<P> During phase 3 (the "rapid repolarization" phase) of the action potential, the L - type Ca channels close, while the slow delayed rectifier (I) K channels remain open as more potassium leak channels open . This ensures a net outward positive current, corresponding to negative change in membrane potential, thus allowing more types of K channels to open . These are primarily the rapid delayed rectifier K channels (I) and the inwardly rectifying K current, I . This net outward, positive current (equal to loss of positive charge from the cell) causes the cell to repolarize . The delayed rectifier K channels close when the membrane potential is restored to about - 85 to - 90 mV, while I remains conducting throughout phase 4, which helps to set the resting membrane potential </P> <P> Ionic pumps as discussed above, like the sodium - calcium exchanger and the sodium - potassium pump restore ion concentrations back to balanced states pre-action potential . This means that the intracellular calcium is pumped out, which was responsible for cardiac myocyte contraction . Once this is lost the contraction stops and myocytic cells relax, which in turn relaxes the heart muscle . </P> <P> During this phase, the action potential fatefully commits to repolarisation . This begins with the closing of the L - type Ca channels, while the K channels (from phase 2) remain open . The main potassium channels involved in repolarization are the delayed rectifiers (I) and (I) as well as the inward rectifier (I). Overall there is a net outward positive current, that produces negative change in membrane potential . The delayed rectifier channels close when the membrane potential is restored to resting potential, whereas the inward rectifier channels and the ion pumps remain active throughout phase 4, resetting the resting ion concentrations . This means that the calcium used for muscle contraction, is pumped out of the cell, resulting in muscle relaxation . </P> <P> In the sinoatrial node, this phase is also due to the closure of the L - type calcium channels, preventing inward flux of Ca and the opening of the rapid delayed rectifier potassium channels (I). </P>

Role of sodium calcium and potassium in contraction of the myocardium