Components of myofibrils

The most obvious structures within the muscle fiber are the myofibrils which are the units responsible for contraction and relaxation of the fiber.

Each myofibril consists of two sets of filaments: a thick set of filaments containing the protein myosin and thin set of filaments containing the protein actin. These two sets are arranged within the myofibril.

It is the light and dark bands of the myofibrils that give the muscle fiber its striated appearance. The dark, anisotropic, A-Bands correspond to the presence of myosin (thick) filaments, while the light, isotropic, I-bands contain the actin (thin) filaments.

In the center of the light I-band the line is called Z-line. Actin filaments are attached to these Z lines (or Z disc) and extend in both directions.

The thin filament (I bands) extend longitudinally from Z lines, which the thick filament (A band) span the gaps between the tips of the opposing actin units.

Components of myofibrils