the plateau phase of the cardiac potential is caused by
Cardiac action potential This phase is also known as the "plateau" phase due to the membrane potential remaining almost constant, as the membrane very, very slowly begins to repolarize. This is due to the near balance of charge moving into and out of the cell. During this phase delayed rectifier potassium channels allow potassium to leave the cell whilst L-type calcium channels (activated by the flow of sodium during phase 0), allow the movement of calcium into the cell. This calcium, binds to and opens more calcium channels (called ryanodine receptors) located on the sarcoplasmic reticulum within the cell, allowing the flow of calcium out of the SR. This calcium is responsible for contraction of the heart. Not only that, but it also activates chloride channels called Ito2, which allow Cl− to enter the cell. Together the movement of both Ca2+ and Cl− oppose the voltage change caused by K+. As well as this the increased calcium concentration increases the activity of the sodium-calcium exchanger, and the increase in sodium entering the cell increases activity of the sodium-potassium pump. The movement of all of these ions results in the membrane potential remaining relatively constant.[17][8] This phase is responsible for the large duration of the action potential and is important in preventing irregular heartbeat (cardiac arrhythmia).