CVM Week 2 Synthesis Session PDF

DONE Week 2 CVM GLS: Basic Electrophysiology Learning Objectives By the end of this week, you should feel comfortable describing the electrical behaviour of the heart from the level of individual ion channels in single myocytes, through the conduction system and the electrical syncytium, to the whol...

DONE Week 2 CVM GLS: Basic Electrophysiology Learning Objectives By the end of this week, you should feel comfortable describing the electrical behaviour of the heart from the level of individual ion channels in single myocytes, through the conduction system and the electrical syncytium, to the whole organ level. Deep understanding of these concepts will facilitate you in understanding how this electrical activity produces the ECG (which we will cover in Week 3), and how aberrant electrical behaviour gives rise to the many dysrhythmias your patients may suffer (which will be introduced in Week 4). Exercise 1: The Action Potential, Inactivation, and Refractory Periods The action potential is the result of the currents flowing across the plasma lemma of the individual myocyte being studied. Refractoriness arises from ion channels which have the property of inactivation. In the heart, myocytes may be conductile or contractile, and have different populations of ion channels, and so, different electrical properties. Question 1: Draw an action potential from a conductile myocyte in the SA node. i. ii. iii. Be sure to label the axes and provide units. Label the phases of this action potential. Identify the ion channel(s) responsible for each phase. Question 2: What are the inherent rates of automaticity, under resting parasympathetic input, in (i) the SA node, (ii) the AV node, and (iii) the Bundle Branches/Purkinje System? Why is there this difference? What would happen if the AV node conductile cells reached their threshold and produced action potentials more rapidly than the SA node conductile cells? If the SA node cells were injured during a heart attack and no longer capable of producing action potentials? Question 3: Draw an action potential from a contractile myocyte. i. ii. iii. Be sure to label the axes and provide units. Label the phases of this action potential. Identify the ion channel(s) responsible for each phase. Question 4: Explain why resting membrane potential (Phase 4) of a contractile myocyte is stable, while in a conductile myocyte, it spontaneously depolarises. What would happen if resting potential in a contractile myocyte became unstable? Question 5: Using fast voltage-gated sodium channels as your model, explain activation and deactivation. 1|Page Week 2 CVM GLS: Basic Electrophysiology Question 6: Explain inactivation and recovery from inactivation, using voltage-gated sodium channel of Phase 0 of a contractile myocyte action potential as your model. (Note, understanding inactivation is important to understanding ‘re-entry’, the mechanism behind the most serious dysrhythmias, (week 4), and also ‘use dependence’ in anti-arrhythmia drugs.) Question 7: Explain the refractory periods (absolute and relative) in cardiac myocytes. Relate inactivation and recovery from inactivation to the absolute and relative refractory periods of the myocyte, first considering the contractile myocytes. Then, describe the cellular basis for the absolute and relative refractory periods in conductile myocytes. Exercise 2: The Anatomy of Excitation Successful ejection of blood depends on each myocyte in the heart depolarising and contracting in the right sequence. This precise and carefully planned sequence of activation is produced by the anatomical relationship between the contractile and conductile myocytes. Question 1: Delineate the conduction system of the heart, and state the roles of each section. Drawing the heart may help cement the anatomy of the conduction system in your mind. Question 2: Describe the sequence of excitation through the conduction system of a normal heart. Once excitation reaches the free wall of the ventricles, what is the sequence of depolarisation through the contractile myocytes? Of repolarisation? (This knowledge is important to understand how the spread of depolarisation and repolarisation through the healthy heart produce the normal ECG waveforms.) Question 3: Explain why the wave of excitation is unidirectional in a healthy heart, despite crossing a continuous “sheet” of electrically coupled myocytes. (Loss of unidirectional spread of action potentials underlies ‘re-entry’, which gives rise to ventricular fibrillation, among other dysrhythmias.) Exercise 3: Autonomic Modulation of Heart Rate and Contractility Question 1: Describe sympathetic and parasympathetic innervation of the heart, identifying the targets of each type of innervation, the neurotransmitter(s) released, and the specific types of receptors present in each target cell. 2|Page Week 2 CVM GLS: Basic Electrophysiology Question 2: Explain how an increase in sympathetic input to the heart influences chronotropy, including the specific cellular targets and changes in your description. How does increased parasympathetic input affect chronotropy? Question 3: Explain how an increase in sympathetic input to the heart influences inotropy, including the specific cellular targets and changes in your description. Does increased parasympathetic input affect inotropy? Question 4: Explain how an increase in sympathetic input to the heart influences dromotropy, including the specific cellular targets and changes in your description. How does increased parasympathetic input affect dromotropy? Question 5: Lusitropy refers to the rate of relaxation of cardiac contractile myocytes. Based on your knowledge of autonomic control of cardiac contractility, what change in lusitropy would you predict to accompany the changes in chronotropy, dromotropy, and inotropy? Why would this change be important to keep pumping efficiency high? (Lusitropy will be discussed in subsequent sessions. This is a thought exercise, to help you consolidate your understanding of autonomic control of pumping efficiency of the heart.) 3|Page Normal Depolarisation & Repolarisation of the Ventricles Dr. Lisa Chilton [email protected] Building DB087, Room TV222, 4781 5195 Normal Sequence Basic Rules: 1) All contractile myocytes are at resting membrane potential during diastole, waiting for the signal from the conduction system to tell them to produce an action potential, and then to contract 2) The wave of excitation (action potentials; depolarisation) goes in the following sequence: 1) Conduction system (SA node to AV node to Bundle of His to Bundle Branches (faster down the left branch than the right) to the Purkinje fibres) 2) Purkinje fibres to the contractile myocardium from apex to base, and for the majority of the ventricular free walls, from endocardium to epicardium 3) The wave of repolarisation goes in the following sequence: 1) Ventricular contractile myocytes from apex to base and epicardium to endocardium 2) Purkinje fibres (still absolutely refractory when myocardium repolarises) Refractory Periods http://www.blaufuss.org/SVT/index2.html Think of the AP as the boat & the refractory period as the wake it kicks up. SA Node → AV Node → Bundle of His → Bundle Branches → Purkinje Fibres → contracle myocytes Endocardium in this direction Artery Epicardium in this direction White = resting membrane potential; red = action potential (depolarised) SA Node → AV Node → Bundle of His → Bundle Branches → Purkinje Fibres → contracle myocytes Endocardium in this direction Artery Epicardium in this direction Red = action potential Endocardium in this direction Artery Epicardium in this direction Red = action potential Endocardium in this direction Artery Epicardium in this direction Red = action potential Endocardium in this direction Artery Epicardium in this direction Red = action potential Endocardium in this direction Artery Epicardium in this direction Red = action potential Endocardium in this direction Artery Epicardium in this direction Red = action potential Endocardium in this direction Artery Epicardium in this direction Red = action potential Endocardium in this direction Artery Epicardium in this direction Red = action potential All ventricular myocytes remain depolarised, triggering coordinated contraction. Artery Red = action potential What is the sequence of repolarisation in a healthy heart? Endocardium in this direction Artery Epicardium in this direction Red = action potential Endocardium in this direction Artery Epicardium in this direction Red = action potential; dark pink = absolute refractory period Endocardium in this direction Artery Epicardium in this direction Red = action potential; dark pink = absolute refractory period Endocardium in this direction Artery Epicardium in this direction Red = action potential; dark pink = absolute refractory period; light pink = relative refractory period Endocardium in this direction Artery Epicardium in this direction Red = action potential; dark pink = absolute refractory period; light pink = relative refractory period; white = resting membrane potential Endocardium in this direction Artery Epicardium in this direction Red = action potential; dark pink = absolute refractory period; light pink = relative refractory period; white = resting membrane potential Endocardium in this direction Artery Epicardium in this direction Red = action potential; dark pink = absolute refractory period; light pink = relative refractory period; white = resting membrane potential Endocardium in this direction Artery Epicardium in this direction Red = action potential; dark pink = absolute refractory period; light pink = relative refractory period; white = resting membrane potential Endocardium in this direction Artery Epicardium in this direction Dark pink = absolute refractory period; light pink = relative refractory period; white = resting membrane potential Endocardium in this direction Artery Epicardium in this direction Dark pink = absolute refractory period; light pink = relative refractory period; white = resting membrane potential Endocardium in this direction Artery Epicardium in this direction Light pink = relative refractory period; white = resting membrane potential All ventricular myocytes are repolarised, allowing coordinated relaxation and filling. Artery White = resting membrane potential Contacts Dr Lisa Chilton (Townsville) Dr Chanika Alahakoon (Cairns). INTRODUCTION SESSION Basic Cardiac Electrophysiology Contact Hours By appointment and during GLS Preparation Review the material on excitable membranes and action potentials from first year (Cells to Life and Musculoskeletal)... Key Words Learning Overview Absolute refractory period Action potential Activation Atrioventricular (AV) node Bundles of His/AV Bundle Branch bundles Chronotropic Conductile cells Contractile cells Deactivation Depolarisation Dromotropic Functional syncytium Gating Inactivation Inotropic Parasympathetic nervous system Pacemaker Pacemaker channel (slow Na+) Phases 0 – 4 of the action potential Purkinje fibres Relative refractory period Repolarisation Resting membrane potential Sinoatrial (SA) node Sinus rhythm Na+/Ca2+ Exchanger (aka NCX) Na+/K+ ATPase (pump) Sympathetic nervous system Threshold Voltage-gated Ca2+ channels Voltage-gated K+ channels Voltage-gated Na+ channels (Fast).. This lecture will focus on the origin of the electrical activity of the heart, the transmission of the electrical signals and the nature of the action potentials that result. We will also introduce innervation of the heart and its effects on heart rate. Relationships between electrical and mechanical events of the heart will also be introduced. What you learn this week will form the basis for understanding the waveforms that comprise a ‘normal’ ECG trace and how these waveforms are established. You will be introduced to the ECG next week. You will then apply your understanding of basic cardiac electrophysiology and the waveform of a normal ECG to an introduction to dysrhythmias, in Week 4. We will consider the basic mechanisms of electrical abnormalities as well as a number of common dysrhythmias in Week 4... Learning Resources Readings Marieb and Hoehn. 2018. Human Anatomy and Physiology, Global Edition. 11th Ed. Chapter 18 (The Cardiovascular System: The Heart) (or comparable section in any anatomy and physiology textbook) Guyton and Hall. 2021. Textbook of Medical Physiology 14th Ed. Chapters 10 (Rhymical Excitation of the Heart) Lilly. 2011. Pathophysiology of Heart Disease, 6th Ed. Chapter 1 (Basic electrophysiology) PLEASE NOTE: Guyton and Hall has more detail than Marieb and Hoehn; it is recommended that you read Guyton and Hall AFTER you have the basics from Marieb and Hoehn, as it will reinforce the information and give you that bit of extra detail. Contacts Dr Lisa Chilton (Townsville) Dr Chanika Alahakoon (Cairns). Contact Hours By appointment and during GLS GUIDED LEARNING SESSION Basic Cardiac Electrophysiology... Preparation Please print out the GLS workbook and bring it to the session. Complete as many of the answers prior to the GLS, to ensure that your time in the GLS is used most effectively discussing concepts with the demonstrators.. Learning Overview. Your goal this week is to work through the physiological implications of the ability of the heart to generate electrical signals, from the cellular through tissue to the organ level. Organisation This session is comprised of a series of exercises. These will take you through (a) the ionic basis of the action potential, channel inactivation, and the refractory periods in conductile and contractile myocytes; and (b), the anatomical sequence by which the wave of depolarisation spreads through the conduction system of a healthy heart; and (c) autonomic modulation of heart rate and contractility.. Learning Activities Exercise 1: The Action Potential, Inactivation and Refractory Periods Exercise 2: The Anatomy of Excitation Exercise 3: Autonomic Modulation of Heart Rate and Contractility Townsville Logistics In order to optimise access to the tutors and to maintain social distancing and COVID-19 safety protocols, students will only attend one of the three hours within the Guided Learning Session. You will be assigned according to your Home Group. Please be sure to attend only in your designated hour, to ensure that we can be safe and keep appropriate social distancing, and to optimise the student to teaching staff ratio. Times will rotate through weeks 1 to 3, so that no group is disadvantaged in the timing of their session. Please check the GLS folder on LearnJCU for weekly allocations. Learning Resources Readings Marieb and Hoehn. 2018. Human Anatomy and Physiology, Global Edition. 11th Ed. Chapter 18 (The Cardiovascular System: The Heart) (or comparable section in any anatomy and physiology textbook) Guyton and Hall. 2021. Textbook of Medical Physiology 14th Ed. Chapter 10 (Rhymical Excitation of the Heart) Lilly. 2011. Pathophysiology of Heart Disease, 6th Ed. Chapter 1 (Basic electrophysiology). PLEASE NOTE: Guyton and Hall has more detail than Marieb and Hoehn; it is recommended that you read Guyton and Hall AFTER you have the basics from Marieb and Hoehn, as it will reinforce the information and give you that bit of extra detail. Also note: if you have more recent editions of these textbooks, look for the chapters/sections which match the concepts discussed in lecture and the GLS. Contacts Dr Lisa Chilton (Townsville) Dr Chanika Alahakoon (Cairns). Contact Hours By appointment and during GLS SYNTHESISING SESSION Review of Basic Electrophysiology... Preparation Please review the material from the GLS before coming to this session. Please print out the SS notes on the web and bring them to class.. Learning Overview We will discuss answers to the questions that formed the workbook from the GLS and will establish the take home messages for the week. Learning Activities This session comprises a presentation and a discussion.. Learning Resources Readings Please ensure that you have read through all the material from the Introductory Session and GLS prior to this class. Week 2 CVM: Basic Electrophysiology Supplemental Information: Basic Electrophysiology Terminology Note: Not for test purposes. Please use this guide if you are uncertain of the terminology. A. Membrane Potential Definition: The separation of charge across the membrane which forms an electrical difference or driving force for the flow of ions, once ion channels open. At rest, membrane potential is more negative inside than outside, to the tune of -90 mV in contractile myocytes and -60 mV in SA node cells. During an action potential, membrane potential transiently becomes positive, to about 0 to ~+20 mV, evoking action potentials in neighbouring resting cells, and signalling to the working muscle that it is time to contract. This is plotted as the y axis on the action potential and the ECG. Associated terminology: if the interior of the myocyte becomes more positive, then the myocyte is depolarised. If the interior of the myocyte becomes more negative, then the myocyte is repolarised. Hence, the upstroke of the action potential (Phase 0) is a depolarisation from resting membrane potential, while Phase 3 is repolarisation to resting membrane potential. These terms are therefore relative and used to describe a change in membrane potential. Hyperpolarisation is to become more negative than resting membrane potential, and is observed for example when parasympathetic input to the SA and AV nodes is increased, causing ACh-sensitive K+ channels to open. B. Current Definition: The movement of ions, by definition in the direction of cations. Hence flow of sodium or calcium ions into the myocyte produced an inward current, depolarising the interior of the myocyte (making it more positive). A potassium current is an outward current, as positively charged potassium ions flow from inside the myocyte to the outside, under healthy conditions. This makes the inside of the myocyte less positive (or more negative) due to the efflux of potassium cations, repolarising the membrane (making it more negative). Remember that in the case of cells, currents are only possible through ion channels, due to the lipid core of the membrane. Also remember that the current will change the membrane potential, by making it more positive (depolarising the interior, due to the influx of sodium and/or calcium) or more negative (repolarising the interior, due to the efflux of potassium ions). 1|Page Week 2 CVM: Basic Electrophysiology C. Resistance Definition: Opposition to the flow of ions (to passing a current); in cells, this refers to whether ion channels are open (low resistance) or closed (high resistance). When ion channels are closed, there is high resistance across the membrane and no current flows. When ion channels open, there is low resistance across the membrane and ions will flow according to the electrical and chemical gradients (the driving force). Remember that the lipid core of the membrane prevents free diffusion or flow of ions across the cell membrane; hence, ion channels are required to mediate the current. This allows for control of resistance (whether the ion channel is open or closed) and so, control of under what conditions a current is allowed. D. Conductance Definition: The inverse (opposite) of resistance: the ability for current to flow across the cell membrane; in myocytes, conductance is high when ion channels are open, allowing ions to pass through the membrane. Mathematically, conductance = 1/resistance. Because conductance is the reciprocal of resistance, ion channels may be open (capable of conductance, or having low resistance to current flow) or closed (resistant to ion flow; incapable of conduction). When the ion channel is capable of conduction (open), the current will then flow according to the driving force. Note therefore that if there is no driving force, there will be no current, even if the ion channel is capable of conducting one. E. Activation Definition: The opening of the activation gate of an ion channel, reducing resistance and increasing conductance. (Refer to the figure on Slide 16 of the CVM Week 2 lecture, which shows two gates: the top activation gate and the bottom inactivation gate.) Opening of the activation gate allows a current to flow; however, whether a current will flow depends on the chemical and electrical gradients (driving force) for that ion. Hence, just because an ion channel activates, current may or may not flow. However, under healthy conditions, ion channels activate when there is a driving force for ion flow, and a current will always be produced upon activation. By constraining ions from simply zipping across the lipid core of the plasmalemma, the cell can control when currents may occur. This means