STRUCTURE OF A MYOFIBRIL -
(i) Primary Filaments (= Myosin Filaments). The primary filaments are thicker and confined to the A-bands only.
They are free at both the ends. They are composed of myosin (protein) and bear minute projections called cross- bridges of the protein meromyosin.
(ii) Secondary Filaments (= Actin Filaments). The secondary filaments are thinner and occur in I-bands, but extend for some distance into the A-bands between the primary filaments. This partial over lapping of the primary fila- ments by the secondary filaments gives dark appearance to the Abands. The secondary filaments are composed of the proteins actin, tropomysin and troponin. They are attached to the Z-lines by one end and are free at the other end. The secondary filaments are more numerous than the primary filaments. Six secondary filaments surround each primary filament. "