 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. That is, from one z-disc to the other z-disc, a network of sarcoplasmic reticulum is formed. This sarcoplasmic reticulum is associated with the endoplasmic reticulum which is found in many aspects in other cells. The sarcoplasmic reticulum of muscle cells has a special organization that is extremely important in controlling the muscle contraction. This is the reason why the rapidly contracting muscles in their cells have a very extensive network of sarcoplasmic reticulum. The sarcoplasmic reticulum has two major parts. Number one, the terminal cisternae and number two, the longitudinal tibules. Terminal cisternae are larger chambers which are closely associated with the t-tibules. These two are closely associated with the t-tibules. Terminal cisternae are the primary sites of calcium release. The sarcoplasmic reticulum does not actually do the entire sarcoplasmic reticulum. It is involved in calcium release because this part has special protein channels in it which open. And these channels open with action potential in t-tibules and when they open then calcium release. The second part of the sarcoplasmic reticulum is long longitudinal tibules. They run between the terminal cisternae and surround the myofibrils. There are two side of the terminal cisternae. The network between them is the long longitudinal tibules. And this is the part which surrounds the myofibrils. These longitudinal tibules are the sites where calcium ion absorption occurs because this part has ion channels which are called calcium pumps. These calcium pumps absorb the calcium ions i.e. they are involved in the calcium sequestering. This part of sarcoplasmic reticulum sequesters the calcium ions from the cytoplasm while the terminal cisternae releases the calcium ions during contraction. Dear students, the calcium ion sequestering is the main function of sarcoplasmic reticulum. As we just mentioned, sarcoplasmic reticulum sequesters the calcium ions because of the presence of the calcium pump in the membrane. These pumps actively transport calcium ions from sarcoplasm and concentrate it within the tibules of 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 are binded with a protein which is called calcium sequestering protein. Each molecule of this protein binds about 50 calcium ions with itself. The effect of this binding is that the concentration of free calcium ions is less in the tibules of sarcoplasmic reticulum 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. Number one, to remove calcium ions from the troponin to prevent contraction and to relax the muscle fiber. This is why the calcium ions which are stored in sarcoplasmic reticulum play an important role in the contraction. When an action potential arrives through tibules, it affects the terminal system of sarcoplasmic reticulum. When calcium channels open, they release a large quantity of calcium ions into the cytosol. When calcium ions are released into the cytosol, the concentration of calcium ions increases as a result contraction begins.