Which of the following ions is necessary for the contraction of muscles and nerve impulse transmission?

The functional unit of the contractile system in the striped muscle is

Select the correct option regarding the components of myosin.

In a resting state,the central part of the thick filaments not overlapped by thin filaments is called the .....

Which of the following statements are false regarding the muscle structure?
$I.$ In the centre of each $I$-band is an elastic fibre ($Z$-line) which bisects it.
$II.$ Thin filaments are firmly attached to the $Z$-line.
$III.$ $M$-line is a fibrous membrane in the middle of $A$-band.
$IV.$ $A$ sarcomere comprises one full $A$-band and two half $I$-bands.

In which condition does relaxation occur in muscles?