical forces acting on that ion are equal and opposite. Resting potential refers to the steady potential across the membrane of an excitable cell which is not being depolarized or hyperpolarized, and is the weighted average of the Nernst potentials of all the ions which are able to cross the membrane; although each one of them is not in equilibrium because the resting potential is not at its Nernst potential, overall there is zero net current flowing, a necessary condition for a steady potential. 7) Pacemaker potential – threshold potential A pacemaker potential derives its name from the fact that the cell in which it is occurring sets the pace for other linked cells; its characteristic feature is a non-steady resting potential, so that gradual depolarisation to threshold occurs periodically due to alterations in ion conductances. The threshold potential is the potential at which the net flow of positive charge across the membrane is inward, producing the escalating increase in Na conductance which characterizes the Hodgkin cycle. 24 2) Misconception MCQs This exercise is designed to give you practice in logical thinking. For each pair of statements, choose which one of the linking final statements correctly completes the logical sequence. 1. A. Increasing diameter of nerve fibres increases conduction velocity. B. Measured velocity was that of the fastest fibres. Therefore: C. The largest fibres had the fastest velocities. D. Measured velocity was that of the largest diameter fibres. E The fastest fibres had the greatest velocity. 2. A. Increasing diameter of nerve fibres reduces the length of their refractory period. B. Nerve excitability was reduced to 50% when a test stimulus was given 4 msec after the conditioning one. Therefore: C. The nerve fibres contributing to the test response were those of smaller diameter. D. Shorter refractory periods are associated with reduced excitability. E. Larger diameter fibres are more likely to respond to a test stimulus 4 msec after a conditioning one. [Answers: 1D 2E] 3) Logical fallacies (and how to avoid them) Following are statements written by students which do not pursue their arguments to a correct conclusion. Explain as concisely as possible what you think is wrong with each and how they could best be corrected. a) The population code means that different nerve fibres, with different physical properties such as diameter and myelination, contribute different amounts to a compound action potential. b) Conduction velocity depends on the distance between the stimulating and recording electrodes, so it is greater in longer nerve fibres. c) Measuring the latencies to the beginning of the compound action potentials for different inter-electrode distances gives values from which the conduction velocity at each distance can be calculated. Explanations of errors: a) This statement mixes up two ideas, both of which are correct on their own, but cannot be linked giving one as the explanation for the other. While it IS true that: (i) different fibres have different physical properties 25 (ii) (iii) these result in their contributing different amounts to a compound action potential (since larger fibres have larger extracellular current flows), it is NOT true that: the population code means they contribute DIFFERENT amounts, only that they contribute to a combined response. b) This statement confuses conduction TIME, which does depend on the interelectrode distance, with conduction VELOCITY, which is obtained by dividing distance by time. It is predicated on the assumption that longer fibres “need” faster velocities of conduction of the information in the form of action potentials, presumably so that the “far reaches” of the organism can respond quickly enough. However, for the actual distances involved, given that conduction velocities are of the order of 50-100 metres/sec, or 5-10 cm/msec, the time it would take to reach the ends of the limbs is of the order of a few msec, even in humans. What the conduction velocity does depend on is diameter and myelination, which will be very close to constant along the length of a nerve fibre, so that the velocity will also be constant. c) This statement suffers from an assumption that conduction velocity varies along the length of a fibre, so that dividing inter-electrode distances by the corresponding latencies will give a series of different numbers, when in fact they will give the same number. This is the reason why the two variables are plotted against each other; this yields a straight line and calculation of its SLOPE then gives the velocity. 4) Sequences which make sense Although it is sometimes not very hard to arrange a sequence of steps in logical order, what is difficult is avoiding errors in the correct use of scientific language. The following sentences contain various errors in describing experiments on stimulating and recording from nerves. What are they? 1. Stimuli were applied to the nerve with an inter-stimulus delay of 5 msec. 2. The inter-stimulus was 5 msec. 3. The criteria used was a response bigger than 1 mV. 4. The phenomena investigated was the response to increasing stimulation. 5. The data shows that there was a big increase in response. 6. Less fibres were active during the relative refractory period. 7. The conduction velocity is the time taken to travel from A to B. 8. The latency is the time it takes for the stimulus artifact to illicit a response. 9. The conduction velocity is proportional to the latency. 10. The distance traveled by the stimulus was determined by the length of the nerve. Explanations of errors: 1. It is better to use the term inter-stimulus “interval” rather than “delay”, as this clearly describes the time between the applications of successive stimuli to the nerve, whereas “delay” refers to the time which elapses after a single stimulus until the next one is applied; it also is used to refer to the elapsed time 26 between a stimulus artifact and the beginning of the compound action potential. 2. The word “interval” is missing after “inter-stimulus”. 3. “Criteria” is plural and refers to more than one criterion, so needs a plural verb “were”; however as only one criterion is mentioned the sentence should read “The criterion used was a response bigger than 1 mV”. 4. A similar error is often made with the word “phenomena” which is the plural of “phenomenon”, so it is necessary to write “The phenomenon investigated was…” or “The phenomena investigated were……”. 5. “Data” is also plural, this time of the word “datum” (sorry, but that’s the difference between Latin and Greek!), so “data show….”. 6. “Less” is used for amounts, “fewer” for numbers. 7. A velocity cannot be a time, since it has units of distance divided by time. 8. “Latency” is another word for “delay”, as mentioned in 1. The stimulus artifact doesn’t even “elicit” a response which would be quite “illicit” for it to do, given that it is itself a response! The stimulus elicits the response. 9. The conduction velocity is able to be read off as the slope of the line expressing the relationship between distance and latency, which are proportional to each other if the line goes through the origin. 10. The stimulus didn’t travel anywhere, the action potential did, and what the length of the nerve determined was how long it took for the potential to reach recording electrodes at the other end. 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about compound action potential variation with inter-electrode distance. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 27 When answering some of the questions on nerve activity, the concepts below may be relevant. 1. Structural variation in nerve fibre populations Nerves are composed of nerve fibres or axons, which not only subserve different functions such as motor or sensory (touch, pressure, temperature) but also have different structures. The most important structural parameters with respect to their functioning are the diameter and the presence or absence of myelin. These variables determine a number of their electrical characteristics, which in turn determine how the fibre responds to a stimulus by generating local and propagated potentials. Three important properties relating to this are discussed below – threshold for excitation, conduction velocity of action potentials, and refractory periods. This section also addresses the issue of how the responses of nerves are built up from the responses of their individual contributing fibres. 2. Correlation of structure with electrical properties A simple electrical model of a nerve fibre consists of resistances for membrane and cytoplasmic current flow and a capacitance for membrane storage of charge. When the resting potential is temporarily changed in the direction of becoming less negative i.e. depolarization, the voltage difference between the depolarized and resting sections of the fibre drives a current round the resulting circuit, the magnitude of which depends on the total resistance in the membrane plus cytoplasm. The fraction of this voltage which drops across each resistor is proportional to its contribution to the total resistance (see Extra notes on Resistances). Both the diameter and the myelination of a fibre impact on these electrical properties. a) Diameter An increased diameter results in a decreased longitudinal resistance (inversely proportional to cross-sectional area) and also membrane resistance (inversely proportional to surface area). However the longitudinal decrease is greater than the trans-membrane, so more of the voltage drop occurs across the membrane and less along the fibre length. There is also an increase in the membrane capacitance, since this is proportional to area. b) Myelin Myelination produces an increase in membrane resistance and a decrease in capacitance. c) Time and space constants These changes then impact on two important constants – the time constant and the space constant. Each of these is a measure of the ease of achieving a particular outcome - either the time needed to change the membrane voltage by a certain fraction or the distance for which the membrane voltage is maintained above a certain fraction. Both increased diameter and myelination result in a smaller time constant and a bigger space constant. The net result of this is that large fibres reach threshold quicker and the electrotonic potentials travel further than in small fibres. These have obvious 28 implications for the speed of propagation of the action potential along these fibres (see 6. Conduction velocity). The implications for spread along myelinated fibres are more complicated because the myelin is interrupted periodically by non-myelinated areas known as nodes of Ranvier. Hence the comparison of properties includes different sections of the same fibre, as well as different fibres. The increased membrane resistance and decreased capacitance due to myelin means that reaching threshold will be more difficult than in an unmyelinated region; the converse of this is that little current will leak out where myelin is present. In addition, at the nodes there is an approximately 50-80-fold increase in the number of voltage-gated Na channels compared to internodal regions, while totally unmyelinated axons have a maximum of a tenth of the number of these Na channels that are present at the nodes. The increased space constant due to myelin increasing membrane resistance means that current is effectively forced to exit the cytoplasm at the nodes only. Because of the distances between these depolarized regions, this makes for much faster spread, a phenomenon called saltatory conduction (see 6.). The shorter time constant is less relevant here because the threshold in the inter-nodal regions is significantly raised due to the paucity of voltage-gated Na channels and the simultaneous increase in membrane capacitance necessitating a much larger current flow to produce a given depolarisation. Myelin & size 29 Myelin/size & activity • Time constant less √Rm x √Rc x Cm • Faster to threshold • (Faster spread) • Size – √less x √much less x more = less • Myelin – √more x same x less = less 3. • Space constant more √ (Rm /Rc) • Faster spread • Size – Membrane & Cytoplasmic R less, but Rm drops less than Rc (surf.vs.XS) • Myelin -Membrane R Rm more Threshold for excitation The main determinant of the threshold for excitation is the ease with which a given stimulus is able to produce the necessary depolarization of the membrane potential from its resting value to the value at which flow of positive charge inward initiates the Hodgkin cycle of increased Na conductance leading to further depolarization in an irreversible manner. The threshold is thus dependent on the degree to which a membrane is depolarized, which in turn will vary with the fractional voltage change which occurs here compared to along the length of the fibre. Since larger diameter fibres have lower internal resistances, more current will flow and more of the voltage drop will happen across the membrane, yielding a lower threshold of stimulus voltage necessary to reach a particular membrane voltage and trigger the Hodgkin cycle. 4. Population code When explaining the shape of the stimulus-response curve, it is necessary to refer to the different thresholds of fibres of different sizes making up the sciatic nerve. Sub-threshold stimuli, giving no AP, are too weak to bring any fibres to their threshold. At the threshold stimulus, when there is a tiny response, the most sensitive fibres of lowest threshold have been activated. As the stimulus strength increases more and more fibres, of progressively increasing threshold, are recruited, until all the fibres are contributing to the CAP with a maximal stimulus, and the curve reaches a plateau for supra-maximal stimuli. This section also refers to the distinction between the frequency code and the population code. It is the population code which is exemplified in the stimulusresponse curve, and even if the fibres were increasing their frequency of firing with increasing stimulus strengths (i.e. the frequency code was also operating), the method of recording did not display this. 30 5. Shape of CAP The amplitude of the CAP is not physiologically significant, as it is not an absolute measure of any particular event of physiological significance as say the blood pressure is, and varies with the recording conditions. However it gives an indication of the amount of activity in a particular group of nerve fibres, so that relative changes reflect variations in the number of contributing fibres. As the CAP is an envelope of potential change over time, it reflects the addition of all the APs generated at the stimulating electrodes and travelling down fibres of different sizes and myelination, and hence having different velocities of conduction. This means that the AP in the fastest fibres reaches the recording electrodes first and hence contributes to the beginning of the CAP, the average velocity fibres contribute to the peak, and the slowest contribute to the tail. Since the duration of the CAP is longer than that of a single AP, several single ones travelling at different velocities could sum to give a wider, but not a higher, CAP. It is therefore necessary to divide the area under the curve of the CAP by that under a single AP to obtain an estimate of the number of fibres contributing. The summation of subthreshold responses to successive stimuli which are sufficiently close together in time illustrates that only the complete membrane potential reversals which occur during an AP are picked up by the external recording electrodes, but that depolarisations are still occurring, which outlast the duration of an AP, as charge is stored on the membrane, which acts as a capacitor. Since local anaesthetics first affect the smallest pain fibres - which is their whole raison d’etre – they alter the shape of the CAP. Such drugs block action potential transmission by blocking voltage-gated sodium channels. Since surface area of the membrane is proportional to size, smaller fibres will have fewer channels needing blocking and hence respond to lower doses. Can you think of any other reasons for the differential responsiveness e.g. drug accessibility? AMPLITUDE of CAP from NERVE No. of nerve fibres depolarised to threshold for AP Threshold for depolarisation and EC current & voltage Strength of stimulus Myelination and size distribution of nerve fibres Spread of CAP i.e.duration Spread of conduction velocities 31 6. Conduction velocity The speed with which nerve axons transmit action potentials is another property which is affected by both their diameter and their myelination. There are 2 quite separate events contributing to the total time which elapses between the beginning of stimulation and the recording of a response: the time needed to depolarize the membrane to threshold, and the time it takes for the action potential to spread along the axon. Both of these are faster in larger fibres, and myelin additionally allows for a specialized form of conduction called “saltatory”, where the action potential “jumps” between nodes of Ranvier, giving a faster transmission. Saltatory Conduction • Nodes of Ranvier are only part of axon which is not myelinated • Cm here is much greater than in myelinated inter-nodal parts • Rm here is much less than in myelinated inter-nodal parts • Na channels here are much more frequent • Thus threshold reached only at nodes One reason why the velocity is slower in toad than human nerves is their lower body temperature. CONDUCTION VELOCITY in axons of NERVE AP jumps from node to node (gNa) Saltatory conduction ↑Space constant and spread ↓Time constant to threshold Speed of chemical reactions Myelination (↑Rm ↓Cm) Size (↓Rm ↓↓Rc ↑Cm) Temperature 32 7. Reduced excitability (refractory periods) of single fibres For a very short period of time after firing an action potential nerve fibres are unable to fire again, being “refractory” to repeated stimulation. For the period when they cannot respond at all, no matter how large the stimulus size, the term “absolute” refractory period is used, and this is followed by a period when a response can be obtained, but only with a larger stimulus than before, hence the term “relative” refractory period. The mechanisms which underpin this variation in excitability have been covered in your lectures. Basically they are the conductance properties of the voltagegated channels for Na and K, which undergo a cycle of availability followed by nonavailability each time an action potential occurs. The diagram and discussion below explain this. A nerve fibre cannot fire a second AP until it is just outside its absolute refractory period, so the maximum frequency of its firing is a little less than the reciprocal of this period e.g. if the ARR is measured as 1 ms, the fibre cannot fire every ms or 1000 times per second, but slightly less frequently. Published frequencies should be obtained by referring to graphs of nerve firing rates in your textbooks, and will vary depending on the type of nerve. As mentioned above, it is changes in sodium and potassium channel conductances which underlie absolute and relative refractoriness. The diagram below is taken from Dr. W. Phillips’ Supplementary Notes on Single Cells. Membrane potential sodium conductance potassium conductance 0 1 2 Time (msec) 3 Fig. 3 Conductance changes contributing to the action potential The sequence of events is as follows: (i) Depolarisation of the membrane potential to threshold (see 3.) results in the opening of voltage-gated Na channels and the Na conductance rises. (ii) Net inward flow of positive ions pushes the membrane towards the Na equilibrium potential. (iii) As the depolarisation approaches the peak of the action potential these Na channels close and are inactivated. (iv) There is a delayed opening of the voltage-gated K channels at this time and thus a rise in the K conductance. 33 (v) The Na channels became capable of re-opening as the membrane becomes less depolarized due to outward flow of K, with this process being complete approximately when the membrane potential has fallen to the threshold value. This marks the end of the absolute refractory period, and it is now possible to depolarize to threshold for another action potential. (vi) However, the increased K conductance eventually results in membrane hyperpolarisation, so that a greater stimulus voltage is required to bring the membrane to threshold as the net outward flow of positive ions pushes the membrane towards the K equilibrium potential. (vii) It is therefore now in its relative refractory period. (viii) The voltage-gated K channels close and the membrane potential returns to its resting level, as determined by the relative conductances of the K and Na leakage channels. It has now fully recovered its excitability and is no longer refractory to stimuli of the original voltage strength. During the period of reduced responsiveness of the whole nerve, a second AP is obtained from that fraction of the population which is not refractory. The length of both the absolute and the relative refractory periods varies between different nerve fibres, being shortest for the largest (presumably because the swing in favour of available voltage-gated Na channels over available voltage-gated K channels occurs sooner in the latter). For an individual fibre to fire an AP, it MUST be out of its absolute refractory period, but can be in its relative if the stimulus strength is increased. Accordingly, at any time when whole nerve excitability is reduced, but not zero, the contributing fibres will be partly in their relative refractory periods, partly completely beyond even this, i.e. completely recovered. 34 Concept Maps for Nerve prac Map the nerve characteristics which influence the shape of a CAP and how they influence it. Trigger: Did the CAP change shape when the responses were obtained at different stimulus strengths? How do structural variations between groups influence their functional behaviour? Map the nerve characteristics which influence the conduction velocities derived from a CAP and how they influence it. Trigger: Did the CAP change shape when the responses were obtained with different conduction distances? How do structural variations between groups influence their functional behaviour? 35 MUSCLE PRACTICAL 1) Confusing terms glossary This exercise is an extension of Appendix E on Confusing Terms in the No Frills Generic Skills Guide. Read the introduction there, then scroll down to find the terms which are relevant to the skeletal muscle practical. Below are some from that list, but you should be able to add more from your lectures and textbooks. Constriction – contraction Relaxation - dilation Smooth muscle cells – cardiac muscle cells Here are some additional ones: Active force – passive force Cross-bridge – filament overlap Isometric contraction – isotonic contraction Optimal length – sarcomere length Twitch – tetanus Write down your own definitions of these 5 pairs of terms. Check your answers by referring to the programmed text for the muscle practical. Topics covered there include: the sarcomere thick filament and thin filament cross-bridges optimal sarcomere length excitation-contraction coupling power stroke twitch tetanus isometric contraction resting length active tension passive tension elastic components optimal length isotonic contraction force-velocity curve fast and slow fibres 36 2) Misconception MCQs Choose the correct option which best completes the statement in the stem. Q1 a) b) c) d) Q2 a) b) c) d) Passive force recorded from a stretched muscle: is inversely proportional to the degree of stretch is generated in the series-elastic components only is generated in the parallel-elastic components only is generated in both the series- and the parallel-elastic components Active force can be recorded during a muscle contraction: only when the actin and myosin filaments slide past each other when the muscle is constrained so that its length cannot change if there is no filament overlap when sarcomere length is less than 2 μm [Answers: 1d 2b] 3) Logical fallacies (and how to avoid them) Explain what is wrong with each of the following statements. a) When the muscle was stimulated electrically, maximum optimal force was produced at the optimal length of the muscle. Explanation: This illustrates a typical confusion between “optimal” and “maximal”. Maximal or maximum force means the greatest value for this measurement; optimal means the best outcome, which in this case refers to the maximum force achieved. Hence “optimal length” is the length at which maximum force occurs, although the force may not be optimal for a particular purpose e.g. minimum energy consumption. b) Stretching the muscle increased the overlap between actin and myosin and hence decreased the sarcomere length. Explanation: Since the actin and myosin filaments slide past each other, stretching the muscle will DECREASE their overlap and INCREASE sarcomere length. c) Measurements of sarcomere length at two muscle lengths showed that there was a linear relationship between sarcomere and muscle length. Explanation: A relationship will always be linear when only two pairs of (x,y) values are plotted, since one can always join two points by a straight line. However it may not be proportional and go through the origin, if there is a constant which displaces the line on either the x- or the y-axis. An example of this would be if the measurement of muscle length included tendons at the ends which are not included in the laser diffraction measurement of sarcomere length. 