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

Contraction of a muscle is caused by

Which muscle is responsible for changes in body posture?

Action potential in the sarcolemma of muscles causes the release of:

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.

$ATPase$ enzyme needed for muscle contraction is located in