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. And these are known as T tubules or transverse tubules that is shown in orange. So basically, these T tubules go inside the cell and they are perpendicular to our myofiable. So if the myofiables'extend this way, our T tubules go inside the cell perpendicular at a 90 degree angle.