In a skeletal muscle fiber,what is the $H$-zone?

Choose the correct properties of muscle fibres.
$I.$ Muscle fibre is lined by the plasma membrane called sarcolemma.
$II.$ Cytoplasm of the muscle fibre is called tonoplasm.
$III.$ Sarcolemma of the muscle fibre encloses the sarcoplasm.
$IV.$ Muscle fibre is a syncytium.
Select the correct option.

'Gastrocnemius' is a muscle of

Which ion is essential for muscle contraction?

Relaxation of the muscle takes place due to
$I.$ Pumping of $Ca^{2+}$ ions into the sarcoplasmic reticulum
$II.$ Presence of $ATP$
$III.$ Conformational changes in troponin and masking of the actin filament
Which of the following options contains the correct statements?

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