So let's remember how the muscle contraction actually takes place. So our nervous system basically generates this action potential and electrical signal that moves into that muscle cell. And once it moves into the muscle cell, what happens is within the membrane of the sarcoplas reticulum, we have these protein channels, not protein pumps, but protein channels. And as a result of the action potential, these protein channels basically open up and then calcium begins to move down its electrochemical gradient. So inside the lumen of the sarcoplasm reticulum. So this side, we have a very high concentration compared to the cytoplasm of the cell.