ATPases use ATP to pump calcium back into the lumen, thereby reestablishing that gradient. And once this process takes place, we essentially establish a gradient in which in the cytoplasm, we have a much smaller concentration than in the lumen. And so we have about 0.1 millimolar in the cytoplasm and about 1.5 millimolar of calcium in the lumen. And clearly, this is a much higher concentration than this. So the pump uses ATP to establish this gradient. And then when an action potential actually comes into that muscle cell, the protein channels not shown in this diagram open up.