We have many of these red regions which are aromiofibels. Now notice the cytoplasm has its own name. The cytoplasm of the muscle fiber is known as arosarco plasma. 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.