In a skeletal muscle fibre,the nuclei are located:

$ATPase$ enzyme needed for muscle contraction is located in

The plasma membrane of the muscle fibre is called

The storehouse of calcium ions in the muscle fibre is

$A$: All muscles follow the 'All or None' principle.
$R$: All muscles contract either fully or not at all depending upon the availability of a threshold stimulus.

Name the ions responsible for unmasking the active sites on myosin for cross-bridge formation during muscle contraction.