Muscle is rich in proteins. Most of these proteins occurs two types of filaments arranged longitudinally in myofibrils. The thick filaments are made up of the protein myosin. Myosin filaments are located inside A bands. Thin filaments are more numerous. They are composed of the protein Actin.

Actin and Myosin

From a fine, dense, dark Z  band at the centre of each I band, actin filaments extend through the I band and encroach between myosin filaments up to a considerable distance into the A band. Each segment of the myofibril from one Z band to the next, functions as a contractile unit and is called  sarcomere.
Actin filaments vs Myosin Filaments
Actin filaments
Myosin Filaments
Found in both A and I bands.
Found only in A band of sarcomere.
Consists of three proteins: actin, tropomyosin and troponin.
Consists of two proteins: myosin and meromyosin.
Thinner (0.005 µm) but shorter (2-2.6 µm) than myosin filaments.
Thicker (0.01 µm) but longer (4.5 µm) than actin filaments.
Cross bridges absent, hence have smooth surface.
Cross bridges present, hence have rough surface.
More numerous than myosin filaments, six of them surround each myosin filament.
Fewer than actin filaments.
Free at one end and are joined to Z line by other end.
Free at both the ends.
Slide into H zone during muscle contraction.
Do not slide during muscle contraction.
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