Skeletal muscles are composed of many individual cells known as muscle fibers. Each individual muscle fiber is in turn composed of a number of smaller myofibrils. Three types of proteins form myofibrils: contractile, regulatory, and structural.
Contractile proteins generate force for muscle contraction. The two contractile proteins in myofibrils are actin (part of the thin filament) and myosin (part of the thick filament). The regulatory proteins troponin and tropomysium, which are also part of the thin filament, are involved in allowing or inhibiting muscle contraction. The structural proteins (the most notable of which is titin) are involved in the stability and elasticity of the myofibrils.
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.