Drugs affecting the<unk> . The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing efflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels. The<unk>  is a brief change in voltage (membrane potential) across the cell membrane of heart cells. This is caused by the movement of charged atoms (called ions) between the inside and outside of the cell, through proteins called ion channels. The<unk>  differs from action potentials found in other types of electrically excitable cells, such as nerves. Action potentials also vary within the heart; this is due to the presence of different ion channels in different cells (see below). Unlike the action potential in skeletal muscle cells, the<unk>  is not initiated by nervous activity. Instead, it arises from a group of specialized cells, that have automatic action potential generation capability. In healthy hearts, these cells are found in the right atrium and are called the sinoatrial node (SAN; see below for more details). They produce roughly 60–100 action potentials every minute. This action potential passes along the cell membrane causing the cell to contract, therefore the activity of the SAN results in a resting heart rate of roughly 60-100 beats per minute. All<unk>  muscle cells are electrically linked to one another, by structures known as gap junctions (see below) which allow the action potential to pass from one cell to the next. This means that all atrial cells can contract together, and then all ventricular cells. Rate dependence of the action potential is a fundamental property of<unk>  cells and alterations can lead to severe<unk>  diseases including<unk>  arrhythmia and sometimes sudden death. Action potential activity within the heart can be recorded to produce an electrocardiogram (ECG). This is a series of upward and downward spikes (labelled P, Q, R, S and T) that represent the depolarization (voltage becoming more positive) and repolarization (voltage becoming more negative) of the action potential in the atria and ventricles (see electrocardiography for more details).
Cardiac action potential
End-plate potential
Pacemaker current
Compound muscle action potential
Threshold potential
Myocardial contractility
Parafunctional activity
Intermuscular coherence