The heart relies on a series of electrical currents to beat, which are regulated by calcium, potassium, and sodium ions. Myocardial action potential refers to the membranes of cardiac cells undergoing a process called depolarization, when negatively charged ions inside of a cell travel out through the cell membrane and positive ions move in. Certain ion channels that let substances pass into and between cells can open and close. Once a cell is depolarized, a threshold is reached that typically opens channels for sodium ions, creating a positive charge inside the cell. In contrast, the inside of a cell has a negative charge while there is a resting potential, which is caused by an outward flow of potassium when the associated channels are opened.
Myocardial action potential does not only occur between one cell and another, but across the heart as a whole. Depolarization can occur throughout regions surrounding specific cells. A continuous electrical signal can be produced along muscle fibers that extend across the heart. Whole fibers can be depolarized at once and then trigger the same effect on others, which typically occurs in a wave-like effect.
There are five phases to myocardial action potential. When a cell is at rest and in a depolarizing state, it is often said to be at phase zero. Sodium enters cells until a certain voltage is reached, and calcium also starts to flow. During phase one, the sodium current stops which generally causes a re-polarizing of the cell. Calcium continues to flow during phase two, which counteracts the loss of potassium as the voltage remains continuous.
Phase three is characterized by a stop in calcium flow, but the potassium current increases until the heart cell goes into a resting state. The sodium and potassium levels are continuously regulated. A cell remains at rest during phase four until triggered by signals from other cells, or in some cases spontaneously.
Myocardial cells contract in a matter of milliseconds. In between, refractory periods can be classified as absolute, which is when sodium and calcium channels stay open. Relative refractory periods are when potassium currents are sufficient for triggering the rest state. The communication between heart cells, even with myocardial action potential, occurs in pulses similar to nerve impulses between neurons.
A network of nerves and nodes runs through the heart, which includes the sinoatrial node that acts as a pacemaker. Heart muscles can sometimes depolarize without any signal from the general nervous system. The sinoatrial node is often the starting point for such reactions. Various proteins in the nervous system can also trigger signals that affect myocardial action potential.