Each myofibril of a muscle contains:

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

Select the correct option for the following statements:
$(i)$ The most important respiratory organ in frogs is the skin / lungs.
(ii) Skeletal muscle / Cardiac muscle exhibits rhythmic contraction.

Contraction of muscle fibre takes place by the sliding of $:-$

Black bands of myofibrils are known as

During muscle contraction in a skeletal muscle fibre,$Ca^{2+}$ combines with