that activation is under rigorous control in everything but leak channels, which are always activated, regardless of cellular conditions. In all other ion channels, activation is gated (triggered) by specific cellular changes. For example, voltage-gated ion channels only open at a trigger membrane potential. Ion channels may also be activated when specific ligands are present, as with the acetylcholine2|Page Week 2 CVM: Basic Electrophysiology activated potassium channels of SA and AV node conductile myocytes, which mediate parasympathetic negative chrono- and dromotropic effects. Note: closing of the activation gate upon repolarisation is deactivation (G below). F. Inactivation Definition: This is the closing of the inactivation gate of an ion channel, preventing a current from flowing despite the conditions being favourable to open the activation gate. For example, the fast voltage-gate sodium channels activate upon depolarisation to ~-60 mV, but then rapidly inactivate when the membrane remains depolarised. Hence, even though the threshold potential for activation (opening of the activation gate) has been achieved, and the activation gate remaining open, no current can flow due to the closing of the inactivation gate (refer to figure on Slide 16 of the lecture). The inactivation gate will only reset upon repolarisation. Note that the terminology is ‘recovering from inactivation’ not closing/deactivation (which refers specifically to closing of the activation gate). Note that the L type voltage-gated calcium channels also undergo inactivation, bringing about the end of Phase 2, the plateau potential. Inactivation in these channels is slow, while in the fast voltage-gated sodium channels, the closure of the inactivation gate is very rapid. Repolarisation and time are required to allow the voltage-gated calcium channels to recover from inactivation. The key voltage-gated potassium channels which are activated upon a depolarisation to ~-40 mV and are responsible for repolarisation (Phase 3) do not have inactivation gates. Hence, these channels do not inactivate and continue to mediate potassium current until they deactivate at resting membrane potential (Phase 4). G. Deactivation Definition: This refers to the activation gate of an ion channel closing once the myocyte returns to resting conditions. During deactivation, the activation gate resets into the closed position, once the trigger for activation is no longer present. In voltage-gated sodium, potassium, and calcium channels, repolarisation to resting membrane potential causes the activation gate to close, deactivating (closing) the channel. This occurs because membrane potential repolarises to a more negative value than the membrane potential required to trigger the activation gate to open. Note that deactivation is not the same process as recovery from inactivation – activation and deactivation are the terms for opening and closing of the activation gate, while inactivation and recovery from inactivation are the closing and opening of the inactivation gate. 3|Page MD2011 Cardiovascular Medicine Dr Lisa Chilton [email protected] Building DB087, Room TV222, 4781 5195 Cardiac Electrophysiology Week 2 = The Basics Week 3 = The ECG Week 4 = Dysrhythmias Learning Objectives – Week 2 1. Differentiate between contractile and conductive myocytes 2. Delineate the anatomy of the conduction system 3. Discuss the events associated with a wave of excitation spreading through the conduction system, explaining why there is a delay the AV node 4. Describe ‘automaticity’ and evaluate the underlying ionic currents 5. Compare and contrast action potentials in myocytes from different parts of the heart 6. Describe the 4 phases of the action potential 7. Compare and contrast activation, inactivation, recovery from inactivation and closing of ion channels (deactivation) 8. Explain the absolute and relative refractory periods 9. Describe the role of intercalated discs and gap junctions in cardiac excitation 10. Explain how the parasympathetic and sympathetic nervous systems modulate heart rate, conduction velocity through the AV node, and contractility Conduction System The signal for the muscle to contract is electrical Conductive (electrical) muscle cells (myocytes) are specialized to conduct action potentials and cannot contract Contractile (working) myocytes contract when an action potential depolarises them Contractile myocytes also conduct electricity through the syncytium created by gap junctions at intercalated discs Conduction System SA Node interatrial band internodal pathways AV Node delay – why? Bundle of His (aka A-V bundle) Bundle Branches Purkinje Fibres Purkinje fibres Figure 10-1, Guyton & Hall, 13ed Automaticity Conductile myocytes spontaneously depolarise and produce action potentials SA node cells have fastest rate of spontaneous depolarisation (60 – 100 AP/min at rest with parasympathetic input) Other portions of the conduction system may also spontaneously depolarise, but at slower rates & only if not driven by the SA node AV node: 40 – 60 AP/min at rest Purkinje Fibres: 15 – 40 APs/min at rest inactivate SA Node Cells Figure 18-13, Marieb and Hoehn Figure 18-14, Marieb and Hoehn, 7ed Note: Atria first AV node - delay Left to right in the septum + base to apex Apex to base in the free walls of the ventricles + endocardium to epicardium (with more complex final sequence right by base) Cardiovascular Physiology Concepts, Figure 2-11. Contractile Myocytes Contraction is stimulated by the arrival of APs from the conduction system and other myocytes Electrical syncytium via intercalated discs Membrane potential of autorhythmic cell Membrane potential of contractile cell Cells of SA node Contractile cell Intercalated disk with gap junctions Depolarizations of autorhythmic cells rapidly spread to adjacent contractile cells through gap junctions. Figure 14-7, Silverthorn, Human Physiology, 5ed Ventricular Contractile Myocytes inactivate Figure 14-13, Silverthorn, Human Physiology, 5ed Phase 0: rapid voltage-gated Na+ channels open at a threshold of ~ -60 mV Depolarisation inactivates the Na+ channels Voltage-gated Ca2+ & K+ channels start opening at ~-40 mV These channels open slowly, compared to the Na+ channels Ventricular Contractile Myocytes inactivate inact Figure 14-13, Silverthorn, Human Physiology, 5ed Phase 1: Sodium channels have inactivated; brief dominance of K+ current Phase 2: The plateau is produced by equal inward Ca2+ and outward K+ currents, until Ca2+ channels inactivate Ca2+ is needed to evoke muscle contraction Phase 3: Rapid repolarisation due to voltage-gated K+ currents Ventricular Contractile Myocytes inactivate inact Figure 14-13, Silverthorn, Human Physiology, 5ed Phase 4: Resting membrane potential is maintained at ~ -87 mV by another type of K+ current, “K1” or “Kir” Normal Na+ and K+ gradients are restored by the Na+/K+ ATPase Normal Ca2+ gradients are restored by Ca2+ ATPases