Differentiate between Red muscles and White muscles.

The contraction of muscle of shortest duration is seen in

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.

According to the sliding filament theory:

At which site do muscle fibers receive stimulation from a motor neuron?

In the center of each $I$-band is an elastic fiber called the $......$ which bisects it.