The Cross Bridge Cycle Is A Series Of Molecular Events That Occur After Excitation Of The Sarcolemma. What Is A Cross Bridge?The globular head of a myosin molecule that projects a myosin filament in the muscle and a sliding filament hypothesis of muscle contraction is held to temporarily attach it to the adjacent actin filament and draw it into the A band of the myosin filament sarcomer.
On the thick filaments extending towards the thin filaments in the overlap region, tiny bridgelike structures can be seen at high magnification. They are called cross bridges and are thought to be responsible for the movement and force developed during contraction (see below for the relation of cross bridges to thick filaments molecular architecture). In the middle of the A band, where only thick filaments are present, is a region called the H zone; the H zone looks somewhat lighter than the overlap region of the A band. A narrow, lightly stained region that contains bare thick filaments without cross bridges is also in the A band and is called the pseudo-H zone.
A narrow, darkly stained region called the M band is at the centre of the A band, in which fine bridges occur between the thick filaments. These bridges distinguish between the thick and thin filaments from the cross bridges and are actually made up of a completely different protein.
When cross sections of the myofibril are examined by electron microscope at different levels of the sarcomer, the filaments can be seen end-on, and the three-dimensional nature of the filament lattice can be appreciated. The I band contains only thin filaments, between 6 and 8 nm in diameter. In the A band, in the overlap region, the thin filaments appear in an extremely regular pattern or lattice with thick ones (diameter of 12 nm). The thick filaments are arranged in a hexagonal lattice in vertebrates, and the thin ones are at the centre of the equilateral triangles formed by the thick filaments.
Sections through the H zone only contain thick filaments arranged in the same hexagonal pattern that they form in the area of overlap. The hexagonal array of thick filaments can be seen in the M band with M bridges running in between.