and the Na+/Ca2+ exchanger Note the Na+/K+ ATPase and the Na+/Ca2+ exchanger contribute to RMP Refractory Periods ARP www.studyblue.co Opening of Na+ channels trigger the action potential Na+ channels inactivate when depolarised When all Na+ channels have inactivated, the cell cannot produce another AP Absolute Refractory Period (ARP) Inactivation of voltage-gated Na+ channels +++ a + + +++ -87 mV i Rapid depolarisation (Phase 0) + + Adapted from Martini and Nath, 8ed + + +++ -60 mV +20 mV Resting (Phase 4) Inactivation (Phases 0 – mid-to-late 3) Refractory Periods ARP www.studyblue.co When the myocyte starts to repolarise, the Na+ channels recover from inactivation Starts in mid-to-late Phase 3 Full recovery occurs in Phase 4 Relative Refractory Period (RRP) Unidirectional Action Potential Propagation How? The action potential in cell 1 depolarises cell 2 Cell 2 then depolarises cell 3 Cell 2 cannot depolarise cell 1 as cell 1 is refractory Travel is unidirectional Refractory 1 RMP 2 3 action potential http://outreach.mcb.harvard.edu/animations/actionpotential.swf Parasympathetic Nervous System (PNS) - 1 Innervates the SA and AV nodes Makes SA node cells more negative at rest by activating muscarinic cholinergic receptors which then activate ACh-sensitive K+ channels The pacemaker current in the SA node is also reduced (slope is not as steep) Both effects make it harder to depolarise to threshold (bradycardia) Negative chronotropic effects Figure 18-16, Marieb and Hoehn, 7ed Parasympathetic Nervous System (PNS) - 2 KACh current-induced resting hyperpolarisation and reduced pacemaker current slow conduction velocity across the AV node, delaying ventricular excitation Negative dromotropic effect No direct effect on force of contraction Figure 18-16, Marieb and Hoehn, 7ed Sympathetic Nervous System (SNS) - 1 Innervates the SA and AV nodes Norepinephrine activates 1adrenergic receptors Increases ability of Ca2+ channels to open, making SA node cells produce action potentials more rapidly (tachycardia) Positive chronotropic effect Increases AV node conduction velocity Positive dromotropic effect Figure 18-16, Marieb and Hoehn, 7ed Sympathetic Nervous System (SNS) - 2 Innervates ventricular muscle directly Norepinephrine activates 1adrenergic receptors Increases force of contraction by increasing size of Ca2+ currents, allowing greater Ca2+ entry per depolarisation Positive inotropic effect Figure 18-16, Marieb and Hoehn, 7ed Chronotropic Changes Acetylcholine and norepinephrine influence the rate of spontaneous depolarisation of the pacemaker cells Pacemaker cell resting membrane potential is also influenced Note that these are Phase 4 effects Figure 20-22, Martini and Nath, 8ed Dromotropic Changes Voltage-gated Ca2+ channels open more readily when sympathetic tone is increased, allowing AV node cells to depolarise more rapidly (increased Phase 0) This increases conduction velocity of action potentials through the AV node Increased parasympathetic tone has the opposite effect http://www.cvphysiology.com/Arrhythmias/A003.htm Summary Contraction is stimulated by electrical depolarisation Specialized conductive pathway Rhythmicity due to pacemaker Na + current Action potential characteristics differ throughout the heart Activation of the ventricles occurs after a delay Activation of the ventricular free wall is (mostly) from endo- to epicardium & from apex to base Autonomic innervation influences heart rate, conduction velocity and contractility Week 2 CVM GLS: Basic Electrophysiology Learning Objectives By the end of this week, you should feel comfortable describing the electrical behaviour of the heart from the level of individual ion channels in single myocytes, through the conduction system and the electrical syncytium, to the whole organ level. Deep understanding of these concepts will facilitate you in understanding how this electrical activity produces the ECG (which we will cover in Week 3), and how aberrant electrical behaviour gives rise to the many dysrhythmias your patients may suffer (which will be introduced in Week 4). Exercise 1: The Action Potential, Inactivation, and Refractory Periods The action potential is the result of the currents flowing across the plasma lemma of the individual myocyte being studied. Refractoriness arises from ion channels which have the property of inactivation. In the heart, myocytes may be conductile or contractile, and have different populations of ion channels, and so, different electrical properties. Question 1: Draw an action potential from a conductile myocyte in the SA node. i. ii. iii. Be sure to label the axes and provide units. Label the phases of this action potential. Identify the ion channel(s) responsible for each phase. i. see figure below ii. Phase 0 = upstroke (Ca current); Phase 3 = repolarisation (K current); Phase 4 = pacemaker potential (predominantly Na current, activated by hyperpolarisation) iii. See above, from the lecture slides. 1|Page Week 2 CVM GLS: Basic Electrophysiology Question 2: What are the inherent rates of automaticity, under resting parasympathetic input, in (i) the SA node, (ii) the AV node, and (iii) the Bundle Branches/Purkinje System? Why is there this difference? What would happen if the AV node conductile cells reached their threshold and produced action potentials more rapidly than the SA node conductile cells? If the SA node cells were injured during a heart attack and no longer capable of producing action potentials? Rates (at rest, with normal parasympathetic input): SA Node: 60 – 100 AP/min (if excitation-contraction coupling is healthy, = a HR of 60 – 100 bpm) AV Node: 40 – 60 AP/min & bpm (note, as long as the SA node is healthy, the AV node is under its control and runs at the faster SA node rate) Bundle of His/Bundle Branches/Purkinje Fibres: 15 – 40 AP/min & bpm (note, as long as the SA node is healthy, the rest of the conduction system is under its control and runs at the faster SA node rate) These differences ensure that the part of the conduction system which initiates the wave of excitation is the SA node, followed by the AV node and then the system in the ventricles. This activates first the contractile myocytes of the atria, while the ventricles are still relaxed, and then the contractile myocytes of the ventricles, from apex to base and endocardium to epicardium. This ensures efficient ejection of blood. If the AV node was the fastest to reach the threshold to produce an action potential, then it would become the pacemaker and set the heart rate. However, contraction of the atria and the ventricles would either be simultaneous (if excitation spreads retrograde into the atria from the AV node) or atria would not become excited (if excitation does not spread into the atria from the AV node). If the SA node fails, then the AV node takes over and resting heart rate drops to 40 – 60 bpm. This is an escape rhythm, and will be covered when we go over