Cardiac Action Potentials & Conduction Pathway PDF 2025

Cardiac action potentials & conduction pathway Daniel N. Meijles Senior Lecturer in Cardiovascular Biology Room: 2.27 (Corridor 10), Level 2, Jenner Wing Please email me feedback or questions: [email protected] Original Slides by Prof. A. Alberts RE...

Cardiac action potentials & conduction pathway Daniel N. Meijles Senior Lecturer in Cardiovascular Biology Room: 2.27 (Corridor 10), Level 2, Jenner Wing Please email me feedback or questions: [email protected] Original Slides by Prof. A. Alberts RECAP: Intro to Medicine module sessions Overview of the Cardiovascular system (CVS) Blood pressure and its Regulation Treatment of Hypertension Introduction to ECG Related Anatomy, Clinical, and Communication Skills + Muscle contraction, Pharmacology, ANS content will be relevent  Resources Levick JR. ‘An Introduction to Cardiovascular Physiology’ The definitive cardiovascular textbook, but goes into detail which can be beyond scope of MBBS Years 1 and 2 content Great reference book Rang & Dale ‘Pharmacology’ – Sections on heart and vascular systems Good combination of physiology and pharmacology Aaronson & Ward. ‘Cardiovascular System at a glance’ textbook Useful summary book – one page per subject  Learning objectives (LOBs) For this session:  Describe the ionic basis of the pacemaker potential at the sino- atrial node (SAN) and the atria/ventricular action potential  Outline the electrical conduction pathway from sino-atrial node to ventricular myocytes and outline how this conduction pathway relates to the pattern of an electrocardiogram (ECG)  Describe electrical-contraction coupling in the heart, how an action potential induces contraction, and the importance of the plateau/refractory phase in this process  Lecture Outline: How we will achieve the LOBs The heart generates electrical activity and contraction without external stimulation, e.g., from nerves or hormones  Termed: Myogenic Different to  Skeletal muscle: needs motor nerve input  Smooth muscle: needs ANS input This session explores how the heart achieves this? Part 1 Pacemaker potential at sino-atrial node  Initiates electrical activity to regulate rate and rhythm of heartbeat Electrical conduction pathway  Forms basis of ECG pattern BREAK Part 2 Atrial/Ventricular action potential  Electrical activity coupled into contraction  Heartbeat initiation begins at the SAN Sino-Atrial Node (SAN) 1s SAN pacemaker potentials firing at 1 per second = Heart rate of 60 beats min SAN generates action potentials called pacemaker potentials in absence of any external stimulation, e.g. nerves or hormones  Firing rate of these action potentials equates to heart rate  Rhythm of these action potentials equates to heart rhythm SAN cells are not contractile  How SAN produces pacemaker potentials unstable/non-equilibrium resting membrane potential Phase 0 Phase 3 Voltage-gated Voltage-gated Ca2+ channels K+ channels 0 3 Ca2+ influx K+ efflux Depolarisation Repolarisation Threshold Resting membrane Phase 4 4 4 potential If channels – 60 mV If (funny channels) : Hyperpolarisation-activated Na+ channels Na+ influx produces depolarising slope  SAN electrical activity conductance through heart Low resistance pathways Initiated at between atrial and ventricular SAN cells called 1 intracalated discs - Allows fast conduction - Concluded at 2 Ventricles 3 (1) Conduction across both atria to atrial-ventricle node (AVN) (2) AVN is non-contractile tissue which slows electrical conduction between atria and ventricles  This allows atrial ejection to correctly fill ventricles before electrical activity is conducted to ventricles and contraction occurs (3) Bundle of His to purkinje fibres is a fast conduction pathway which allows both ventricles to be stimulated and contracted together Conductance pathway & ECG relationship  P: atrial depolarisation  QRS: ventricle depolarisation  ST: Interval between ventricle depolarisation and repolarisation  T: ventricle repolarisation ECG: provides information on the electrical conduction through the heart  Does not provide information on contraction Modulation of pacemaker potential frequency and conduction pathway have profound action on heart rate and rhythm - See later sessions - Break  Electrical-Contraction coupling through Heart (1) Electrical activity generated in SA node (non-contractile tissue)   spreads out into atria to produce (2) contraction Electrical activity enters AV node (non- contractile tissue), which delays conduction before stimulating ventricles - allows ventricles to fill proper from   atrial contraction - (3) Excitation conducts rapidly through bundle of His into ventricles (4) Excitation through purkinje fibres is conducted throughout the ventricles producing contraction Ventricular excitation and contraction begins at the apex then spreads to base of heart  Atrial/Ventricular action potentials Action potentials in atrial/ventricular cells are ultimately generated from SAN electrical activity  atrial/ventricular cells have a stable resting membrane potential Phase 1 Phase 2 Phase 2 into Phase 3 Plateau phase VGCCs switch off and 200-400 ms K+ channels switch on Phase 0 Voltage-gated Phase 3 Na+ channels Voltage-gated Voltage-gated Ca2+ channels (VGCCs) K+ channels Na+ influx Depolarisation Sustained Ca2+ influx K+ efflux Depolarisation Repolarisation Threshold for firing Phase 4 Phase 4 IMPORTANT : – 90 mV – 90 mV Function of plateau phase Tissue is unexcitable or REFRACTIVE = CANNOT fire more action potentials Voltage-gated Na+ channels are inactivated No twitching  One action potential produces ONE contraction ESSENTIAL for proper ejection of blood from heart How do atrial/ventricular action potentials couple to contraction? Contraction caused by an INCREASE in cytosolic Ca2+ levels In a healthy resting individual Ca2+ rises from about 0.1 µM to about 1 µM Phase 2 ICa Voltage-gated Ca2+ channel Ca2+ influx Ca2+ Ca-induced Ca release Contractile proteins (CICR) (Troponin system) Ca2+ Ryanodine receptor Ca2+ store Contraction (RyR) Sarcoplasmic reticulum Modulation of these processes have profound action on heart contractility - See later sessions -  Example SAQs Compare and contrast the action potentials in SAN with ventricles What is the role of AVN? Outline steps coupling ventricular action potential to contraction What is the function of ventricular action potential plateau phase?