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chapter 22 - 1 Cardiac Action Potentials Membrane...

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1 Cardiac Action Potentials Membrane potentials are created when ions move into and out of the cell, an electrical (voltage) difference across the membrane is created. So drugs that alter ion movement (ex: calcium) have profound effect s of action potential and can alter heart rate Various phases of cardiac action potential are related to changes in permeability in cell membrane, particularly sodium and potassium Threshold: point at which the cell membranes selective permeability to sodium and potassium is disrupted, leading to depolarization Resting Membrane Potential --Myocardial cells: -80 & -90 mV --SA node: -50 & -60 mV --AV node: -60 & -70 mV Depolarization : caused by the movement of electrically charged solutes (ions) across cardiac cell membranes. Electrical state (acting) Inside of cell becomes less negatively charged Repolarization : occurs in the same way but is the deactivation. Resting state Refractory period Effective or absolute refractory period: period which no new cardiac action potential can be potentiated; corresponds to time needed for reopening of channel that permit sodium and potassium influx (follows depolarization) Relative refractory period: membrane can be depolarized again but only by a greater than normal stimulus; occurs near the end of repolarization The normal electrocardiogram The P wave represents atrial depolarization. Atrial dysrhythmia affects the P wave
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2 ** PR Interval: measure of time from onset of atrial activation to onset of ventrical activation; represents the time necessary for blood to flow through atrium, AV node, and His Purkinje system to activate ventricle myocardial cells (0.12-0.20 seconds) QRS complex represents the sum of all ventricular muscle cell depolarization (contraction) and atrial repolarization. (0.06-0.10 seconds) Configuration and amplitude vary considerably among individuals ** ST Interval: entire ventricular myocardium is depolarized The T wave represents ventricular repolarization (??term) (0.4 seconds but varies inversely with heart rate) ** QT Interval: sometimes called the electrical systole of the ventricles (under .38 seconds) Automatically the property to generate spontaneous depolarization to threshold, enables the SA and AV nodes to generate cardiac action potentials without any stimulus. Cells capable of spontaneous depolarization are called automatic cells . Automatic cells of cardiac conduction can stimulate the heart to beat even when it is removed from body Spontaneous depolarization is possible because the membrane potential doesn’t rest during return to resting membrane potential; instead it slowly creeps towards threshold in diastolic phase Because threshold is approached during diastole, return to the resting membrane potential in automatic cells is called diastolic depolarization .
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