Talk about the ultrastructure of myofibrils.

They are formed by the assembly of protein myofilaments and are differentiated for contraction. In the myoplasm several hundreds of myofibrils are observed, which constitutes the majority of cell volume. They are cylindrical, parallel, elongated in the cell direction clamped against each other, and extend from one end to another. In longitudinal section, they present a transverse striation due to the alternation of bands (Disks) -Dark, anisotropic (A band) of non-homogenous appearance -Light isotropic (I band) of homogenous, divided in the middle by a Z line, the portion between two Z lines is called the sarcomere.