37 d) A fatter muscle produces a greater maximum force because each sarcomere is able to form a larger number of cross-bridge attachments. Explanation: As the myofilaments have constant lengths in a given muscle as well as across different muscles, the number of possible cross-bridge connections in a sarcomere will be determined only by the degree of filament overlap i.e. muscle, and hence sarcomere, length (providing Ca supply is maintained). The only way for more cross-bridges to attach and produce a greater force maximum force (providing the muscle is at its optimal length) is if there are more sarcomeres lying in parallel. This is what occurs in a fatter muscle with a greater cross-sectional area. 4) Sequences which make sense Rearrange the following 10 concepts relating to isotonic force production in skeletal muscle into a numbered chronological sequence which shows how one leads to the next (as in a concept map). Indicate by arrows (↑↓) the direction of each change. Loaded muscle Shortening of muscle DHP receptor conformational change Development of isometric force to match load Generation of muscle action potential Electrical stimulation Sliding of filaments Ryanodine receptor Ca release Actin-myosin cross-bridge detachment Actin-myosin cross-bridge attachment 1. Loaded muscle 38 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about active isometric force variation with muscle length. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 39 When answering some of the questions on muscle activity, the concepts below may be relevant. Factors influencing muscle force What did you learn from both the isotonic and the isometric experiments about the factors which influence the force produced by a muscle when it contracts? Are there other structural properties or characteristics of the muscle which also have an influence? (Hint: how do muscles in different parts of the body or in different people compare?) The isometric experiment showed that maximum isometric force is achieved when a muscle is at its optimal length (greatest number of possible myosin-actin crossbridges formed) and is stimulated at a frequency which produces a fused tetanic contraction (greatest Ca availability). The isotonic experiment showed that the force produced also depends on the load, so that the maximum isometric force is achieved only when the muscle is contracting against a load which equals or exceeds that capacity. With smaller loads it begins to shorten before it has generated its maximum isometric force, as cross-bridges detach more easily and fewer maintain the tension. Muscles also differ in size or cross-sectional area with larger muscles having a greater number of sarcomeres in parallel. Whether the total population is activated depends on how many motor units are recruited for a particular movement and in real life this is achieved unconsciously as we match the effort to the load. When attempting to move big loads we also position our joints in such a way that the muscles are at or near their optimal length. Factors influencing muscle velocity What did you learn from the isotonic experiment about the factors which influence the velocity produced by a muscle when it moves? Are there other structural properties or characteristics of the muscle which also have an influence? (Hint: how do muscles in different parts of the body or in different people compare?) The isotonic experiment showed that the velocity produced depended on the load, with maximum velocity being achieved when the external load was zero. Muscles also differ in length with longer muscles having a greater number of sarcomeres in series. In the experiment and in real life a muscle must first overcome any internal loads or resistance to movement before it is able to shorten and move an external load, and this is achieved by operating at the optimal length so as to produce the necessary force. Again we use our muscles to our best advantage in an unconscious way. As well, different muscles are adapted to different tasks as their myosin ATP-ase activity is matched to their role in the body. Postural muscles are slow muscles which need to maintain body position against gravity, whereas fast twitch muscles have appropriate ATP-ase to enable rapid cross-bridge cycling and hence rapid movement. In vitro versus in vivo muscle stimulation How are muscles normally stimulated to contract in a person? Which of the above factors can be voluntarily controlled? Normally a motoneurone in the ventral horn of the spinal cord stimulates a motor unit. This is under voluntary or conscious control via descending pathways from the CNS, as well as being influenced by inputs from other reflexes, which all converge on the cell body. As noted above, joint position determines initial length, while conscious effort impacts on motor unit recruitment. 40 Concept Maps for Skeletal Muscle Prac Map the muscle characteristics that influence force production and how they influence it. Trigger: In what ways were the experimental conditions manipulated so that the muscle would produce its maximum possible active force? Are there any structural variations between muscles which could increase this? Map the muscle characteristics that influence velocity of a muscle contraction and how they influence it. Trigger: In what ways were the experimental conditions manipulated so that the muscle would produce its fastest possible velocity? Are there any structural variations between muscles which could increase this? 41 REFLEXES PRACTICAL (including tonic vibration reflexes and pupillary reflexes from SENSES PRACTICAL) 1) Confusing terms glossary This exercise is an extension of Appendix E on Confusing Terms in the No Frills Generic Skills Guide. Read the introduction there, then scroll down to find the terms which are relevant to the reflexes practical. Below are some from that list, but you should be able to add more from your lectures and textbooks. Constriction - contraction Refractoriness – refraction Relaxation – dilation Here are some additional ones: Extrafusal fibres – intrafusal fibres Final common pathway – motor unit Muscle end-plate – muscle spindle Write down your own definitions of these 3 pairs of terms. 2) Misconception MCQs Choose the correct option which best completes the statement in the stem. Q1 a) b) c) d) Q2 a) b) c) d) A decrease in reflex muscle contraction can occur by all of the following mechanisms EXCEPT: inhibition of the muscle end-plate by inhibitory motoneurones hyperpolarisation of its alpha-motoneurone cell body evoking a stretch reflex in its antagonist increased activity of inhibitory interneurones in the spinal cord The efferent neural pathway for constriction of the pupil is: pre-ganglionic fibres to superior cervical ganglion, postganglionic fibres to radial iris muscle pre-ganglionic fibres to ciliary ganglion, post-ganglionic fibres to radial iris muscle pre-ganglionic fibres to superior cervical ganglion, postganglionic fibres to circular iris muscle pre-ganglionic fibres to ciliary ganglion, post-ganglionic fibres to circular iris muscle 42 Q3 a) b) c) d) Vibrating both Achilles tendons of a blind-folded subject produces: The subjective feeling that they are falling backwards The experimental observation that they are falling backwards The experimental observation that they are falling forwards Stimulation of the Golgi tendon reflex in their Achilles tendons [Answers: 1a 2d 3b] 3) Logical fallacies (and how to avoid them) Explain what is wrong with each of the following statements. a) As the ankle is further away from its spinal cord synapse than the knee, the conduction velocity of action potentials travelling along its stretch reflex pathway will be faster. Explanation: The underlying assumption here is that a longer distance requires a faster propagation, which is a type of teleological argument saying that the time to react should be the same irrespective of which muscle is involved. Although this might be true, the determinants of conduction velocity in nerve fibres are primarily their size and myelination. If the fibres in the ankle and knee jerk pathways are similar in both these respects, then their velocities will also be very similar, and reflex times will in fact differ. b) Since there are two synapses in the reflex pathway, it is necessary to add on two synaptic delays to the latency of the EMG in order to calculate the total time taken by the nerve action potentials. Explanation: Since the two synaptic delays due to the two synapses are included in the latency of the EMG, they are already part of the total time, and must therefore be subtracted from this in order to give just the time taken by the nerve action potentials. c) Type Ia and Type Ib sensory fibres from skeletal muscles carry information from primary and secondary spindle endings respectively. Explanation: Type Ia and Type II fibres are the ones which do this, and their central connections are part of the reflex pathways which eventually result in contraction of the same stretched muscle. Type Ib fibres carry information from Golgi tendon organs in the joint capsules, and their central connections produce inhibition of muscle contraction as a protective mechanism; this occurs when they have been over-stretched by either external pulling on the tendon or excessive internal contraction of the muscle. d) Vibrating the Achilles tendon and hence stretching the gastrocnemius muscle spindle causes a subject to compensate by leaning backwards to contract the muscle back to normal length. 43 Explanation: This implies some sort of intent on the part of the subject, as it they were responding voluntarily. However their response is involuntary, and is a result of a message being sent to the CNS in the form of an increased firing rate of action potentials originating in the gastrocnemius muscle spindles. This initiates the reflex response of an increased output from the alpha-motoneurones to that muscle, whose contraction causes ankle flexion which makes the body lean back. e) When light is shone in the eye, it causes the circular iris muscle to constrict and the pupil to contract. Explanation: This illustrates a typical confusion between “constrict” and “contract”. Muscles contract, and this may cause a space to constrict. Light produces a reflex contraction of the circular iris muscle, leading to constriction of the pupil. 4) Sequences which make sense This exercise lists 7 steps which occur in a stretch reflex sequence, in chronological order. Each step is preceded by questions designed to focus your thinking on factors influencing that event. When you have written down your answers to these you may then read an explanation of what would happen if they were not optimized. Question a) What difference does either stretching or contracting the muscle prior to tapping the tendon make to the functioning of the different components of the reflex pathway? Answer: 1.Knee joint in “neutral position” so quadriceps muscle is neither over-stretched nor over-contracted Explanations: If the muscle is stretched or contracted i.e. the joint is not in its “neutral” position, then i) the intrafusal fibres (muscle spindles) will be less sensitive to stretch if they are slack due to extrafusal muscle fibre contraction ii) the extrafusal fibres will either not be at their optimal length when stimulated (if stretched) or resist the stretch (if contracted) Questions a) What influences the sensitivity of the spindle response (hint: related to 1)? b) What regulates the sensitivity of the spindle response? Answer: 2. Tendon tap produces stretch of muscle spindle 44 Explanations: a) The spindle is most sensitive at an intermediate length b) The gamma-efferent motoneurones regulate the sensitivity of the spindle response by producing contraction of the ends of the intrafusal fibres i.e. taking up the slack Questions a) What determines how many sensory nerve fibres are activated? b) What controls the sensitivity of individual motoneurones to activation (i.e. depolarization to threshold)? c) What else determines the relationship between total input to, and total output from, the spinal cord? Answer: 3.APs travel along afferent sensory nerve fibres to somas of alphamotoneurones Explanations: a) The greater the intensity of the tendon tap, the larger the number of sensory fibres activated b) The sensitivity of individual motoneurones, or stimulus strength required for depolarization to threshold, depends on the net of the total excitatory and inhibitory inputs from descending neural pathways c) Input from antagonistic muscles via inhibitory interneurones also influences the alpha-motoneurone potential Questions a) What determines how many muscle fibres are activated (hint: related to 3b&c)? b) Where in the pathway, spinal cord or neuromuscular junction, can this activation be inhibited? Answer: 4. APs travel along efferent motor nerve fibres to end-plates of extrafusal muscle fibres Explanations: a) The number of muscle fibres activated depends on the number of motor units activated i.e. the number of alpha-motoneurones brought to threshold b) This activation can be inhibited ONLY at the spinal cord level i.e. the soma of the motoneurone, as there are no inhibitory synapses from motoneurones onto skeletal muscle fibres 45 Questions a) What recorded response reflects the number of extrafusal muscle fibres activated? b) What aspect of this response would change if a different number of fibres had been activated? Answer: 5. APs generated in extrafusal muscle fibres Explanations: a) The electromyogram or EMG reflects the number of extrafusal muscle fibres activated b) The amplitude of this response would change if a different number of fibres had been activated i.e. it gives an indication of the muscle response Questions a) What are the parameters of the stimulus from a motoneurone to an individual muscle fibre which influence the size of the contractile force? b) What are the parameters of the muscle fibre which influence the size of the contractile force? c) What are the parameters of the whole muscle which influence the size of the contractile force (hint: related to 4)? Answer: 6. Contraction occurs in extrafusal muscle fibres Explanations: a) The only parameter is the frequency of the action potentials, since this determines the extent of summation producing a tetanus b) The contractile force developed by muscle fibres is greatest if i) the muscle is contracting at its optimal length ii) its movement is restricted to an isometric contraction c) Number of motor units contributing and cross-sectional area of the muscle both influence the size of the contractile force Questions a) What type of muscle contraction is occurring in the quadriceps muscle? b) What has to happen in which other muscles to allow this to occur? c) What are the pathways which achieve this? Answer: 46 7. There is extension of knee joint Explanations: a) The contraction is isotonic b) There has to be concurrent relaxation of the antagonistic hamstrings c) The pathway involves interneurones in the spinal cord being activated by the 1A afferent from the quadriceps and inhibiting the alpha-motoneurones innervating the hamstring muscle fibres (reciprocal inhibition) NB Whenever a muscle contracts, its antagonist is stretched (e.g. biceps and triceps, quadriceps and hamstrings). This would then initiate its own stretch reflex, causing it to contract and oppose the movement of the joint, unless its own alpha-motoneurones were inhibited. Although the Golgi tendon organ is traditionally thought to be activated by EXCESS FORCE, and to function to protect THE SAME MUSCLE from excessive further contraction, it here plays a role in controlling contraction of the antagonist. 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about muscle CAP variation with stimulus strength. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 47 When answering some of the questions on reflex activity, the concepts below may be relevant. EMG interpretation It is vitally important to appreciate what is being recorded on the screen, and what physiological events are responsible for each component of the EMG. Before the tutorial, see if you can answer the following questions (some of them relate to your skeletal muscle class). a) At what time is the tendon being tapped? Zero time, as the microswitch in the hammer triggers the computer sweep. b) What is occurring during the latency period or delay to the beginning of the muscle compound action potential? This period is the sum total of the time taken to generate action potentials in the 1A afferent fibre endings in the muscle spindles, transmit them along the axon to the synapse in the spinal cord with the somas of the alpha-motor neurones, synaptic transmission here, generation and transmission of action potentials in the alphamotor neurones to the muscle end-plates of each motor unit, neurotransmission here, generation of end-plate, then action potentials in the muscle fibres. Therefore, to calculate the conduction velocity along the afferent and efferent nerve axons it is necessary to subtract the time taken by the two neurotransmissions. c) What events follow the muscle action potential? Excitation-contraction coupling follows the muscle action potential, as voltage-gated calcium channels are opened and the sarcoplasm is flooded with calcium, which then triggers the cross-bridge cycle. Note that we are not measuring the contraction here. d) What type of muscle contraction is occurring? Since the joint is constrained but is free to move, the muscle contraction is isotonic. e) a What is the basic structural unit which is recruited by the stretch to bring about contraction? As mentioned in b), the basic structural unit recruited is the motor unit i.e. the muscle fibres innervated by one alpha-motor neurone, which includes their muscle spindles. f) How can more of these units be recruited? A more effective/stronger stimulus may stretch more muscle spindles more effectively. g) What experimental parameters could be varied to make the extrafusal muscle fibres produce more force? There are two main variations which could be tried – a sharper tap with the hammer and having the muscle in as ideal a starting position as possible, fully relaxed. This is because the main extrinsic factors influencing muscle force are its initial length (which should be close to rest-length to be optimal) and the frequency of stimulation from its alpha-motor neurone (so as to produce a tetanic contraction); 48 this latter condition is most likely to be achieved with a sharper tap. Unloading the muscle so that there is no external resistance to overcome also results in the maximum force being produced. h) What intrinsic properties of the extrafusal muscle fibres influence the amount of force they are capable of producing? The main structural characteristic influencing force is muscle cross-sectional area, since the force generated by sarcomeres in parallel is additive. Pupillary Reflexes (Senses practical) The pupillary reflexes, particularly the light reflexes, are of great clinical importance. The length of the reflex pathways, as well as their location, makes the reflexes vulnerable to intracranial damage; abnormal light reflexes are an important indicator of brain damage after head injuries. Light activates an afferent pathway from the retinal photoreceptors to the Edinger-Westphal nucleus, then the efferent pathway goes via the ciliary ganglion. Darkness, or emotions such as fear, can cause reflex pupillary dilation, via a sympathetic nervous system pathway. The pupillary reflexes: a) are triggered by changes in the intensity of light entering the eye, and improve the quality of the resultant image via mechanisms illustrated in the acuity experiment b) are part of the triad of accommodation, convergence and pupillary constriction, triggered when focusing on an object nearer to the eyes, when the lens, visual axes and pupil diameter all alter in ways which improve image quality (here the stimulus is not a change in light intensity) c) are mediated by autonomic nervous system pathways, as detailed above; parasympathetic motoneurons contract the circular muscle and sympathetic ones, the radial muscle, resulting in opposite changes in pupil diameter. In accommodation, ciliary muscle contraction results from parasympathetic motoneurons, allowing the lens to become more rounded (powerful), whereas sympathetic stimulation of the muscle causes it to relax and the lens to flatten. Knowing these actions you can predict the consequences of applying drugs to the eye, as is done in visual examinations. What will happen to pupil diameter and lens power if (i) atropine (ii) adrenaline is used? (i) (ii) Atropine is a muscarinic cholinergic antagonist, so will block transmission from all the motoneurons onto the smooth muscles in the parasympathetic pathways. The circular muscle of the iris of both pupils will relax, resulting in dilation of both pupils. The ciliary muscle of the lens will also relax, resulting in flattening of the lens and less power. Adrenaline is a sympathetic agonist, so will stimulate the smooth muscles activated by sympathetic pathways. The radial muscle of the iris of both pupils will contract, also resulting in dilation of both pupils. The ciliary muscle of the lens will also relax, producing the same lens flattening and decreased power as with atropine. 