Black bands of myofibrils are known as

$A-$ Actin filaments occur in two forms: the monomeric $G$-actin and the polymeric $F$-actin.
$R-$ Tropomyosin is a rod-shaped fibrous protein. Tropomyosin forms two helical strands,which are wrapped around the $F$-actin filaments.

What is a sarcomere?

Give scientific reasons: During flight,birds do not fatigue easily.

Which of the following statements is correct regarding visceral muscles?

Striated muscles contract by