The fundamental contractile unit is the sarcomere. Each muscle cell apart from the nucleus, mitochondria and other structures contains parallel fibrils. Sarcomere is a subunit of the fibril. It consists of parallel actin and myosin filaments. The thin actin filaments are attached to its limiting membrane or the Z line and interdigitates with  the thicker myosin filaments which are placed centrally. The myosin filaments have flexible heads which can come into contact with the actin filaments. The filaments are propelled past each other by the repeated making and breaking of cross bridges between the actin and myosin filaments. In the resting state when the calcium level is low the myosin binding sites on the actin filament are blocked by tropomyosin which is a regulatory protein. This prevents cross bridge formation. When there is activation and a rise in calcium, the myosin actin binding sites are exposed. The energy required for the cross bridge recycling is provided by adenosine tripshophate (ATP). The number of cross bridges depends on the available calcium molecules. The greater the number of bridges, the more forceful the resulting contraction. During systole there is a 50 fold increase in intracellular calcium concentration.