Myosin head separates from actin when:

Define the sliding filament theory of muscle contraction.

Select the correct option for skeletal muscle.
$(I)$ In human body,it is made of a number of muscle bundles.
$(II)$ Muscle bundles contain a number of muscle fibers and each fiber is lined by the sarcolemma.
$(III)$ In a muscle fiber,the sarcoplasm contains only one nucleus.
$(IV)$ In a muscle fiber,the sarcoplasmic reticulum is the storehouse of calcium ions.

Arrange the following steps of muscle relaxation in the correct chronological sequence:
$I.$ $Ca^{2+}$ ions are pumped back to the sarcoplasmic reticulum.
$II.$ $Z$-lines return to their original position.
$III.$ Masking of actin filaments occurs.
$IV.$ Relaxation of muscles.

The special contractile protein actin is found in

In striated muscle contraction: