Who propounded the "Sliding filament theory" for muscle contraction?

The functional unit of the contractile system in the striped muscle is

What does $HMM$ stand for in the context of muscle structure?

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.

Cardiac muscle fibers are........

Cardiac muscle fibers differ from skeletal muscle fibers in that they are -