 Dear students, in this topic we shall discuss the role of calcium in contraction. The calcium ions play a crucial role in regulating the contractile activity of the muscle. Calcium Amelitoar ke upar kya karti hai? It helps to expose the active sites of actin filament which bind the myosin heads. As a result, cross bridges are formed which are necessary for contraction. Calcium ions induce contraction with the help of two regulatory proteins which are associated with the actin-thin filament. These proteins are troponin and tropo-myosin. In a relaxed myofibril, tropo-myosin coiles around the actin filaments. During this coiling, these actin-thin-myosin binding sites are physically covered. This causes the actin-thin-myosin to stop interacting. In addition, troponin is attached to every 40 nanometer distance with tropo-myosin. This helps to keep the tropo-myosin in its place. Troponin has high affinity for calcium ions. Every tropo-myosin complex can bind 4 calcium ions with it. When these calcium ions bind with troponin molecules, as a result, troponin undergoes a change in its conformation. This conformational change in troponin causes a shifting in the position of tropo-myosin. When tropo-myosin moves from its place, the actin-thin filaments expose the myosin binding sites. The calcium binding to troponin removes that inhibition that was present for binding of myosin with actin. When this inhibition is over, myosin heads are attached to the actin and cross bridges are formed. When cross bridges are formed, thick and thin filaments slide past each other as a result. The muscle fiber is able to contract. Dear students, for the role of calcium ions, it is necessary that the concentration of calcium ions in the myoplasm of cells is more than 10 raised to the power of minus 7 molar.