Actin filament is made up of

Arrange the following steps of muscle contraction in the sequence of events occurring first:
$I.$ Receptor sites on sarcolemma
$II.$ Nerve impulse
$III.$ Release of $Ca^{2+}$
$IV.$ Acetylcholine release
$V.$ Shortening of sarcomere
$VI.$ Synaptic cleft
$VII.$ Spread of impulse over sarcolemma on $T$-tubule
The correct option is:

In a relaxed muscle fibril,the $H$-zone,a lighter region of low density,can be seen in the centre of:

Who propounded the "Sliding filament theory" for muscle contraction?

The sliding filament theory to explain muscular contraction was given by

Identify the bands $P, Q,$ and $R$ in the given diagram.
$\quad\quad\quad P \quad\quad\quad Q\quad\quad\quad R$