dysrhythmias. Question 3: Draw an action potential from a contractile myocyte. i. ii. iii. Be sure to label the axes and provide units. Label the phases of this action potential. Identify the ion channel(s) responsible for each phase. Please refer to the figure provided in the lecture slides. Phase 0 = fast voltage-gated Na channels triggered to activate at -60 mV (at -40 mV, the voltage-gated L-type Ca channels and voltage-gated potassium channels also open, but slowly, such that they aren’t open until phase 1) Phase 1 = brief dominance of K current, can be ignored for test purposes Phase 2 = balance of inward voltage-gated L-type Ca current and outward voltage-gated potassium current creates the plateau 2|Page Week 2 CVM GLS: Basic Electrophysiology Phase 3 = voltage-gated L-type Ca channels inactivate at the end of Phase 2, leaving voltagegated potassium current which repolarises the myocyte back to resting membrane potential/Phase 4. Phase 4: K1 (also known as the “inward rectifying” K channel, or Kir), the pumps (Na/K ATPase, plasma membrane Ca ATPase or PMCA, sarco/endoplasmic Ca ATPase, or SERCA), the Na/Ca exchanger and leak channels for Na, K and Ca keep the membrane stable at ~-87 mV. Question 4: Explain why resting membrane potential (Phase 4) of a contractile myocyte is stable, while in a conductile myocyte, it spontaneously depolarises. What would happen if resting potential in a contractile myocyte became unstable? Stable resting membrane potential in healthy contractile myocytes prevents spontaneous depolarisation and production of action potentials in the working muscle of the heart, ahead of the appropriate signal from the conduction system. In the conduction system, the spontaneous depolarisation produced by the pacemaker sodium current ensures that the conductile myocytes spontaneously depolarise and produce action potentials, initiating the wave of excitation that causes each heartbeat. If RMP becomes unstable in one or more contractile myocytes, these myocytes may produce APs before the signal to depolarise comes from the conduction system. These contractile myocytes then become the new pacemaker of the heart, causing the whole heart (or just the ventricles, depending on the location of the sick contractile myocytes) to contract out of sequence. This may be for one beat or occur repeatedly, producing dysrhythmias. Such premature excitation and abnormal spread of action potentials will be considered in more detail in Week 4. Question 5: Using fast voltage-gated sodium channels as your model, explain activation and deactivation. Ion channels such as the voltage-gated sodium channel have “gates”; physical barriers within the channel which prevent ions from passing through the channel. The activation gate is voltagesensitive, opening upon depolarisation to the threshold of ~-60mV. Opening of this voltage-sensitive activation gate is called “activation”. Once the myocyte repolarises below the threshold for activation, this gate closes, thereby deactivating the ion channel. Note that the activation/deactivation gate is a separate physical gate from the inactivation gate (refer to next question). Question 6: Explain inactivation and recovery from inactivation, using voltage-gated sodium channel of Phase 0 of a contractile myocyte action potential as your model. (Note, understanding inactivation is important to understanding ‘re-entry’, the mechanism behind the most serious dysrhythmias, (week 4), and also ‘use dependence’ in anti-arrhythmia drugs.) Refer to the SS. 3|Page Week 2 CVM GLS: Basic Electrophysiology Question 7: Explain the refractory periods (absolute and relative) in cardiac myocytes. Relate inactivation and recovery from inactivation to the absolute and relative refractory periods of the myocyte, first considering the contractile myocytes. Then, describe the cellular basis for the absolute and relative refractory periods in conductile myocytes. Refer to the SS. Exercise 2: The Anatomy of Excitation Successful ejection of blood depends on each myocyte in the heart depolarising and contracting in the right sequence. This precise and carefully planned sequence of activation is produced by the anatomical relationship between the contractile and conductile myocytes. Question 1: Delineate the conduction system of the heart, and state the roles of each section. Drawing the heart may help cement the anatomy of the conduction system in your mind. Note: the internodal pathways and interatrial pathways are represented by the dashed lines but not labelled. These are shown in other diagrams from the lecture in week 2. Roles: SA Node = first to reach threshold and produce action potentials, therefore, this is the pacemaker of the heart. Internodal and interatrial pathways = conduction system rapidly spreading the wave of action potentials/activation between the SA and AV nodes, and to the myoyctes of both atria. This signals the atria to contract. AV Node = electrical gateway to the ventricles. There is a delay in conduction through the AV node, to allow the atria time to contract before the ventricles depolarise and contract. Note that the annulus, the fibrous skeleton of the heart formed by the valves and the connective tissue at the junction between the atria and ventricles (and where the great arteries arise) doesn’t conduct action potentials, so the only route between the atria and the ventricles in the healthy heart is via the AV Node and Bundle of His. AV Bundle/Bundle of His = conduction pathway delivering the wave of action potentials into the ventricles, once it clears the AV node. This is in the base of the septum. Left and Right Bundle Branches = conduction pathway spreading the wave of action potentials to the septum, travelling from base to apex. The left bundle branch is much bigger than the right, as the left 4|Page Week 2 CVM GLS: Basic Electrophysiology ventricle has far more muscle than the right, to excite and trigger contraction in. Hence, the rate of conduction of the wave of action potentials is faster in the left than the right bundle branch, and the vectors (directions) of the waves of action potentials is (a) zooming down the left bundle branch from base to apex, with slower base to apex depolarisation down the right bundle branch, (b) action potentials spreading left to right from the already depolarised left side of the septum toward the still resting cells (both contractile and conductile) on the right side of the septum. Contraction of the septum stabilises the core of the heart against the force of contraction by the ventricles free walls. Purkinje fibres = final conduction pathway which spreads the waves of excitation into the papillary muscles and the contractile myocytes of the free walls. These run along the endocardial surface from apex to base for most of the free wall, then penetrate toward mid-myocardial. Conduction along these fibres is the fastest of any part of the conduction system, allowing very rapid