What roles do contractile, regulatory, and structural proteins play in muscle contraction and relaxation?

Muscle fibers or myofibrils are formed of three types of
proteins which are (1) contractile, (2) regulatory, and (3)
structural.

Contractile proteins are the force generators
of muscle contraction. The two contractile proteins in myofibrils are actin which is
part of thin filament and myosin, which is part of the thick
filament.

The regulatory proteins troponin and tropomysium,
which are a part of the thin filament, are involved in starting or stopping muscle
contraction.

When the muscle is relaxed, tropomysium blocks
the myosin-binding sites on the actin proteins; this prevents the muscle from
contracting. Troponin holds the tropomysium proteins in place. This is changed when
calcium enters the muscle fibers; it binds to the troponin molecule, and moves the
tropomysium away from the myosin-binding site on the actin
molecule.

The structural proteins are involved in the
stability and elasticity of the myofibrils. The most important structural protein is
titin. It spans half of the muscle fiber. The titin protein is very elastic and serves
to help a stretched or contracted muscle return to its relaxed
length.