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. And so when the protein channels open up and these protein channels are not shown in this diagram, when they open up, the calcium begins to move naturally from a high to a low concentration. And what that means is once the calcium moves into that cytoplasm, the calcium can then react with the actin filaments found within our muscle cells that exposes special type of mice and binding sites and then coupling with ATP, that muscle contraction actually takes place. Now, to relax the muscle, what has to happen is these protein pumps must be able to very quickly and effectively take these calcium and move them back into the inside, into the lumen of the sarco plasma ticulum.