excitation of the ventricles. Question 2: Describe the sequence of excitation through the conduction system of a normal heart. Once excitation reaches the free wall of the ventricles, what is the sequence of depolarisation through the contractile myocytes? Of repolarisation? (This knowledge is important to understand how the spread of depolarisation and repolarisation through the healthy heart produce the normal ECG waveforms.) Depolarisation: SA node – internodal pathways (and interatrial pathways, to the atrial contractile myocytes in both atria) – AV node – Bundle of His – Left and right bundle branches of the septum (note that conduction is faster down the left bundle branch, moving from base to apex and left to right down the septum) – Purkinje fibres – contractile myocytes of the free wall of the ventricles (apex to base and endocardial to epicardial for most of the free wall, until the Purkinje fibres move more mid-myocardium and excitation radiates out through the contractile myocytes in all directions, up by the base) Repolarisation of the free wall of the ventricles (the contractile myocytes): From apex to base and epicardium to endocardium through the contractile myoyctes of the ventricle and septum. Question 3: Explain why the wave of excitation is unidirectional in a healthy heart, despite crossing a continuous “sheet” of electrically coupled myocytes. (Loss of unidirectional spread of action potentials underlies ‘re-entry’, which gives rise to ventricular fibrillation, among other dysrhythmias.) Under healthy conditions and with a normal heart rate, myocytes behind the wave of excitation are always absolutely refractory and unable to produce a second action potential when their ‘downstream’ neighbours are depolarised and producing action potentials. Hence, only the ‘downstream’ myocytes at resting membrane potential will be capable of producing action potentials when the wave of excitation occurs. 5|Page Week 2 CVM GLS: Basic Electrophysiology Exercise 3: Autonomic Modulation of Heart Rate and Contractility Question 1: Describe sympathetic and parasympathetic innervation of the heart, identifying the targets of each type of innervation, the neurotransmitter(s) released, and the specific types of receptors present in each target cell. Sympathetic – sympathetic cardiac nerve to the SA node, the AV node and the contractile myocytes (focus on those in the ventricle, where the key pumping occurs). These nerves release 1)adrenergic receptors on the target cells. Parasympathetic – Vagus nerve to the SA and AV nodes (note unlike sympathetic innervation, not the contractile myocytes) where acetylcholine is released to interact with muscarinic cholinergic receptors on the target cells. Question 2: Explain how an increase in sympathetic input to the heart influences chronotropy, including the specific cellular targets and changes in your description. How does increased parasympathetic input affect chronotropy? Refer to the SS. Question 3: Explain how an increase in sympathetic input to the heart influences inotropy, including the specific cellular targets and changes in your description. Does increased parasympathetic input affect inotropy? Refer to the SS. Question 4: Explain how an increase in sympathetic input to the heart influences dromotropy, including the specific cellular targets and changes in your description. How does increased parasympathetic input affect dromotropy? Refer to the SS. Question 5: Lusitropy refers to the rate of relaxation of cardiac contractile myocytes, and is modulated by the sympathetic nervous system. You learned this week that increased sympathetic nervous tone in the heart is associated with positive chronotropic, positive dromotropic, and positive inotropic effects. What change in lusitropy would you predict to occur if sympathetic input to the heart increased? Why would this change be important to keep pumping efficiency high? (Lusitropy will be discussed in subsequent sessions. This is a thought exercise, to help you consolidate your understanding of autonomic control of pumping efficiency of the heart.) Refer to the SS. 6|Page MD2011 CVM Week 2 Synthesis Session Dr. Lisa Chilton [email protected] Building DB087, Room TV222, 4781 5195 Parasympathetic Nervous System (PNS) Innervates the SA and AV nodes Negative chronotropic and dromotropic effects Figure 18-16, Marieb and Hoehn, 7ed Sympathetic Nervous System (SNS) Innervates the SA and AV nodes as well as the contractile myocytes (ventricle primarily) Positive chronotropic, dromotropic, inotropic, and lusitropic effects Note, as SNS tone increases, PNS decreases, and vice versa Herring et al. 2002 DOI:10.1152/nips.01386.2002 0 Chronotropic Changes 3 4 Acetylcholine and norepinephrine exert opposite effects on the rate of spontaneous depolarisation of the SA Node myocytes Resting membrane potential is influenced via activation of AChsensitive K+ channels by PNS (or lack thereof when SNS is activated and therefore PNS is depressed) Note difference in pacemaker potential slope: slow sodium “funny” current decreases with PNS and increases with SNS stimulation Note that these are phase 4 effects Figure 20-22, Martini and Nath, 8ed Chronotropic Changes Norepinephrine also affects the voltage-gated L-type Ca channels, allowing them to open at a more negative membrane potential (lower voltage threshold) and increasing calcium current Earlier, faster upstroke/ Phase 0 = positive chronotropy This effect is lost when PNS stimulation increases, as SNS stimulation automatically decreases Note that these are Phase 0 effects Figure 20-22, Martini and Nath, 8ed Note that the SNS modulates the depolarising currents (Na+ and Ca2+) while the PNS modulates K+ current, which makes the sarcoplasm more negative Dromotropic Changes Voltage-gated L-type Ca2+ channels open more readily when sympathetic tone is increased, allowing AV node cells to depolarise more rapidly (increased Phase 0) RMP is also more positive due to lack of parasympathetic input Pacemaker potential is steeper due to SNS (and lack of PNS) input This increases conduction velocity (positive dromotropy) http://www.cvphysiology.com/Arrhythmias/A003.htm Increased parasympathetic tone has the opposite effect Concept Check Chronotropy = heart rate - Determined by the SA node in a healthy heart Dromotropy = conduction velocity - Rate of action potentials propagating from one myocyte to the next, like a series of dominos Inotropy = force of contraction - Relates to the amount of sarcoplasmic calcium, triggering crossbridge formation http://www.cvphysiology.com/Blood_Pressure/BP010.htm Lusitropy = rate of relaxation - How rapidly do repolarisation and resequestration via SERCA & PMCA occur, returning [Ca2+]i to resting levels? Explain inactivation, using voltage-gated sodium channel of Phase 0 of a contractile myocyte action potential as your model. Explain the refractory periods (absolute and relative) in cardiac myocytes. Relate