Veterinary Physiology 1: Cardiovascular System #1 PDF

VETM 1502: VETERINARY PHYSIOLOGY 1 LECTURE: CARDIOVASCULAR SYSTEM #1 Kavita R. Lall, B.Sc. (Hons.), D.V.M. (Hons.), M.Sc. (Dist.) OUTLINE  Learning objectives  Introduction - morphological characteristics  Systemic excitability  Conduction...

VETM 1502: VETERINARY PHYSIOLOGY 1 LECTURE: CARDIOVASCULAR SYSTEM #1 Kavita R. Lall, B.Sc. (Hons.), D.V.M. (Hons.), M.Sc. (Dist.) OUTLINE  Learning objectives  Introduction - morphological characteristics  Systemic excitability  Conduction and transmission  Action potentials  Definitions LEARNING OBJECTIVES  Understand what makes cardiac cells unique and be able to explain the different terms  Briefly describe the circulatory systems that work together in the cardiovascular system  Explain the conduction system of the heart – systemic excitability  Describe how depolarization and repolarization occurs in ventricular cells and pacemaker cells; be able to compare and contrast  Differentiate between latent pacemakers and the pacemaker INTRODUCTION  Review anatomy! artery INTRODUCTION CONT’D  Three circulatory systems work together in the cardiovascular system: 1. Coronary circulation 2. Pulmonary circulation 3. Systemic circulation CORONARY CIRCULATION PULMONARY CIRCULATION SYSTEMIC CIRCULATION SYSTEMIC EXCITABILITY  Excitability: Intrinsic membrane property that allows a cell to generate an electrical signal or AP in response to stimuli of sufficient magnitude  Cardiac excitability: The ability of cardiac cells to depolarize and repolarize during an AP, as well as the ease with which electrical activity propagates from cell to cell SYSTEMIC EXCITABILITY CONT’D  Electrical activity of the heart involves:  The generation of pacemaker potentials  The generation of action potentials  The conduction of action potentials through the heart SYSTEMIC EXCITABILITY CONT’D Basic mechanisms of membrane potential 3 major factors cause the membrane potential 1. Permeability of the membrane to diffusion of ions  Resting state (resting membrane potential) - cell membrane is more permeable to K+ than Na+ due to more K+ leak channels SYSTEMIC EXCITABILITY CONT’D 2. Na+-K+ ATPase  Generates positive membrane potential outside the cell by actively pumping 3 Na+ out of the cell for every 2 K+ pumped into the cell against their concentration gradients SYSTEMIC EXCITABILITY CONT’D 3. Fixed anions inside the cell  Many intracellular anions, e.g. proteins, are found within the cell - fixed  Generate negative electrical potential inside the cell SYSTEMIC EXCITABILITY CONT’D  Modified cardiac muscle is involved in electrical activity of the heart:  Sinoatrial node (SA node)  Atrioventricular node (AV node)  Atrioventricular bundle (AV bundle)  Purkinje fibers SYSTEMIC EXCITABILITY CONT’D  The cyclic nature of cardiac activity depends on normal conduction of electrical impulses from the SA node through the atrial and ventricular myocardium CONDUCTION AND TRANSMISSION  The function of the cardiac conducting system is to coordinate the contraction and relaxation of the four cardiac chambers ACTION POTENTIALS SA node/pacemaker cells  No true resting potential - generate regular, spontaneous action potentials  Depolarization is due to Ca2+ instead of fast Na+ ACTION POTENTIALS CONT’D  At the end of repolarization (membrane potential is very negative ~ -60 mV), ion channels open that conduct slow, inward (depolarizing) Na+ currents called "funny" currents (If)  membrane potential begins to spontaneously depolarize  initiate Phase 4 ACTION POTENTIALS CONT’D  Phase 4:  As the membrane potential reaches about -50 mV, a transient/T-type Ca2+ channel opens  increases calcium conductance (gCa)  As Ca2+ enters the cell through these channels down its electrochemical gradient, the inward directed Ca2+ currents further depolarize the cell ACTION POTENTIALS CONT’D  When the membrane depolarizes to about - 45 mV, long-lasting/L-type Ca2+ channels open, which further increases gCa  Opening of these channels causes more Ca2+ to enter the cell and to further depolarize it until the threshold is reached (about -40 mV) ACTION POTENTIALS CONT’D  Phase 0: Depolarization phase  Primarily caused by increased gCa through the L-type Ca2+ channels  If and Ca2+ currents through the T-type Ca2+ channels, decline during this phase as their respective channels close ACTION POTENTIALS CONT’D  Phase 3: Repolarization phase  K+ channels open (increased gK), which increases outward directed  L-type Ca2+ channels close, which decreases gCa  Once the cell is completely repolarized (about -60 mV), the cycle is spontaneously repeated ACTION POTENTIALS CONT’D Ventricular cells  Phase 4: Resting phase/resting membrane potential  RMP is about −90 mV ACTION POTENTIALS CONT’D  Phase 0: Depolarization  An AP triggered in a neighbouring cardiomyocyte causes the transmembrane potential to rise above −90 mV  Fast Na+ channels open  Na+ enters into the cell  approaches −70mV (threshold potential) ACTION POTENTIALS CONT’D  The large Na+ current rapidly depolarizes the transmembrane potential to 0 mV and slightly above 0 mV for a transient period of time (overshoot); fast Na+ channels close  L-type Ca2+ channels open (when the transmembrane potential is greater than −40 mV)  small but steady influx of Ca2+ down its concentration gradient ACTION POTENTIALS CONT’D  Phase 1: Early repolarization  Transmembrane potential is now slightly positive  Some K+ channels open and an outward flow of K+ returns the transmembrane potential to approximately 0 mV ACTION POTENTIALS CONT’D  Phase 2: Plateau phase  L-type Ca2+ channels are still open  small, constant inward current of Ca2+  K+ continues to moves down its concentration gradient ACTION POTENTIALS CONT’D  These two countercurrents are electrically balanced, and the transmembrane potential is maintained at a plateau ACTION POTENTIALS CONT’D  Phase 3: Repolarization  Ca2+ channels are gradually inactivated  K+ continues to moves down its concentration gradient  RMP (Phase 4) to prepare the cell for a new cycle of depolarization ACTION POTENTIALS CONT’D Latent pacemakers  Other areas of the heart can undergo spontaneous depolarization – AV node, AV bundle and Purkinje fibers  Rate of depolarization of the SA node is faster than those of latent pacemakers  They take over the function of initiating action potentials of the heart only when the SA node is unable to generate impulses or when these impulses fail to propagate DEFINITIONS  The physiological characteristics of the cardiac conduction cells:  Automaticity: Ability of the heart to initiate its own action potentials and subsequent contractions  Excitability: Ability to respond to an electrical impulse  Conductivity: Ability to transmit an electrical impulse from one cell to another  Rhythmicity: Refers to the regularity or pattern of heartbeats THE END