Cardiac Cells: Contraction PDF

Cardiac Cells: Contraction Cardiac muscle, like skeletal muscle and neurons, is an excitable tissue with the ability to generate action potentials. The heart can contract without an outside signal because the signal for contraction is MYOGENIC or originating within the heart itself. The...

Cardiac Cells: Contraction Cardiac muscle, like skeletal muscle and neurons, is an excitable tissue with the ability to generate action potentials. The heart can contract without an outside signal because the signal for contraction is MYOGENIC or originating within the heart itself. The heart contracts, or beats, rhythmically as a result of action potentials that it generates by itself, a property called AUTHORHYTHMCITY (auto means “self”) The signal for myocardial contraction comes from the PACEMAKER CELLS (SA Node, AV Node, Bundle of His, Purkinje Fibers) Cardiac Cells: Contraction The Myocardium Two specialized types of cardiac muscle cells: Each of these 2 types of cells have a distinctive action potential Cardiac Cells: Contraction  Cardiac cells: Cardiac Action Potential Remember… Action potentials can be defined as the rapid changes in the membrane potential. These changes occur due to the changes in membrane permeability to ions Threshold Potential: It is a critical level to which the membrane potential must be depolarized to initiate action potential  Cardiac cells: Cardiac Action Potential Remember… In the extracellular fluid, we see a high concentration of sodium ions with a low concentration of potassium ions Intracellularly there is a high concentration of potassium ions and a low concentration of sodium ions The sodium potassium pump present on the membrane, regulates the respective concentration of these ions  Cardiac cells: Cardiac Action Potential Remember… As we know from before, this pump uses ATP to extrude 3 NA+ ions out of the cell and 2 K+ into the cell thus creating an environment in which intracellularly the cell is negatively charged and extracellularly it is positively charged Cardiac Cells: Action Potentials Cardiac Action Potentials are brief changes in voltage membrane potentials across the cell membrane of the heart Caused by movement of charged ions between the inside and outside of the cell through transmembrane ion channels Action potentials in the heart are initiated by SA Node SA Node produces roughly 60-100 action potentials every minute  Cardiac Cells: Pacemaker Action Potentials The Intrinsic Conduction system as we have identified is Autorhythmic….WHY? Depolarization of the heart is rhythmic and spontaneous Pacemaker cardiac cells are self-excitable Pacemaker cells are autorhythmic and initiate and distribute impulses to coordinate the depolarization and contraction of the heart  Cardiac Cells: Pacemaker Action Potentials Pacemaker Cells Membrane Potential = -60mv These cells DO NOT stay at -60mV, constantly firing At -60mV, membrane potential drifts back up to - 40mV Threshold is reached spontaneously at -40mV Peak depolarization is +10mV Repolarization back to - 60mV, cycles again  Cardiac Cells: Pacemaker Action Potential Channels Cardiac muscle Action Potentials are caused by opening 3 types of channels Ionic movement during Pacemaker Action Potential’s: Membrane Potential (-60mV) drifts towards threshold on it own: Cardiac Cells: Pacemaker Action Potential Cardiac Cells: Pacemaker Action Potential Cardiac Cells: Pacemaker Action Potential Cardiac Cells: Pacemaker Action Potential Cardiac Cells: Pacemaker Action Potential Action Threshold potential 2 2 3 1 1 Pacemaker potential 1 Pacemaker potential 2 Depolarization The 3 Repolarization is due to This slow depolarization is action potential begins when Ca2+ channels inactivating and due to both opening of Na+ the pacemaker potential K+ channels opening. This channels and closing of K+ reaches threshold. allows K+ efflux, which brings channels. Notice that the Depolarization is due to Ca2+ the membrane potential back membrane potential is influx through Ca2+ channels. to its most negative voltage. never a flat line.  Cardiac Cells: Cardiac Muscle Action Potentials Action Potentials in the Pacemaker cells will spread to the cardiac muscle cells (Atria and Ventricles) and cause them to contract  Cardiac Cells: Cardiac Muscle Action Potentials Begins with Phase 4 (Resting Potential) Occurs when cell is at rest (diastole) Resting potential for cardiomyocytes is -90mV Na+ and Ca+2 channels are closed at RMP  Cardiac Cells: Cardiac Muscle Action Potentials Phase 0 (Depolarization) Depolarization opens voltage-gated fast Na+ channels in the sarcolemma Drives Na+ into cell, changing membrane potential  Cardiac Cells: Cardiac Muscle Action Potentials Phase 1 (Early Repolarization) Begins with rapid inactivation of Na+ Channel Reduction of Na+ Movement into the cell At the same time K+ channels opens and closed rapidly  Cardiac Cells: Cardiac Muscle Action Potentials Phase 2 (Plateau Phase) This phase is responsible for prolonging the Cardiac Action Potential Ca+2 channels open to keep the cells polarized Increased Ca+2 concentration increases the activity of Na+/Ca+2 exchanger  Cardiac Cells: Cardiac Muscle Action Potentials Phase 3 (Rapid Repolarization) Outflow of potassium brings membrane potential back to RMP of -90mV to prepare cell for new cycle of depolarization  Atrial and Ventricular Muscle Action Potential K+ Channels Ca Channels K+ Channels ++ Open Open More Open More Slow Ca++ Channels Open 1 +20 2 Membrane Potential 0 -20 h ase 3 p (mV) -40 0 -60 4 -80 -100 Fast Na+ Channels Open 0 1 2 3 4 phase 0- Fast Na+ channels open then slow Ca++ channels phase 1- K+ channels open Seconds phase 2- Ca++ channels open more phase 3- K+ channels open more phase 4- Resting membrane potential