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

In the mechanism of muscle contraction or shortening of muscle,the . . . . . . get reduced whereas the . . . . . . retain the length.

Visceral muscles are involved in which functions?

Which protein is present in muscles?

$A$: Application of stimuli repeatedly just at the start of relaxation of a muscle fibre shows an increase in the extent of contraction initially and results in treppe or staircase phenomenon.
$R$: It can be attributed to the summation effect of the sub-threshold stimuli being given again and again.

Which of the following statements about the molecular arrangement of actin and myosin are correct?
$I.$ Each actin (thin filament) is made up of $2$ $F$ (filamentous) actins.
$II.$ $F$-actin is the polymer of $G$ (globular) actin.
$III.$ $2$ $F$-actins are twisted into a helix.
$IV.$ Two strands of tropomyosin (protein) lie in the grooves of $F$-actin.
The correct option is: