Remember, the cytoplasm of our muscle cell is also known as our sarcoplasm. So inside our sarcoplasm reticulum, we have a high concentration of calcium. On the outside, in the cytosol, we have a low concentration. So these calcium ions will travel down their electrochemical gradient end into our cytosol. Now, inside the muscle cell, we have many, many of these myofibrils. And what these myofibrils are, are basically our sarcomeres connected end to end.