Considering the ultrastructural properties, the phenomenon of transverse striation is based on regular alternation of pale (isotropic or I) and dark (anisotropic or A) bands. They contain contractile fibrillar proteins (myofilaments) of two types: actin (thin strands) and myosin (thick strands). The contractile filaments are fixed inside myofibrils by a system of specialized proteins that form Z lines and M lines.

Pale (isotropic or I) bands exhibit no birefringence. They consist mainly of thin actin filaments; every pale band is crossed by a fine dark Z line in its center (the Z line is a protein complex for thin filament attachment).

Dark (anisotropic or А) bands exhibit birefringence; they are constituted by thick myosin filaments #1, #2. Thin actin strands enter between them; the distance of their advance depends on contractile force. Every A band contains a pale H band with a dense М line in the center.

Structural units of myofibrils are sarcomeres (S), which are patterns between two Z lines, including an А band and two adjacent halves of neighboring I bands. A contraction involves interaction between molecules and results in mutual sliding of actin and myosin strands (with no length change in any of them). Then, the total length of sarcomeres decreases because of I band constriction.

This is a longitudinal section (39,000X) of a fragment in a myofibril. It includes a Z line (attachment site for thin actin filaments) and an A band where myosin and actin filaments overlap while free ends of myosin filaments form an edge of I band. Between alternating thin and thick filaments, there are transverse junctions ( М-bridges), which are formed by the heads of myosin molecules.

This transverse section of a myofibril passes by the peripheral zone of an A band (60,000X). There is a well-marked hexagonal orientation of thin actin filaments around thick myosin strands #1, #2; some М-bridges may also be seen.