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Anatomy & Physiology

Background:
This page is a continuation of the page about the structure of a muscle cell - which is a continuation of the more general page about the structure of muscle.

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 then to work together.
The structures are illustrated below:

Thick 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 illustrated in the diagram above - indicating the two parts of the myosin molecule referred to in Advanced Textbooks about Muscles
These are the myosin tail, and the myosin heads, or "crossbridges" .

Thin Filaments
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 illustrated above).
The other two protein molecules that form the thin filaments are called troponin and tropomyosin.
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.

Recall (from the previous page) that the thick filaments and the thin filaments together form units called sacromeres.
The diagram of a sacromere is repeated below:

The extent to which the thick filaments and the thin filaments overlap 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.