49 Concept Maps for Reflexes Pracs Map the sequence of events in the stretch reflex pathway leading to muscle contraction. Trigger: As there are a number of different tissues and physiological mechanisms involved in a stretch reflex, in what way could changes at different points in this pathway produce a better response? Map the sequence of events in the pupillary light reflex pathway leading to pupillary constriction. Trigger: As there are a number of different nerves and muscles involved in the light reflex, in what way do these become either stimulated or inhibited when light is shone into one eye? 50 SENSES PRACTICAL (including cutaneous and visual sensation) 1) Confusing terms glossary Write down your own definitions of these 5 pairs of terms. Receptor – receptive field Frequency code – population code Adaptation – accommodation Blind spot – optic disc Myopia - hyperopia 2) Misconception MCQs Choose the correct option which best completes the statement in the stem. Q1 When the temperature of the skin rises from 30 to 35OC: a) Heat loss to the environment will always be possible b) Cold receptors do not change their firing rate c) Warm receptors increase their firing rate d) Pain receptors are stimulated Q2 When plotting visual fields by the hand perimetry method: a) Each blind spot is easily found b) The plotted outline of the visible field is exactly the same for both eyes c) The temporal field extends beyond the plot d) The nasal field exceeds the temporal in extent [Answers: 1c 2c ] 51 Logical fallacies (and how to avoid them) Explain what is wrong with each of the following statements. a) Because there are both cold and warm receptors which sense the skin temperature, the brain is able to determine the absolute temperature of an object placed there very accurately. Explanation: Although there are both cold and warm receptors, which are defined by the direction of the temperature change which excites them, it depends on the initial temperature of the skin as to whether the new object causes heat to flow away from or towards them, and hence the change in the firing rate of their afferent nerves. Starting at a temperature within the range to which both the cold and warm receptors are able to respond produces an increase in firing frequency of one and a decrease in the other, but this signals the amount and direction of change, not the actual temperature i.e. the new temperature relative to the old one. b) A myopic/short-sighted person has a smaller range of accommodation because their near point is so much closer to the eye. Explanation: Range of accommodation is given by 1/dn -1/df. Although the near point of a myopic person is less than that of an emmetrope (e.g. 1-2 cm compared to 9-10 cm), and hence the first term in the equation is larger, this has much less of an influence on the equation than do the differences in the second term, since it is approximately zero in an emmetrope whose far point is effectively infinity, while a myope may not be able to see clearly beyond half a metre, making the inverse of this term very large and the difference between the two terms very small. c) The range of accommodation decreases with age because the far point gets closer. Explanation: As people age their lens becomes less able to accommodate to view near objects, so their near point increases. This decreases their range of accommodation without there necessarily being any decrease in their far point, although it may also eventually decrease. Hence, like the myope, their near and far points end up much closer together than for an emmetrope. d) As the size of the optic disc does not change when viewing different objects, neither does the size of the blind spot. Explanation: While is it correct that the size of the optic disc does not change, because it is an anatomical structure, the size of the blind spot is entirely dependent on the distance at which an object is viewed, since the light rays which fall on the disc, and hence cause “blindness” to what is at that position, form a cone which increases in cross-sectional area as the object is moved further away from the eye. 52 4) Sequences which make sense Rearrange the following 10 concepts relating stimulation of a Pacinian corpuscle into a numbered chronological sequence which shows how one leads to the next (as in a concept map). Indicate by arrows (↑↓) the direction of each change. 1. Vibrator applied to skin Firing of action potentials down axon Generator potential develops at initial segment Rapid adaptation of membrane potential Na influx and K efflux occurs Synaptic transmission occurs in thalamus Deformation of connective tissue layers Threshold for action potential reached in axon Cation channels mechanically opened Awareness of sensation occurs in somatosensory cortex 1. Vibrator applied to skin 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about the variation of two-point discrimination with location. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) 53 Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 54 Concept Maps for Senses Pracs Map the anatomical characteristics which influence the size of the receptive field of a cutaneous sensory neuron for touch/pressure and how they influence it. Trigger: Did the use of different tests for cutaneous sensation give uniform results across different body locations? How do the structures of the pathways explain this? Map the functional characteristics which determine the perception of temperature when the hand is immersed in a beaker of hot water, left there for a while, then removed. Trigger: Which receptors sense temperature, how are they stimulated/inhibited, and how do they interact? Does time of exposure make any difference? 55 RESPIRATION PRACTICAL 1) Confusing terms glossary This exercise is an extension of Appendix E on Confusing Terms in the No Frills Generic Skills Guide. Read the introduction there, then scroll down to find the terms which are relevant to the respiratory practical. Below are some from that list, but you should be able to add more from your lectures and textbooks. Constriction - contraction Flow – flux Renal medulla – medulla of brainstem Resistance of arterioles - resistance of bronchioles Vasodilator – bronchodilator Add your own additional pairs: Write down your own definitions for the following pairs of terms, and give their units and typical values where appropriate. Consider any mathematical relationships which are relevant. i) total pulmonary ventilation – alveolar ventilation ii) hyperventilation – hypoventilation iii) tidal volume – residual volume iv) end-tidal alveolar gas – dead space gas v) central chemoreceptors – peripheral chemoreceptors 2) Misconception MCQs Choose the correct option which best completes the statement in the stem. Q1 a) b) c) d) Q2 a) b) c) d) Under steady state conditions of metabolism: alveolar pCO2 is directly proportional to alveolar ventilation alveolar pO2 is directly proportional to alveolar ventilation alveolar pCO2 is inversely proportional to alveolar ventilation alveolar pO2 is inversely proportional to alveolar ventilation The greatest impact of the diversion of gases into and out of body stores when holding one’s breath is in producing: a faster rise in alveolar pO2 a slower fall in alveolar pO2 a faster fall in alveolar pCO2 a slower rise in alveolar pCO2 [Answers: 1c 2d] 56 3) Logical fallacies (and how to avoid them) Explain what is wrong with each of the following statements. a) Alveolar ventilation is proportional to tidal volume, dead space volume and respiratory rate. Explanation: Alveolar ventilation is equal to (tidal volume – dead space volume) times respiratory rate. Since proportional means that the two variables change together by the same fraction, alveolar ventilation will change proportionally to respiratory rate change, but only to the difference between the two volumes and not each individually. Furthermore, if a simultaneous change in this volume difference were to occur when the rate was changed, then alveolar ventilation would be equal to a product in which no term had been kept constant, and hence would not be proportional to any variable. b) During end-tidal expiration, due to diffusion of oxygen and carbon dioxide down pressure gradients, the pressure in blood equates that of the alveoli, producing an equilibrium. Explanation: This statement contains a number of common errors. End-tidal expiration should rather be expressed as end-tidal volume in order to make a claim about equality of gas pressures. The pressure in blood must be defined as a particular gas partial pressure (and not hydrostatic or osmotic pressures). Diffusion of oxygen and carbon dioxide down their pressure gradients does produce an equilibrium between alveolar and dissolved blood gas, so that the values for each gas in the two compartments are equal. It is this equality which allows one to use a sample of endtidal gas as a proxy for arterial blood, as the dead space gas has already been exhaled. c) The amounts of pCO2 and pO2 are different in different subjects, as these are dependent on age, sex, weight etc. Explanation: Although one might hypothesize that physical differences between individuals would result in different blood gas levels, counteracting this are the physiological homeostatic mechanisms which maintain pCO2 and pO2 at approximately the same values within the normal range, so that metabolism can proceed efficiently. d) Breathing pure oxygen containing a small fraction of carbon dioxide causes all alveolar nitrogen to be replaced within a couple of breaths. Explanation: As the amount of air remaining in the lungs at the end of a normal expiration (functional residual capacity) is about 2300 ml, and only about 350 ml of new air is brought into the alveoli with each normal inspiration, it obviously requires many more than a couple of breaths to completely replace the alveolar air, including the major nitrogen fraction, with pure oxygen. 57 4) Sequences which make sense Rearrange the following 10 concepts relating to ventilatory change into a numbered chronological sequence which shows how one leads to the next (as in a concept map). Indicate by arrows (↑↓) the direction of each change. 1. Rebreathing own expired air Alveolar pCO2 Arterial pCO2 Alveolar pO2 Arterial pO2 Oxyhaemoglobin Carbaminohaemoglobin Plasma bicarbonate concentration Alveolar ventilation rate Medullary chemoreceptor stimulation 1. Rebreathing own expired air 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about alveolar gas composition variation with ventilation pattern. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 58 When answering some of the questions on respiratory activity, the concepts below may be relevant. Gaseous equilibria The respiratory system is the site of exchange of gases between the outside air and the body. Cellular respiration uses O2 and produces CO2 when food is oxidised/metabolised in the tissues, with O2 being supplied by the systemic arterial blood and CO2 being removed by the venous blood. At the venous end of the pulmonary capillaries which surround the alveoli, blood enters with a pO2 of about 40 mm Hg and a pCO2 of about 46 mm Hg. When inspiration occurs and a tidal volume of room air enters the respiratory system, about a quarter remains in the dead space, where no exchange is possible, and three-quarters mixes with the alveolar air (functional residual capacity). As end-expiratory alveolar air had a pO2 of about 100 mm Hg and a pCO2 of about 40 mm Hg, this extra volume alters the alveolar gas concentrations so that the pO2 rises above 100 mm Hg (though much below inspired) and the pCO2 falls below 40 mmHg. Under these conditions there are pressure gradients for O2 to diffuse into, and CO2 to diffuse out of, the capillary blood. Equilibration with alveolar air is very rapid and results in the blood at the arterial end of the pulmonary capillaries leaving at partial pressures for O2 and CO2 of 100 and 40 mm Hg respectively. When air is now breathed out, the first fraction will be dead space gas, while the air at the end of the expiration will be pure alveolar gas. Hence taking an end-tidal sample avoids contamination with dead space gas while at the same time providing the closest approximation to arterial blood gas values. It is important to appreciate that the pressure gradient created by the active contraction of muscles during inspiration results in bulk flow of air into the lungs, and is different from the simple diffusion down partial pressure gradients which results in equilibration between alveolar air and blood. [This is obvious when you consider that pCO2 is almost zero in room air, but this does not result in CO2 in lung air flowing out along the CO2 gradient.] During a breath-hold, gas exchange between the body and the outside air is prevented, but gas exchange continues between the lungs and the blood as the tissues continue to metabolise. Assuming that the metabolic rate does not change, CO2 will accumulate in, and O2 will be withdrawn from, alveolar air, so that pCO2 will rise and pO2 will fall. During hyperventilation, gas exchange between the body and the outside air is increased above the level which normally balances metabolism. As a result extra CO2 will be lost from the body and extra O2 will accumulate in the body, so that pCO2 will fall and pO2 will rise. With hypoventilation the pCO2 increase can result in cerebral blood vessels dilating, which may produce a headache because of the inrush of blood. Conversely, with hyperventilation the pCO2 decrease can constrict cerebral blood vessels, with the reduced blood flow then producing a tissue hypoxia in the brain, again causing a headache. Additionally in hyperventilation, the alkalosis which accompanies the blowing off of the extra CO2 from stores exacerbates the arteriolar vasoconstriction. It may also cause "pins and needles" and muscle cramps due to a decrease in free Ca2+ as more becomes bound to albumin, and may produce disturbances of cardiac function. Some or all of these symptoms may develop simply as a result of anxious over-breathing. 