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. So this green portion inside the muscle cell is our sarcoplasmic reticulum. Now, what exactly is so special about the membrane, the plasma membrane of our muscle cell? Well, the sarcolema basically consists of these deep tunnels, these deep in vaginations that tunnel all the way inside our cell.