Basically the sarcoplasmic reticulum releases the calcium and that induces the contraction of our muscle. So around the entire muscle cell, we also have a specialized type of cell membrane. So basically, the surrounding region is our cell membrane that is known as the sarcolema. So this is our entire muscle fiber, it's the muscle cell. And this portion that is covering our muscle cells, shown in red, is our sarcoplasm, or our sarcolema, the cell membrane. Now, if we peel off a bit of this sarcolema, we basically expose our sarcoplasmic reticulum that is shown in green.