59 Gas stores Since the oxidation of carbohydrate uses up one O2 for every CO2 produced, the respiratory quotient RQ is 1, and when ventilation matches metabolism, the respiratory exchange ratio is also 1, i.e. volume of CO2 produced equals volume of O2 consumed. This implies that a change in alveolar partial pressure of one gas should be paralleled by an equal, though opposite, change in the alveolar partial pressure of the other gas. Even though assuming an RQ of 0.8 because a mix of food types is being oxidised would change alveolar pCO2 by only 0.8 of the amount pO2 changes, the data obtained when hypo- and hyper-ventilating show that the absolute changes in pO2 are very much greater than those in pCO2, typically 2-3 times as big. The explanation for this is that the CO2 being generated in the tissues when breath-holding does not all escape into the lungs, but is diverted into chemical stores in the form of bicarbonate ion and carbaminohaemoglobin. The CO2 enters into reversible chemical reactions as follows: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3CO2 + Hb ↔ Hb-CO2 Under opposite conditions, when CO2 is escaping from the body faster than it is being generated by metabolism, these reactions are reversed. In both situations the change which would occur in CO2 concentration in plasma, and hence in alveolar gas, is mitigated by an additional sink or source of the gas. The greater the size of the stores which can function as either sources or sinks, the more possible is a temporary imbalance between the metabolic and alveolar gases. Body stores for O2 are much more limited, and in this experiment the lungs were deliberately not over-filled before breath-holding to keep this store constant. Oxyhaemoglobin is the chemical store which can buffer changes in the plasma O2 concentration, according to the reversible chemical reaction: O2 + Hb ↔ Hb-O2 However the position of the oxyhaemoglobin dissociation curve (or Hb saturation curve) is such that Hb is about 90% saturated above a pO2 of 60 mm Hg and almost fully saturated at the normal arterial pO2 of 100 mm Hg, leaving little extra room for O2 to be added as the pO2 rises during hyperventilation. Note that the correct explanation of the failure of Hb saturation to rise above 100% is NOT that it has a limited carrying capacity (which it does) but that whatever its capacity, 100% means full saturation and no more possibility of more O2 being added. Despite this store being full, the extra O2 can dissolve in the plasma, thereby raising the pO2. It is important to bear in mind that the two gases will not be present in equal concentrations in the plasma when their partial pressures are the same. This is because their solubilities in aqueous solution are very different, with CO2 being about 20 times as soluble as O2. In contrast to alveolar-plasma gas exchange, tissueplasma CO2 uptake and O2 release is not usually given as much emphasis, but exactly the same principles apply. Breathing higher oxygen concentrations Normally the O2 in inspired air constitutes 20% of the total gases, the CO2 is negligible, and the remaining 80% is N2 (minus the very small amounts of H2O and rare gases). However, in the Respiration practical class, two different gas mixtures are used in addition to room air. In the Alveolar Ventilation experiment, pure O2 is inhaled for several minutes. In the Demonstration of Control of Ventilation experiment, a 5% CO2 – 95% O2 mixture is initially inhaled, and the addition of the subject’s expired CO2 then progressively increases the contribution of CO2 to the inhaled mixture, while the O2 is simultaneously being removed from it. 60 It has been shown that, with a normal tidal volume, it requires about 16 breaths to completely replace alveolar air with new room air. Thus, it would take the same number of breaths to replace a subject’s alveolar air with a new inspired gas mixture, such as 100% O2. If this is the case, then several minutes of breathing pure O2 will be more than enough time to achieve this total replacement, and to increase the pO2 towards 760 mmHg. A much lower value therefore implies that the mask being used was inadequate to prevent air as well as pure O2 being inhaled. Of course CO2 being produced by metabolism will still be added to the alveolar gas, but will constitute only a tiny fraction of the total (about 40 mmHg). If the subject experiences discomfort or anxiety and over-breathes as a result, the pCO2 will drop below this. Note that, despite the extremely high pO2 levels in the blood perfusing the carotid and aortic bodies (peripheral chemoreceptors), there is no evidence that ventilation is reduced. Note also that the increase in body levels of O2 is not being achieved by greater amounts being bound to haemoglobin, since saturation is almost 100% to start with. Where the O2 is being increased is in the plasma, where amounts of O2 dissolved can rise indefinitely. The O2 will be progressively replacing mainly N2 in the lungs, but as gas exchange between the alveolus and the pulmonary capillary is so efficient, it will also be replacing N2 dissolved in the plasma, which will be diffusing out along its large gradient caused by the absence of any N2 in the inhaled gas. Whatever the composition of the air being breathed, the principle of mixing applies, with the total pool of gases being made up of the lung and plasma volumes, and the alveolar endtidal gas pressures being in equilibrium with, and thus giving a measure of, the arterial gas pressures. In the rebreathing experiment, the first few breaths are deliberately made much larger than a normal tidal volume, so as to speed up the mixing process. Thus the recording begins at a pCO2 of about 50 mmHg, and this then increases to about 60 mmHg over a period of a couple of minutes. As there is no possibility in this experiment for N2 to be lost from this closed system, the final gas composition will be determined by the extent to which the original N2 in the lungs and plasma has equilibrated with the bag, the original O2 in the bag, lungs and plasma has been depleted by metabolism, and the original CO2 in the total system has been increased by metabolism. As predicted, the recording demonstrates the increase in CO2 being accompanied by a decrease in O2. The high pCO2 levels in the circulating blood produce high H+ levels at the medullary chemoreceptors, stimulating ventilation. This is evident in the gradually increasing tidal volumes, but the durations of breaths (and hence the instantaneous frequencies) do not appear to increase as systematically. The arterial pCO2 is monitored directly by the peripheral chemoreceptors, contributing 20% of the response to this gas, and indirectly by the central chemoreceptors in the medulla, contributing 80% of the CO2 response. As the medullary CO2 is first converted to H+, the central response is somewhat slower. H+ ions themselves are unable to cross the blood-brain barrier, so blood pH is monitored by the peripheral chemoreceptors. However their main stimulus is a low arterial pO2, to which they respond even more when arterial pCO2 is above normal; conversely the pO2 needs to be very low if the pCO2 is below normal. Likewise the central chemoreceptor response to pCO2 is increased under hypoxic conditions, until the brain becomes so deprived of O2 that respiration fails. Note that even when pO2 is extremely high, this does not of itself suppress ventilation, but very high pCO2 (above 80 mm Hg) does depress respiration. 61 Concept Maps for Resp. Prac Map the influence of ventilation pattern on alveolar oxygen and carbon dioxide partial pressures. Trigger: How do hypo- and hyperventilation alter the normal equilibrium between cellular respiration (metabolism) and respiratory gas exchange? Do gas stores play any role? Map the influence of inhaled gas composition on alveolar ventilation. Trigger: What is the result of mixing a small tidal volume of a certain composition with a much larger lung volume of different composition? Which gas compositions would make a significant difference to the drive to breathe? 62 ENDOCRINE PRACTICAL 1) Confusing terms glossary This exercise is an extension of Appendix E on Confusing Terms in the No Frills Generic Skills Guide. Read the introduction there, then scroll down to find the terms which are relevant to the endocrine practical. Below are some from that list, and you may be able to add more from your lectures and textbooks. Aldosterone – antidiuretic hormone Diuresis – diabetes Dysmenorrhea – premenstrual tension Glucose tolerance – immunological tolerance Glycogen-glucagon Granular cells – zona granulosa Inhibin – inulin Inulin – insulin Lacteal – lactation Melatonin – melanotropin Milk secretion – milk ejection Proliferative – progestational Renal cortex – adrenal cortex Renal medulla – adrenal medulla Thyroglobulin – thyroxine binding globulin Thyrotropin releasing hormone- thyroid stimulating hormone Trophic – tropic Zona granulosa – zona glomerulosa 2) Misconception MCQs Choose the correct option which best completes the statement in the stem. Q1 In a glucose tolerance test: a) glucose absorption in the small intestine occurs by a secondary active transport process b) only one hormone, insulin, changes its secretion rate c) peak plasma glucose concentration reached is lower after prior carbohydrate restriction d) plasma glucose concentrations never go below the fasting level Q2 a) b) c) d) Abnormalities of thyroid hormone secretion: occur only when TSH secretion is abnormal do not usually produce significant clinical signs or symptoms may be the result of abnormal antibody stimulation of the gland cannot be treated by pharmacological means but require surgery Q3 In relation to the use of arginine to stimulate growth hormone secretion: a) this depends on the arginine first stimulating insulin secretion b) a positive response can occur in the absence of hypoglycemia c) this does not mimic what happens physiologically d) any rise in growth hormone levels is considered significant [Answers: 1a 2c 3b] 63 3) Logical fallacies (and how to avoid them) Explain what is wrong with each of the following statements. a) As insulin and growth hormone are the two main anabolic hormones, they have similar effects on all aspects of carbohydrate metabolism. Explanation: Insulin and growth hormone both have anabolic actions on protein metabolism leading to protein synthesis, but have opposite actions on both fat and carbohydrate metabolism. Insulin is anabolic for both of these, producing glycogen and triglyceride synthesis, but growth hormone is catabolic, resulting in a higher blood glucose level by opposing its uptake and promoting gluconeogenesis, and a higher blood fatty acid level by promoting lipolysis. b) As adrenaline and noradrenaline are both secreted by the adrenal medulla in response to stress, their actions on the cardiovascular system are the same. Explanation: Being secreted together in response to a common stimulus such as stress does not mean that adrenaline and noradrenaline act in identical ways to counteract that stress. Although they both stimulate beta-1 adrenergic receptors in the heart and hence increase both heart rate and contractility, resulting in a rise in cardiac output, their actions on the blood vessels are different. Noradrenaline acts primarily on alpha adrenergic receptors which contract vascular smooth muscle, causing vasoconstriction and a rise in total peripheral resistance, while adrenaline is more potent on beta-2 adrenergic receptors which relax vascular smooth muscle and result in vasodilatation and a fall in total peripheral resistance. Obviously it will depend on the balance between these opposing actions as to the final direction of change of the vascular resistance, and then on the total effect of this and the changed cardiac output as to how the blood pressure changes. Note that the adrenal cortex also secretes hormones in response to stress, including cardiovascular stress such as a drop in arterial blood pressure; they are aldosterone and cortisol. However their actions are not as immediate and direct as for the adrenal medullary hormones, although cortisol is permissive for the vasoconstrictor effect. Do not make the mistake of including angiotensin in any discussion of ADRENAL hormones, except to state that it is one of the stimulators of aldosterone secretion from the zona glomerulosa, which then results in distal tubular salt and eventually water retention, after ADH is also released, thereby raising extracellular volume and hence BP. ATII promoting thirst and causing proximal tubular salt reabsorption and vasoconstriction is NOT an adrenal hormone response. c) As positive feedback would result in an ever-increasing level of hormone activity, it does not occur anywhere in the endocrine system. Explanation: There are a few examples of positive feedback in the reproductive system e.g. estrogen surge producing LH surge before ovulation, oxytocin being released in ever-increasing amounts during labour, prolactin and oxytocin being secreted in proportion to the degree of suckling by the baby. 64 4) Sequences which make sense Rearrange the following 10 concepts relating to carbohydrate ingestion into a numbered chronological sequence which shows how one leads to the next (as in a concept map). Note: Food and water ingestion produces combined endocrine, gastrointestinal and renal effects, and each sequence contains only a small selection of possible physiological consequences. 