Sarcoplasmic reticulum is present in which structure:

The structural and functional unit of the striated muscle fibre is called

'Oxygen debt' is the amount of oxygen required for

Where are $ATPase$ enzymes located in a muscle fiber?

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.

$M-$line passes through the centre of