Potassium channels are chemically regulated by the presence of free calcium ions in the cytoplasm. When present, these ions bind to potassium channels causing them to open (separated oval pair). The efflux of potassium ions along their electrochemical gradient ( block arrow) prevents an accumulation of positive charge due to the calcium influx. This is reflected in section C in the action potential; it is called the plateau. This cell contracts during this period of high calcium levels.
The calcium channels close after a few tenths of a second (not shown here). Calcium ions are removed from the cytoplasm by the action of calcium pumps located throughout the cell membrane (ATP circle with twisted arrow). Because free and bound calcium ions are equilibrated (double-headed dashed arrow) the loss of calcium ions from the cytoplasm also causes the potassium channels to assume their closed state (dark arrow).
Most of the cytoplasmic calcium is removed by similar pumps located throughout the membrane of the sarcoplasmic reticulum (not shown here). The overall result is reflected in section D of the action potential. This loss of positive charge is called repolarization. The resting membrane potential (RMP) is re-estalished when all gated channels are closed and the exchange of sodium and potassium ions are equal.