Cardiac action potential
The cardiac action potential is a short-lasting event in which the membrane potential (the difference of potential between the interior and the exterior) of a cardiac cell rises and falls following a consistent trajectory, similar to the action potential in other types of cells.The cardiac action potential differs significantly in different portions of the heart. The heart is provided with a special excitatory system and a contractile system necessary to differentiate action potentials in the heart, which allow it to function at a constant rate.This differentiation of the action potentials allows the different electrical characteristics of the different portions of the heart. For instance, the specialized excitatory system of the heart has the special property of spontaneous depolarization. This means the heart depolarizes without any external influence via a slow, positive increase in voltage known as the pacemaker potential across the cell's membrane (the membrane potential) that occurs between the end of one action potential and the beginning of the next action potential. This increase in membrane potential (depolarization) typically allows it to reach the threshold potential at which the next pacemaker potential is fired. Thus, it is the pacemaker potential that drives the self-generated rhythmic firing, known as cardiac muscle automaticity or autorhythmicity.Pacemaker potentials are fired by sinoatrial node (SAN), but also by the other foci. However, the last ones have firing frequencies slower than the SAN's. When other foci attempt to fire at their intrinsic rate, they can not, because they have been discharged by the previous electric impulse coming from the SAN before their pacemaker potential threshold is reached. This is called ""overdrive suppression"". Under certain conditions (if pacemaker cells become compromised) non-pacemaker cells can take over and set the pace of the heart (become pacemakers). Rate dependence of action potential is a fundamental property of cardiac cells. This is important for the QT interval, measured from the beginning of the QRS complex to the end of the T wave. This interval must be corrected for the cardiac rhythm QTc. A prolonged QTc, long QT syndrome, induced by drugs or disease congenital or acquired, increases the possibility of developing severe ventricular arrhythmias and sometimes sudden death.The electrical activity of the specialized excitatory tissues is not apparent on the surface electrocardiogram (ECG). This is due to the relatively small time duration. It is not possible, for example, to see on the ECG the sinus node activity, but the resulting atrial myocardium contraction is apparent as a wave – the P wave. The electrical activity of the conducting system can be seen on the ECG (for example the AV node delay and the so-called PR segment).