Which of the following is the contractile protein of a muscle?

Provide the full names for the following structures found in muscle fibers:
$(1)$ $Z-$ disc
$(2)$ $H-$ zone

$A$: Muscle tetanus is the phenomenon of sustained contraction of a muscle due to a succession of nerve impulses/stimuli being received by it.
$R$: Many of our daily activities are due to tetanic contraction of muscles,such as holding a book.

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

Analogy type questions:
$(1)$ The portion between two successive $Z$-lines is the functional unit of contraction,which is called the sarcoplasmic reticulum / sarcomere.
$(2)$ The thick filaments in the $A$-band are held together in the middle of this band by a thin fibrous membrane. It is called the $Z$-line / $M$-line.

$A$ motor unit is a