The region between two successive $Z-$lines in a myofibril is

$A -$ Calcium ions $(Ca^{2+})$ activate the interaction between myosin and actin filaments.
$R -$ The cross-bridge is formed in the presence of $ATP,$ and the breaking of the cross-bridge also requires the binding of a new $ATP$ molecule.

Function of $ATP$ in muscle fibres is

The $H$-zone in the skeletal muscle fibre is due to

Proprioceptors are present in . . . . . . .

In the given diagram,which label represents the $I$-band?