The sliding filament theory

To understand how a muscle contracts it is necessary to analyse the molecular event that occur in each single muscle fiber.

Viewed under the microscope a skeletal muscle fibre shows striations.

These straitions result from a specif arrangements of proteins inside the myofibre. The two pricipal type of proteins are myosin (forming the thick filament) and actin (forming thin filaments)

The diagram below shos the structure of a sarcomere: a functional unit along the muscle fiber.

The sarcomere is the distance between two Z lines which are formed by the joining points of the thin filaments. The lighter I band represents areas wher the thin filaments are present while the H band is where both actin and myosin run parallel to each other.

The diagram below describe the arrangements of actin and myosin. Please note that actin filaments are made of chains of globular actin molecules. Myosin chins instead have characteristic globular heads and long tails.

At rest actin and myosin are prevented from contacting each other by two other proteins: tropomyosin and the Ca++ binding protein troponin. Upon stimulation, Ca++ is released from internal stores and binds to troponin which induces a conformational change of tropomyosin and allows actin-myosin interaction.

The myosin head can bind hydrolise ATP (loss of one phosphate)to ADP. This gives energy to the myosin head to bind to actin and to bend pushing the acting filaments along. ADP is then resubstituted with ATP and actin and myosin come apart. Hydrolisis of ATP will start another binding and sliding cycle.pushing the actin filament along and resulting in contraction of the cell

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