What is the sliding filament theory and how is tropomyosin involved?

The sliding filament model shows the overlapping myosin and actin fibres that are found within a sarcomere. When a muscle contracts, the actin and myosin form cross-bridges, increasing the amount of overlap, and make the sarcomere shorter. These cross-bridges can only form when the cross bridge binding sites are exposed on the actin fibre. Tropomyosin is a protein that covers the cross-bridge binding sites on the actin fibre, preventing contraction.

The sliding filament theory is the mechanism by which muscles contract. When a muscle is stimulated by a motor neuron there is a release of CA^2+ ions from the sarcoplasmic reticulum. The Ca^2+ ions then bind to the troponin molecules which are attached to the tropomyosin filament on the actin filament. This binding triggers a conformational change in tropomyosin revealing myosin binding sites on the actin filament. Myosin is then able to bind to the actin filament and can 'nod' along the filament using ATP-hydrolysis. This movement causes the Z line to come closer together causing a muscle contraction.

The theory is about how muscle contracts.

The process of muscle contraction is as below:

1) action potential stimulates muscle cells and depolarises sarcolemma and sarcoplasmic reticulum through T-

tubules;

2) sarcoplasmic reticulum releases calcium ions into sarcoplasm;

3) calcium ions bind to troponin to change its shape to pull tropomyosin out of actin myosin binding site;

4) binding site is exposed for myosin head to bind to form actin-myosin cross bridge;

5) calcium ions activate ATPase to break down ATP into ADP and Pi to provide energy for muscle contraction;

6) energy is used to move myosin head which pulls actin filament along rowing action;

7) myosin head detaches from actin filament and reattach to different binding site further along actin filament.

Tropomyosin is involved in muscle contraction.

1) tropomyosin at first blocks myosin binding site;

2) tropomyosin is pulled by troponin out of myosin binding site, when calcium ions bind to troponin to change its

shape;

2) binding site is exposed for myosin head to bind to form actin-myosin cross bridge;

3) myosin head pulls actin filament along rowing action;

4) myosin head detaches from actin filament and reattach to different binding site further along actin filament.

The sliding filament theory describes the mechanism that allows muscles to contract. According to this theory, myosin (a motor protein) binds to actin. The myosin then alters its configuration, resulting in a "stroke" that pulls on the actin filament and causes it to slide across the myosin filament. Calcium is required by two proteins, troponin and tropomyosin, that regulate muscle contraction by blocking the binding of myosin to filamentous actin. In a resting sarcomere, tropomyosin blocks the binding of myosin to actin.