Now inside the sarcoplasm around each one of the myofibrils we have a specialized type of endoplasmic reticulum known as the sarcoplasmic reticulum. Now what's so special about the sarcoplasmic reticulum is that it contains a high concentration of calcium. And calcium is involved in the contraction of muscle, as we'll see in the next lecture. 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.