 Dear students, in this topic, we shall discuss the sarcoplasmic reticulum and its role in muscle contraction. The sarcoplasmic reticulum is a network of membrane-bound tibules which extend throughout the muscle cell. This network extends on either side of the z-disc and extends from one z-disc to the other. The sarcoplasmic reticulum is the endoplasmic reticulum which is found in the other cells. The sarcoplasmic reticulum of muscle cells has a special organization that is extremely important in controlling the muscle contraction. The sarcoplasmic reticulum of muscle cells is extremely important in controlling the muscle contraction. The sarcoplasmic reticulum of muscle cells is two major parts of the sarcoplasmic reticulum. And number two, the longitudinal tubules. Terminal cisterns are larger chambers which are closely associated with the t-tubules. Terminal cisterns are the primary sites of calcium release, because this part has special protein channels in it which open and these channels open with action potential in t-tubules and when they open then the calcium release occurs. The other part of the sarcoplasmic reticulum is the long longitudinal tubules which run between the terminal cisterns and surround the myofibrils. These longitudinal tubules are the sites where calcium ion absorption occurs because this part has ion channels which are called calcium pumps. The calcium pumps absorb calcium ions i.e. they are involved in the calcium sequestering. The sarcoplasmic reticulum of the sarcoplasmic reticulum sequesters the calcium ions from the cytoplasm while the terminal cisterns release the calcium ions during contraction. Dear students, the calcium ion sequestering is the main function of the sarcoplasmic reticulum. Like we just said, the sarcoplasmic reticulum sequesters the calcium ions because of the presence of the calcium pump in the membrane. These pumps actively transport the calcium ions from the sarcoplasmic reticulum and concentrate it within the tubules of the sarcoplasmic reticulum. Now, this sarcoplasmic reticulum contains the calcium ions which are stored not in free form rather they are stored in bound form and for binding these calcium ions bind with a protein which is called calci-questin protein. Each molecule of this protein binds with about 50 calcium ions. The effect of this binding is that the concentration of free calcium ions in the tubules of the sarcoplasmic reticulum is less and as a result it can sequester more and more calcium ions from the cytoplasm. Dear students, the calcium ion sequestering is important because due to this activity of sarcoplasmic reticulum the cytosolic concentration of calcium ions remains below 10 raised to power minus 7 molar. This concentration is actually required for three purposes. 1. To remove calcium ions from the troponin to prevent contraction and to relax the muscle fiber. 2. These calcium ions which are stored in the sarcoplasmic reticulum also play an important role in making this contraction. 3. When an action potential arrives through the tubules then it affects the sarcoplasmic reticulum's terminal cistern. 4. When calcium channels open or when calcium channels open then they release a large quantity of calcium ions into the cytosol. 5. When calcium ions are released into the cytosol the concentration of calcium ions increases as a result contraction begins.