inactivation and recovery from inactivation to the absolute and relative refractory periods of the myocyte, first considering the contractile myocytes. What produces these periods in conductile myocytes? Concept Check: a -87 mV i ~-60 mV (activation gate triggered to open at ~-60 mV; this gate will stay open until the membrane repolarises below this threshold) + + Modified from Martini and Nath, 8ed + + +++ Note: the gates have positive charges, which cause them to move according to the membrane potential (too negative for the activation gate to open; channel deactivated at rest) Activation +++ Activation (a) and inactivation (i) gates of fast voltage-gated Na+ channels RMP +20 mV Inactivation (depolarisation causes the inactivation gate to block channel; repolarisation will allow recovery from inactivation) Activated Activation gate is open Activation gate is open but inactivation gate is closed Modified from Stotz and Zamponi 2001 (DOI: dx.doi.org/10.1016/S0166-2236(00)01738-0) Concept Check: a i +++ -87 mV The activation gate activates above -60mV in phase 0 & deactivates below -60mV in phase 3 Modified from Martini and Nath, 8ed + + +++ ~-60 mV + + +20 mV The inactivation gate inactivates with full depolarisation in phase 0 & recovers from inactivation during midto late phase 3 Absolute Refractory Period ARP www.studyblue.co Fast voltage-gated Na+ channels are activated (Phase 0) Na+ channels are then inactivated upon further depolarisation (Phases 1, 2, and start of 3) The myocyte cannot produce another action potential, regardless of stimulus strength ARP = Phases 0, 1, 2 and the first part of Phase 3 Relative Refractory Period ARP www.studyblue.co During Phase 3 repolarisation, fast voltage-gated Na+ channels start to recover from inactivation Full recovery occurs by Phase 4/resting membrane potential RRP = mid-Phase 3 to start of Phase 4 Note, in Phase 4, the myocyte is no longer refractory Question 11: Explain inactivation, using voltage-gated sodium channel of Phase 0 of a contractile myocyte action potential as your model. Question 12: Explain the refractory periods (absolute and relative) in cardiac myocytes. Relate inactivation and recovery from inactivation to the absolute and relative refractory periods of the myocyte, first considering the contractile myocytes. What produces these periods in conductile myocytes? inactivate 0 3 4 SA Node Cells Figure 18-13, Marieb and Hoehn Concept Check 0 3 Compare the electrophysiology of conductile and contractile myocytes: 4 Parameter Conductile Contractile Phase 0 L-type voltagegated Ca2+ current Fast voltagegated Na2+ current Phase 2 Not present L-type voltagegated Ca2+ and voltage-gated K+ currents Phase 3 voltage-gated K+ currents voltage-gated K+ currents Phase 4 (RMP) -60 mV, unstable (slow Na+ current, aka the “funny” current) -87 mV, stable (K1 current plus membrane pumps and exchangers) Marieb and Hoehn Helpful YouTube video: www.youtube.com/watch?v=v7Q9BrNfIpQ Figure 14-13, Silverthorn, Human Physiology, 5ed Concept Check http://www.zoology.ubc.ca/~gardner/cardiac_muscle_contraction.htm Concept Check Cardiovascular Physiology Concepts www.cvphysiology.com Thought Exercises 1 to 4: Please refer to the separate PPTX/pdf files. Vectorstock.com Thought Exercise 1: Explain what would happen to the action potential in the contractile myocytes if you prescribe a drug which blocks ~25% of the fast voltage-gated sodium channels. Would this drug affect the conductile myocytes? Contractile, not conductile myocytes 25% fast voltage gated Na+ gone VGNa++ involved in phase 0 No K+ involvement. Potentially a slower phase 0, negative dromotropy, negative chronotropy? You would have a lower slope on the uptick, it would mimic PSNS output Thought Exercise 1: Explain what would happen to the action potential in the contractile myocytes if you prescribe a drug which blocks ~25% of the fast voltage-gated sodium channels. Would this drug affect the conductile myocytes? Note: specificity of sodium channel blockers (Class 1 antiarrhythmic drugs) is never 100% for sodium channels – these drugs influence other channels as well, and may have other side-effects. Sources: http://dx.doi.org/10.1016/j.ajem.2006.11.038 (above) http://www.cvpharmacology.com/antiarrhy/sodium-blockers.htm (right) Thought Exercise 2: What would happen if you gave your patient a drug which blocked 25% of their slow “funny” sodium channels? As part of your answer, identify which cardiac myocytes would be affected. it would only affect conductile cardiomyocytes it would decrease the slope of the spontaneous Na+ influx, leading to delayed reaching to threshold potential. Negative chronotropy effect, no other effects in the other parts of the heart Thought Exercise 2: What would happen if you gave your patient a drug which blocked 25% of their slow “funny” sodium channels? As part of your answer, identify which cardiac myocytes would be affected. Source: http://163.178.103.176/Tema1G/Grupos1/GermanT1/GATP15/Nico1.htm On the diagram above, the effect in the SA node would be like “a” control vs. “b” pacemaker channel block. * What is line “c”? Thought Exercise 2: What would happen if you gave your patient a drug which blocked 25% of their slow “funny” sodium channels? As part of your answer, identify which cardiac myocytes would be affected. *Negative chronotropic effect of ACh Source: http://163.178.103.176/Tema1G/Grupos1/GermanT1/GATP15/Nico1.htm On the diagram above, the effect in the SA node would be like “a” control vs. “b” pacemaker channel block. * What is line “c”? Thought Exercise 3: Explain what would happen to the action potential in an SA node myocyte if you prescribed a drug which blocks 50% of the voltage-gated calcium channels. Compare this to the effect of this drug on the action potential in a myocyte in the ventricular free wall. Negative chronotropy, dromotropy, ionotropy, positive lusitropy I Figure 18-13, Marieb and Hoehn Figure 14-13, Silverthorn, Human Physiology, 5ed Thought Exercise 3: Explain what would happen to the action potential in an SA node myocyte if you prescribed a drug which blocks 50% of the voltage-gated calcium channels. Compare this to the effect of this drug on the action potential in a myocyte in the ventricular free wall. Figure 18-13, Marieb and Hoehn Figure 14-13, Silverthorn, Human Physiology, 5ed Thought Exercise 4: Explain what would happen to the action potential if you prescribed a drug which blocks 25% of the voltage-gated potassium channels. Would this affect be different in a conductile vs. a contractile myocyte? 7 Figure 18-13, Marieb and Hoehn Figure 14-13, Silverthorn, Human Physiology, 5ed Thought Exercise 4: Explain what would happen to the action potential if you prescribed a drug which blocks 25% of the voltage-gated potassium channels. Would this affect be different in a conductile vs. a contractile myocyte? Figure 18-13, Marieb and Hoehn Figure 14-13, Silverthorn, Human Physiology, 5ed