Cardiac Arrhythmias: Difference between revisions

During phase 4 of the action potential intracelullair and extracellulair concentrations of ions are restored. 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). 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.