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. And when this is done, these calcium ions are moved against the electrochemical gradient. And ultimately, what this also does is it reestablishes that electrochemical gradient that is needed for the protein channels to move the calcium back into this direction so that the muscle contraction, the next muscle contraction can actually take place. So we see that calcium is needed for muscle contraction.