1. Eat starch Glycogen synthesis in hepatocyte Maltose breakdown by maltase in brush border Starch breakdown by pancreatic amylase Glucose transport into portal vein blood Starch breakdown by salivary amylase Glucose transport into hepatocyte Glucose transport into enterocyte with Na Cholecystokinin stimulation of exocrine pancreas Parasympathetic stimulation of salivary acini 1. Eat starch 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about blood glucose concentration variation with food carbohydrate composition. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 65 The previous two exercises were focused on glucose release from food into blood and its arrival in the liver, while the next two will be focused on growth and metabolism. 4) Sequences which make sense Rearrange the following 10 concepts relating to growth into a numbered chronological sequence which shows how one leads to the next (as in a concept map). 1. Increased somatoliberin secretion Increased bone length Increased muscle protein synthesis Increased somatomedin (IGF1) production Increased width of epiphyseal growth plate Increased height Increased somatotropin (GH) secretion Decreased plasma aminoacid level Decreased adipose tissue triglycerides Decreased body mass index (BMI) 1. Increased somatoliberin secretion 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about hepatic glucose metabolism variation under the influence of different hormones. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 66 When answering some of the questions on endocrine activity, the concepts below may be relevant. Insulin-cation interactions The relationships between plasma and cellular K+ and H+ concentrations and insulin are quite complex, and become even more complicated in situations such as diabetes where a metabolic acidosis and dehydration often occur. As abnormal plasma levels of both these cations can have very damaging effects on excitable cells, and even lead to death, it is important to understand how they interact. For insulin and K+ levels, there is a two-way inter-relationship. The secretion of insulin from β-cells in the pancreas is stimulated not only by a rise in plasma glucose, but also by a rise in plasma K+ concentration. The consequence of both these changes is depolarisation of the β-cell membrane, which raises intracellular Ca2+ and promotes insulin secretion. Insulin then acts on its receptor in muscle and adipose tissue cells to recruit GLUT4 transporters as well as stimulate K+ uptake via the Na/K/ATP-ase and Na/K/2Cl symport. Hence diabetics who lack insulin are prone to hyperkalaemia, a raised plasma K+ concentration. H+ levels also feed into this system, in that a rise in plasma H+ concentration (or drop in plasma pH) leads to an intracellular acidosis which inhibits the Na pump and symport, thus preventing K+ uptake, as well as displacing K+ from proteins and driving it out of the cell. Hence the metabolic acidosis which accompanies diabetes will also produce hyperkalaemia. This is frequently stated as H+ ions “shifting into cells in exchange for” K+ ions, or as a rule of thumb that a rise in plasma K+ accompanies a rise in plasma H+, and that they similarly fall together. The clinical situation is complicated by any dehydration which occurs as a result of the osmotic diuresis caused by the hyperglycaemia and glycosuria (glucose loss in the urine). Since extra K+ can also be lost under these circumstances, the patient’s body may be K+ depleted, and this will have been exacerbated by the rise in aldosterone secretion as a consequence of both the raised plasma K+ level and the reduced extracellular volume stimulating the renin-angiotensin system. Hormones and pregnancy maintenance After conception has occurred with the fertilization of an ovum by a sperm in the upper third of the fallopian tube, the zygote begins a hazardous journey in which it must travel down to implant in the endometrium of the uterus, contribute to the establishment of a placenta, develop and grow until it is ready to be expelled in the process of labour. Each of these events may not proceed normally and the pregnancy may then terminate spontaneously in a miscarriage or premature birth. Apart from fetal genetics, the hormonal environment is critical to pregnancy maintenance and success, and it can be compromised in the following ways: 1. Inadequate hCG secretion by blastocyst trophoblast layer→failure to maintain corpus luteum estrogen (E) and progesterone (P) secretion 2. Incorrect E:P ratio→inappropriate motility of tube →arrival at uterus too early or too late in relation to its chemical changes 3. Inadequate luteal P secretion→failure to develop secretory endometrium with appropriate nutrients 4. Failure of placenta to take over P secretion→inadequate suppression of myometrial contractions→miscarriage 5. Failure to secrete sufficient hCS and/or other maternal endocrine abnormalities→incorrect rate of growth and baby of abnormal size 67 Concept Maps for Endo. Prac Map the mechanisms by which an excess of thyroid hormone causes symptoms of sensitivity to heat. Trigger: How is heat generated and dissipated by the body and how does thyroid hormone normally influence these processes? Map the mechanisms by which an increase in insulin resistance can eventually result in an impairment of glucose tolerance. Trigger: What is meant by insulin resistance and how does it develop? How is impaired glucose tolerance demonstrated and what does it signify? 68 GASTROINTESTINAL PRACTICAL 1) Confusing terms glossary This exercise is an extension of Appendix E on Confusing Terms in the No Frills Generic Skills Guide. Read the introduction there, then scroll down to find the terms which are relevant to the gastrointestinal practical. Below are some from that list, but you should be able to add more from your lectures and textbooks. Absorption – reabsorption Bile pigment – bile acid Constriction – contraction Flow – flux Lacteal – lactation Add your own additional pairs: 2) Misconception MCQs Choose the correct option which best completes the statement in the stem. Q1 a) b) c) d) Q2 Acetylcholine acting on muscarinic receptors on the parietal cell: increases cyclic AMP levels which stimulates the K-H-ATPase can be blocked by H2-antagonists such as ranitidine antagonizes the stimulatory effect of caffeine on acid secretion is released during the cephalic and gastric phases of digestion Neutralization of gastric chyme occurs after: a) gastrin stimulates the parietal cell to secrete bicarbonate into the lumen b) secretin stimulates the pancreatic duct cell to secrete bicarbonate into the lumen c) cholecystokinin stimulates the pancreatic acinar cell to secrete proteolytic enzymes d) fat incorporates the acid into micelles in the duodenum [Answers: 1d 2b] 3) Logical fallacies (and how to avoid them) Explain what is wrong with each of the following statements. a) Secondary active transport of HCO3 out of the parietal cell occurs because of the presence of carbonic anhydrase in the cell. Explanation: Carbonic anhydrase in the parietal cell enables CO2 and H2O to combine and form carbonic acid at a fast rate, which then dissociates into H+ and HCO3-. However the transport of the HCO3- out of the cell in exchange for Clmoving in across the basolateral membrane is a not a secondary active transport process, but occurs because the active removal of H+ via the luminal membrane K-HATPase generates a concentration gradient for HCO3- which drives the antiport for these two anions. The rise in intracellular pH also pushes the reaction between CO2 69 and H2O to the right, generating more of both H+ and HCO3-. It is the Cl- which is secreted by secondary active transport into the lumen, entering the parietal cell from the blood against its concentration gradient and exiting by diffusion down its electrochemical gradient through the luminal Cl- channel. b) K+ and Cl - movement through their channels on the luminal membrane of the parietal cell is an active transport process dependent on their electrochemical gradients. Explanation: There are a number of incorrect associations in this statement. Movement through channels is passive, not active, and active transport processes depend on ATP, not electrochemical gradients, which drive the channel movements. c) Just as water is never secreted into the filtrate in the lumen of the proximal tubule, so it is also never secreted into the lumen of the gut. Explanation: Although the transport mechanisms in the epithelia of the gut and kidney are similar in many respects, the movement of water can be very different. In the nephron the osmotic gradient between the lumen of the tubule and the surrounding capillary blood always promotes reabsorption of water, although whether or not this can occur also depends on how permeable the epithelium is at that point. In the gut the same processes usually establish a net gradient for water reabsorption – active reaborption of Na and metabolites of digestion such as glucose and aminoacids. However there are also secretory processes which form part of the normal physiology of the gut, such as acid secretion by the parietal cells in the stomach, salt and water secretion by the crypt cells of the small intestine, similar secretions by the acini of the salivary and pancreatic exocrine glands, and bicarbonate and other secretions by surface epithelial cells throughout. The final secretions from all of these sites are often isotonic or hypotonic solutions, and in addition, the absorption of solutes usually leads, rather than follows, transepithelial water movement. However when complex food components are broken down in digestion, they generate a much larger number of osmotically active products, hence increasing the luminal osmolality; normally these are the only ones which could potentially lead to secretion rather than reabsorption of water. Yet it is only under circumstances in which they are not being aborbed rapidly enough that the gradient for water will be into the lumen, and hence it will be secreted. This can occur in such situations as dumping syndrome, where removal of part of the stomach produces a loss of negative feedback mechanisms which control gastric emptying, so that volumes in excess of the capacity of the upper small intestine to absorb are suddenly dumped there, which is followed by water being secreted into the lumen. d) The tonicity of saliva is lowest when there is a high rate of secretion by the acini and a high rate of flow through the ducts. Explanation: The acini secrete an isotonic solution independently of the rate at which this occurs, and the tonicity of the saliva is determined by the rate of flow through the ducts, which are impermeable to water. When this is slow, the reaborption of NaCl exceeds the secretion of KHCO3 and hence the tonicity is round its lowest value. Note that this is different from what happens in the exocrine pancreas, whose secretion is always isotonic at the outlet from the pancreatic duct into the duodenum. 