Cardiac Arrhythmias: Difference between revisions

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===Phase 2: Plateau===
===Phase 2: Plateau===
The plateau phase represents an equal influx and efflux of ions in or out of the cell producing a stable membrane potential. This plateau phase is predominantly observed in the ventricular action potential. The inward movement of Ca<sup>2+</sup> through the open L-type Ca<sup>2+</sup> channels (I<sub>Ca-L</sub>) and the exchange of Na<sup>+</sup> for internal Ca<sup>2+</sup> by the Na<sup>+</sup>/Ca<sup>2+</sup> exchanger (NCX) are responsible for the influx of ions during the pleateau phase. The efflux of ions is the result of outward current carried by K<sup>+</sup> (I<sub>Kur</sub> and <sub>Ks</sub>) and Cl<sup>-</sup> ions.
The plateau phase represents an equal influx and efflux of ions in or out of the cell producing a stable membrane potential. This plateau phase is predominantly observed in the ventricular action potential. The inward movement of Ca<sup>2+</sup> through the open L-type Ca<sup>2+</sup> channels (I<sub>Ca-L</sub>) and the exchange of Na<sup>+</sup> for internal Ca<sup>2+</sup> by the Na<sup>+</sup>/Ca<sup>2+</sup> exchanger (NCX) are responsible for the influx of ions during the plateau phase. The efflux of ions is the result of outward current carried by K<sup>+</sup> (I<sub>Kur</sub> and <sub>Ks</sub>) and Cl<sup>-</sup> ions.


===Phase 3: Final Rapid Repolarization===
===Phase 3: Final Rapid Repolarization===
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===Phase 4: Resting membrane potential===
===Phase 4: Resting membrane potential===
Depending on cell type the resting membrane potential is between -50 to -95 mV, sinus node and AV nodal cells have a higher resting membrane potential (-50 to -60 mV and -60 to -70 respectively) in comparison with atrial and ventricular cardiomyocytes (-80 to -90 mV). The resting membrane potential is mainly caused by differences in the the K<sup>+</sup> outside the cell (5mM) and the K<sup>+</sup> inside the cell (140mM). Sinus node cells and AV nodal cells (and to a lesser degree Purkinje fibers cells) have a special voltage dependent channel I<sub>f</sub>, the funny current. Furthermore they lack I<sub>K1</sub>, a K<sup>+</sup> ion channel that maintains the resting membrane potential in atrial and ventricular tissue. The I<sub>f</sub> channel causes a slow depolarization in diastole, called the phase 4 diastolic depolarization, which results in normal automaticity. The frequency of the sinus node discharges are regulated by the autonomous nerve system and due to the relative high firing frequency (60-80 beats per minute) the sinus node dominates other potential pacemaker sites.
Depending on cell type the resting membrane potential is between -50 to -95 mV, sinus node and AV nodal cells have a higher resting membrane potential (-50 to -60 mV and -60 to -70 respectively) in comparison with atrial and ventricular cardiomyocytes (-80 to -90 mV). The resting membrane potential is mainly caused by differences in the K<sup>+</sup> outside the cell (5mM) and the K<sup>+</sup> inside the cell (140mM). Sinus node cells and AV nodal cells (and to a lesser degree Purkinje fibers cells) have a special voltage dependent channel I<sub>f</sub>, the funny current. Furthermore they lack I<sub>K1</sub>, a K<sup>+</sup> ion channel that maintains the resting membrane potential in atrial and ventricular tissue. The I<sub>f</sub> channel causes a slow depolarization in diastole, called the phase 4 diastolic depolarization, which results in normal automaticity. The frequency of the sinus node discharges are regulated by the autonomous nerve system and due to the relative high firing frequency (60-80 beats per minute) the sinus node dominates other potential pacemaker sites.


==Cardiac conduction==
==Cardiac conduction==
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