Now, what's so special about our T tubules? What is the function of these channels known as T tubules? Basically, these tubules extend to our sarcoplasmic reticulum and the T tubules allow for a quick and rampant activation of those sarcoplasmic reticulum. Basically, our T tubules allow for the action potential to actually get inside the cell quickly and efficiently. So once again, we see that the sarcoplasmic reticulum contains a high concentration of calcium that is released during muscle contraction. Now, the plasma membrane that surrounds the muscle cell is called the sarcolema.