Structures of Muscle Filaments
Each muscle cell (also known as a 'muscle fibre') contains many specialised components described on the page about the structure of a muscle cell. Key functional components within muscle cells include myofibrils, which consist of two types of protein filaments called 'thick filaments', and 'thin filaments'.
These two types of filament have different structures that enable them to work together.
Their structures are shown below:
Above: Diagrams of Muscle Filaments
Thick filaments are formed from a protein called myosin which has important properties of elasticity and contractibility.
The shape of the myosin molecules has the apperance of two 'hockey sticks' or 'golf clubs' twisted together. This is shown in the diagram above, indicating the two parts of the myosin molecule referred advanced textbooks about muscles. These are the myosin tail, and the myosin heads, or 'crossbridges'.
The main component of the thin filaments is a protein called actin. Actin molecules join together forming chains twisted into a helix configuration. These molecules are very important to the contraction mechanism of muscles because each actin molecule has a single 'myosin-binding site' (not shown above).
The molecules of tropomyosin cover the myosin-binding sites on the actin molecules when the muscle fibres are relaxed.
Myosin and actin form the main contractile elements of muscles. This is because it is the binding of the thick filaments to the thin filaments and in particular the positions of these points of attachment, that controls the state of contraction / relaxation of the muscle of which they are apart.
The diagram of a sacromere is repeated below:
Above: Diagram of a Sacromere
As shown above, the extent to which the thick filaments and the thin filaments overlap with each other determines the sizes of the H zone, the I band, and the A band of the sacromere formed by these filaments.
For information about how the muscle filaments cause muscles to contract, read about the sliding-filament mechanism.