70 4) Sequences which make sense Rearrange the following 10 concepts relating to carbohydrate ingestion into a numbered chronological sequence which shows how one leads to the next (as in a concept map). Note: Food and water ingestion produces combined endocrine, gastrointestinal and renal effects, and each sequence contains only a small selection of possible physiological consequences. 1. Eat starch Glycogen synthesis in hepatocyte Maltose breakdown by maltase in brush border Starch breakdown by pancreatic amylase Glucose transport into portal vein blood Starch breakdown by salivary amylase Glucose transport into hepatocyte Glucose transport into enterocyte with Na Cholecystokinin stimulation of exocrine pancreas Parasympathetic stimulation of salivary acini 1. Eat starch 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about gastric secretion variation with drugs. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 71 When answering some of the questions on gastrointestinal activity, the concepts below may be relevant. Luminal stimuli in gastric phase Antrum & body/corpus:different outcomes of luminal physical & chemical stimulation • Corpus: distension→+vagal reflexes to G, ECL & parietal cells, - reflex to D cell • Antrum: distension→local reflex to G cell aminoacids/peptides→direct on G cell → G (→ paracrine stimulation of D cell) acid→direct on D cell → SS (→ paracrine inhibition of G cell) &→ECL & parietal cells Chief cell pepsinogen: via stimulation by vagus, G, histamine + acid, CCKA, - by SS Salivary & pancreatic secretion • Acini secrete digestive enzymes • Acini secrete salt & water by same mechanism in both glands →isotonic solution like plasma • Salivary ducts impermeable to water absorb NaCl & secrete KHCO3 →more hypotonic at faster secretion rates • Pancreatic ducts permeable to water absorb Cl (& Na paracellularly) secrete KHCO3 →isotonic (Na&K as in plasma) • Neural stimulation of saliva (symp.¶symp.) neural & hormonal of pancreatic juice (vagus) (CCK – acini secretin – ducts) 72 Concept Maps for Gastric Prac Map the mechanisms which increase gastric secretion when protein is eaten. Trigger: What is the effect of protein on the stomach? What mechanisms result in stimulation of the parietal and chief cells to secrete acid and enzymes? Map the mechanisms which increase luminal pH in response to acidic chyme. Trigger: What is the effect of acidic chyme on gastric and small intestinal neural and hormonal reflex responses? What mechanisms result in neutralization of the acid? 73 RENAL PRACTICAL 1) Confusing terms glossary This exercise is an extension of Appendix E on Confusing Terms in the No Frills Generic Skills Guide. Read the introduction there, then scroll down to find the terms which are relevant to the renal practical. Below are some from that list, and you may be able to add more from your lectures and textbooks. Absorption – reabsorption Aldosterone – antidiuretic hormone Counteract – countercurrent Countercurrent flow – countercurrent multiplication Countercurrent exchange – countercurrent multiplication Diuresis – diabetes Flow – flux Granular cells – zona granulosa Inhibin – inulin Inulin – insulin Juxtaglomerular body – juxtaglomerular cells Mesangial cells – extraglomerular mesangial cells Renal clearance – total clearance from renal blood Renal cortex – adrenal cortex Renal cortex – cerebral cortex Renal medulla – adrenal medulla Renal medulla – medulla of brainstem Resorption – reabsorption Secretion – excretion Urine - urea 2) Misconception MCQs Choose the correct option which best completes the statement in the stem. Q1 Glucose begins to appear in the urine: a) when its Tmax for renal reabsorption exceeds its renal threshold b) when its plasma concentration exceeds its renal threshold c) when its Tmax for renal reabsorption is reduced below its renal threshold d) when the amount of glucose filtered is more than that secreted Q2 a) b) c) d) In relation to water handling by the nephron: an osmotic gradient is necessary, but not sufficient, for reabsorption 80% of the volume filtered is reabsorbed in the proximal tubule 20% of the volume filtered is reabsorbed in the thick ascending limb antidiuretic hormone is required for reabsorption in all segments [Answers: 1b 2a] 74 3) Logical fallacies (and how to avoid them) Although students intuitively feel that GFR “ought to” change after diuretics or ADH treatment, they often have problems with giving a mechanism to justify this prediction, and to explain the link to urinary volume changes. [An alternative argument could be that you would NOT expect GFR to change after any of the treatments because they all act POST-glomerulus.] Wrong Explanation! Furosemide solute excretion water excretion urine volume per min Therefore, to supply the increased volume, GFR must ADH distal tubule permeability to water water secretion urine volume per min Therefore GFR must The above is additionally wrong because water never is secreted, only reabsorbed less Water loading plasma osmolality which in turn urinary osmolarity * Therefore there must be an water excretion and GFR must *This is inadequate without the missing link of plasma ADH water reabsorption water excretion Right Explanation!! The right logic could have been any one of the following: Furosemide water excretion ECV/BP glomerular capillary hydrostatic pressure And glomerular capillary oncotic pressure GFR N.B. Furosemide solute excretion glomerular capillary osmotic pressure GFR is not correct because the only solutes which make a difference to filtration are the plasma proteins, and these would in fact have become more concentrated as just indicated Water loading ECV/BP glomerular capillary hydrostatic pressure And glomerular capillary oncotic pressure GFR N.B. Again, it is not correct to argue that there is a glomerular capillary osmotic pressure GFR Note also that these results pre-suppose that there has NOT been local auto-regulation of GFR maintaining it constant. They also pre-suppose an absence of homeostatic reflexes maintaining MAP. It is therefore clear that there is no one RIGHT ANSWER, but as long as your arguments are physiologically sound, you will be given credit for what you have written. 75 4) Sequences which make sense Rearrange the following groups of concepts relating to water ingestion into numbered chronological sequences which shows how one leads to the next (as in a concept map). You will need to have two parallel sequences converging on the final outcome. 1.Water ingestion Decreased aquaporins in collecting duct Increased net filtration pressure Decreased plasma osmolality Decreased oncotic pressure Increased urine volume Increased urine volume Increased plasma volume Increased plasma volume Decreased water reabsorption Decreased water reabsorption Decreased hypothalamic osmoreceptor Decreased proximal tubular stimulation Na&H2O reabsorption Decreased ADH secretion Increased glomerular filtration rate 1.Water ingestion 76 5) The three most important facts In this exercise you have to make a choice from all the relationships you have covered in the previous exercises, which THREE are the most vital to underpin a hypothesis about urinary osmolality variation with diuretics. Since it is essential that the hypotheses evolve from what you have written in the Introduction, these would constitute the three most important sentences as they would be the ones from which your hypotheses could then be predicted. i) ii) iii) Repeat this exercise now for the Discussion, where the THREE relationships are those which best explain how your data have supported your hypotheses. i) ii) iii) 77 When answering some of the questions on renal activity, the concepts below may be relevant. Mechanisms of salt and water transport in the kidney The kidney is a particularly difficult organ to consider in relation to salt and water movement because of the variability in the properties of different segments of the nephrons and the flow-on effects due to the anatomical proximity of different tubular and vascular structures. As an example, the countercurrent multiplication mechanism for generating the vertical osmotic gradient in the medullary interstitium depends for its maintenance on the countercurrent exchange mechanism of the vasa recta. Yet the permeability of these tubular and vascular elements to different solutes and to water varies uniquely with the structure concerned, as well as their movement being influenced by chemical factors such as hormones or drugs, and physical factors such as pressures. Therefore, when considering the effects of any treatment on urinary composition it is necessary to take into consideration changes in the following: (i) ion transport from lumen to interstitium and vice versa (ii) effects of (i), if any, on osmolality of the filtrate and of the interstitium (iii) potential effects of (ii), if any, on the passive reabsorption of water (iv) water permeability in the part of the tubule where osmotic gradients occur Remember that for a substance to move passively from one compartment to another, there must be BOTH a force or gradient and a pathway. The luminal surface of the epithelial cells is the site of those transport mechnisms which remove from, or add to, the filtrate, whereas the interstitial surface of the epithelial cells is the site of those mechanisms which exchange material with the interstitial space. This in turn can exchange with the capillary blood, which perfuses that part of the kidney. Whatever remains in the tubule becomes urine and whatever remains in the capillaries becomes the renal venous blood which is returned to the systemic circulation. These are, thus, the two routes of excretion of material from the medullary interstitium and that if allowed to accumulate would cause a rise in pressure. Apart from the gradients and available pathways, rate of flow of filtrate or of capillary blood past the exchange sites will influence the time available for material to move between the compartments, as well as the absolute capacity to remove it in urine or renal venous blood. Remember that water movement is always passive, along an osmotic gradient. Also remember that ANTI-diuretic hormone (ADH), which acts in the distal nephron to increase luminal membrane permeability to water, has the exactly opposite effect to diuretics, reducing, rather than enhancing, the excretion of water. Differential effects of diuretics Diuretics exploit known transport mechanisms to block solute absorption and hence the water which follows. Depending on their site of action, they produce urine of varying solute composition and osmolality. The most powerful diuretics are those which block the Na/K/2Cl symport in the thick ascending limb of the loop of Henle. This is because 25% of filtered Na is normally reabsorbed here, and because this site is the driver for the countercurrent multiplier mechanism. Blocking the reabsorption makes the filtrate osmolality higher and the interstitial osmolality lower, hence reducing the osmotic gradient for water reabsorption more distally, where the nephron is permeable. Loop diuretics thus interfere with both the dilution and the concentration of urine, which reaches very large volumes which are not as hypotonic as when pure water is drunk and ADH is completely suppressed. 78 Concept Maps for Renal Prac Map the hydrostatic pressures along the length of the renal vasculature, emphasizing the unique sequence of blood vessel types in the kidney. Trigger: What are the physical factors which influence hydrostatic pressure in blood vessels? How do they produce a differential between the pressure in the afferent arteriole and that in the peritubular capillaries? Map the hydrostatic and osmotic pressures which affect proximal tubular reabsorption of salt and water. Trigger: What are the factors which influence hydrostatic pressure and osmotic pressure? How do they interact to affect reabsorption of salt and water from the proximal tubule via the interstitium? 79