Striated and voluntary muscle fibres are found in

Function of $ATP$ in muscle fibres is

Select the correct statement with reference to muscle structure.
$I.$ Each myosin is a polymerized protein.
$II.$ Many meromyosin constitute one thick filament (myosin).
$III.$ Each meromyosin's tail is called heavy meromyosin $(HMM)$ and head is called light meromyosin $(LMM)$.
$IV.$ The globular head is an active $ATPase$ enzyme and has binding sites for $ATP$ and active sites for actin.
Choose the option with correct statements.

The contractile protein of skeletal muscle involving ATPase activity is

$A : Ca^{2+}$ plays an important role in muscle contraction.
$R : Ca^{2+}$ combines with the troponin chain,displacing tropomyosin and allowing the myosin head to combine with actin to form an actomyosin complex.

The diagram given below represents the structure of a striated muscle fiber. Label the parts $A, B, C, D, E$ and $F$.