13 14 Heart and Circulation I and II (1)

Connection between cardiac muscle cells gap junction

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Connection between cardiac muscle cells Gap Junction : For electrical synchronization of cardiac muscle cells Note : Cardiac muscle cells must contract synchronously in order to generate sufficient ventricular pressure to eject blood into the circulation
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Heart Beat Generation and Electrocardiogram
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Heartbeat Generation SA Node : Pacemaker Cells that generate spontaneous action potentials AV Node : Slow conduction of action potentials from atria to ventricles Bundle of His and Purkinje Fibers : Rapid propagation of Action potentials to all ventricular cardiac muscle cells 1. Slow conduction through the AV node causes the time delay between atrial depolarization and ventricular depolarization. 3. AV nodal delay allows atrial blood to fill the ventricles before ventricular pumping of the blood to the circulation
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Mechanism of Spontaneous Generation of Action Potentials by Sinoatrial Node Pacemaker Potential : Sspontaneous depolarization caused by Na+ and Ca2+ influx through hyperpolarization-activated funny channel. Action Potential : Inward Ca2+ current causes SA nodal depolarization; outward K+ current causes SA nodal repolarization.
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Autonomic Innervation of the Heart
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Effects of Sympathetic and Parasympathetic Stimulation on SA Node Sympathetic Stimulation : Increases rate of spontaneous depolarization by sinoatrial node, thereby increasing heart rate Parasympathetic Stimulation : Induces hyperpolarization of sinoatrial node, thereby decreasing heart rate
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Ionic Basis of Cardiac Action Potential Upstroke Phase of Depolarization : Activation of fast Na+ channels Plateau Phase of Depolarization: Activation of slow Ca2+ channels and inactivation of K+ channels Repolarization Phase : Opening of voltage-sensitive K+ channels
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Cardiac Action Potential and Muscle Contraction 1. Cardiac action potential and cardiac contraction overlap in time; therefore, cardiac muscle is NOT capable of producing tetanic contractions. 2. Ca2+ influx during the plateau phase of cardiac action potential is important for triggering the release of Ca2+ from sarcoplasmic reticulum.
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Origin of the Electrocardiogram 1. Electrical coupling between cardiac muscle cells enables the heart to function electrically as two giant cells - a) atria and b) ventricles 2. AV conduction delay results in temporal separation between atrial depolarization and ventricular depolarization
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Electrocardiograph 1. An electrocardiograph is a voltmeter that measures voltage between two points on the body surface 2. An electrocardiograph has positive and a negative terminals 3. An electrocardiograph records a positive voltage when the positive terminal is more positive than the negative terminal, and vice versa 4. An ECG wave is recorded whenever there is a rapid change in membrane potential of cardiac muscle cells - during the upstroke depolarization and downstroke repolarization of cardiac action potential
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Relationship Between ECG and Ventricular Action Potential Ventricular Depolarization Wave Ventricular Repolarization Wave
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Depolarization Wave of Electrocardiogram Resting - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + ECG + - - - - - - + + + + + + + + + + + Depolarization   + + + + - - - - - - - - - - - - - - - ECG + - Resting 0 mV 0 mV
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