Cardiac Arrhythmias: Difference between revisions

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 Ca²⁺ through the open L-type Ca²⁺ channels (I_Ca-L) and the exchange of Na⁺ for internal Ca²⁺ by the Na⁺/Ca²⁺ 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⁺ (I_Kur and _Ks) and Cl^- ions.

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⁺ outside the cell (5mM) and the K⁺ 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_f, the funny current. Furthermore they lack I_K1, a K⁺ ion channel that maintains the resting membrane potential in atrial and ventricular tissue. The I_f 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.