The available evidence from phase contrast, polarization optical, and electron microscopic studies on vertebrate skeletal muscle, insect skeletal muscle, and dipteran flight muscle is interpreted as favoring the following general structure of striated muscle.
A continuous array of filaments (actin) runs through all bands of the sarcomere. These are linked by an axially periodic system of transverse filamentous bridges. Myosin (and probably other substances) are localized in the A bands. The system of transverse bridges compensates the birefringence of actin and is thus responsible for the isotropy of the I band. Myosin is responsible for the birefringence of the A bands. On strong contraction, A band material migrates to the Z bands to form contraction bands. It is not yet certain whether this migration involves myosin or another A band component.