What are $P, Q,$ and $R$ in the given diagram?
$P \quad\quad Q \quad \quad R$

Black bands of myofibrils are known as

The set of ions necessary for muscle contraction is

Cardiac muscle fibers are........

Action potential in the sarcolemma of muscles causes the release 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: