Cardiovascular Physiology (Lec 2) PDF

Cardiovascular physiology Assist prof. Zainab F. Hassan Lec 2 Key points 1. Action potential in cardiac muscle 2. Conduction system of the heart 3- Phases of cardiac action potential o Action potentials in cardiac muscle All living cells have resting action potential (RMP). Th...

Cardiovascular physiology Assist prof. Zainab F. Hassan Lec 2 Key points 1. Action potential in cardiac muscle 2. Conduction system of the heart 3- Phases of cardiac action potential o Action potentials in cardiac muscle All living cells have resting action potential (RMP). The resting membrane potential is expressed in millivolts (mV) and it is negative inside with respect to outside.Changes in membrane potential are caused by flow of ions into or out of cells. Depolarization occurs when there is a movement of a positive charge into the cell.it causes the membrane potential to become less negative, Conduction system of the heart In the human heart, an excitation signal (an action potential) is created by the sinoatrial (SA) node. Conduction system of the heart consist of:  The SA node is located at the junction of the superior vena cava with the right atrium.  The AV node is located in the right posterior portion of the interatrial septum.  Three bundles of atrial fibers that connect the SA node to the AV node.  the bundle of His and its branches  Purkinje fibers. The conduction system is composed, for the most part, of modified cardiac muscle that has fewer striations and indistinct boundaries. Purkinje fibers, specialized conducting cells, are large with fewer 1 Cardiovascular physiology Assist prof. Zainab F. Hassan mitochondria and striations and distinctly different from a myocyte specialized for contraction. Fig 1. Conduction system of the heart All parts of the conduction system are able to generate a cardiac impulse; (autorhythmicity), but the normal primary pacemaker is the SA node. The AV node rarely initiates contractions for two reasons. First, action potentials originating in the SA node travel through the AV node on their way to the ventricles. When this happens, cells in the AV node go into a refractory period, during which they cannot generate their own action potentials. Second, the SA node has a higher frequency of action potentials than the AV node—about 70 impulses per minute for the SA node, as opposed to 50 impulses per minute for the AV node. 2 Cardiovascular physiology Assist prof. Zainab F. Hassan So the AV node is a secondary pacemaker and the Purkinje system is a tertiary (or latent) pacemaker. The AV node acts only if the SA node is damaged or blocked. If for some reason the AV node is unable to drive ventricular contraction, Certain cells in the Purkinje fibers (sometimes referred to as idioventricular pacemakers) can take over. However, the firing frequency of these cells is only 30–40 impulses per minute. Action potential of ventricle, atria, and the purkinje system Myocardial fibers have a resting membrane potential of approximately - 90 mV. Stimulation of cardiac muscle cells by SA produces a propagated action potential, that is responsible for muscle contraction i.e., excitation- contraction coupling. Phases of cardiac action potential (fig1) a. Phase 0 (depolarization): In this phase Voltage-gated sodium channels (fast sodium channels) open and permit sodium to rapidly flow into the cell and depolarize it. This is the rapid depolarization phase. The membrane potential reaches about +20 millivolts before the sodium channels close. b. Phase 1 (initial repolarization): The sodium channels close, the cell begins to repolarize, and potassium ions leave the cell through open potassium channels. c. Phase 2 (plateau): A brief initial repolarization occurs and the action potential then plateaus as a result of increased calcium ion permeability and decreased potassium ion permeability. The voltage-gated calcium ion channels open slowly during phases 1 and 0, and calcium enters the cell. Potassium channels then close, and the combination of decreased potassium ion efflux and increased calcium ion influx causes the action potential to plateau. Benefits of plateau to cardiac cells: 1. Prolongation of the duration of excitation. 3 Cardiovascular physiology Assist prof. Zainab F. Hassan 2. Prolongation of the exposure to high cytoplasmic Ca2+ concentration, thereby increasing the force of cardiac muscle contraction. 3. Giving more time that all cardiac cells can be activated together and contract as one unit in atria and ventricles. d. Phase 3 (rapid repolarization): The closure of calcium ion channels and increased potassium ion permeability, permitting potassium ions to rapidly exit the cell, ends the plateau and returns the cell membrane potential to its resting level. e. Phase 4 (resting membrane potential): Averages about −90 millivolts. Figure (2) Action potential of cardiac muscle and associated ionic currents Refractory period: 1- Absolute refractory period (ARP): It is the period during which the membrane cannot be re-excited by an outside stimulation whatever the level of external voltage applied. It is prolong in cardiac cell due to 4 Cardiovascular physiology Assist prof. Zainab F. Hassan presence of plateau. It occurs in phase 0, 1, 2 and half phase 3 until membrane reaches approximately – 50 mV during repolarization. This period coincides with the systole (contraction) of the cardiac muscle, so another contraction would never occur except after completion of the systole. 2- Relative refractory period (RRP): It is the time during which action potential can be generated with stimulus larger than normal Action potential of SA node The rhythm originates from the sinus node in normal sinus rhythm (NSR).The resting membrane potential of SA node is of -55 to -60 mV (millivolts) in comparison with -85 millivolts for ventricular muscle fibers. The cause of this reduced negativity "less negative" is that the cell membrane of the sinus fibers are naturally leaky to sodium ions "Na + influx". Therefore; Na+ influx causes a rising membrane potential "gradual depolarization" which when reaches a threshold voltage at about - 40 mV, then the calcium-sodium channels opened, leading to a rapid entry of both Ca+2 and Na+ ions causing the action potential to about 0 mV (zero), to be followed by repolarization which is induced by K+ efflux out of the fiber because of the opening of K+ channels. This repolarization carries the resting membrane potential down to about -55 to -60 mV at the termination of action potential.Fig3 5 Cardiovascular physiology Assist prof. Zainab F. Hassan Fig3 Action potentials of the SA node. Latent pacemaker : SA node pacemaker cells maintain the normal heartbeat whereas latent atrial pacemakers contribute to various types of atrial arrhythmias. Latent pacemaker include the cells of the AV node, bundle of His and Purkinje fibers. They have driving the heart rate only if the SA node is suppressed. Question Define Refractory period 2024 Good luck 6