<P> This phase begins with the rapid inactivation of the Na + channels by the inner gate (inactivation gate), reducing the movement of sodium into the cell . At the same time potassium channels (called I) open and close rapidly, allowing for a brief flow of potassium ions out of the cell, making the membrane potential slightly more negative . This is referred to as a' notch' on the action potential waveform . </P> <P> There is no obvious phase 1 present in pacemaker cells . </P> <P> 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 I, which allow Cl to enter the cell . Together the movement of both Ca 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 . This phase is responsible for the large duration of the action potential and is important in preventing irregular heartbeat (cardiac arrhythmia). </P> <P> There is no plateau phase present in pacemaker action potentials . </P>

The plateau phase of the cardiac potential is caused by
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