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Sliding Filament Theory

Sliding filament theory describes the mechanism of muscle contraction. The steps involved are described below (figure 1).

Diagram illustrating the steps in sliding filament theory. The image is divided into four scenes. The thick filament is depicted as a helix, called tropomyosin, bound to some spheres, the actin, also shaping a helix. There are other types of spheres located at each intersection of the helix, called troponin. There is calcium depicted as smaller spheres. The thin filament is depicted as stripes bound together, with a circular structure reaching out, called the myosin head. In the first scene, the myosin head is approaching the actin and a molecule of ADP, and a molecule of phosphate are close to it. In the second scene, a cross-bridge is formed between the myosin head and the actin. The phosphate molecule is released. In the third scene, the myosin head bends, dragging and sliding the thick filament and creating a power stroke. The ADP molecule is released. In the fourth scene, a new ATP molecule binds to the myosin head and causes the separation between it and the actin.

Figure 1: Sliding Filament Theory

1: When a nerve signal reaches the muscle cell, calcium is released from the sarcoplasmic reticulum surrounding the myofibrils. Calcium causes a conformational change in the tropomyosin molecule which shift in position to expose the binding sites (dark green) of the actin proteins.

2: The myosin heads bind to the binding sites of the actin proteins, to form a cross-bridge as the inorganic phosphate is released.

3: ADP is released which causes initiation of the power stroke, where the thin filament gets pulled closer toward the midline of a sarcomere.

4: A new ATP molecule binds to the myosin head causing the separation of the actin-myosin cross-bridge. The ATP is subsequently hydrolyzed to ADP and inorganic Phosphate (step 1) and the energy transferred from the ATP to the myosin head causes it to "cock" back like the trigger of a gun.

This contraction cycle will continue until the nerve signal stops, and calcium is reabsorbed back into the sarcoplasmic reticulum, which causes the tropomyosin molecules to cover the actin binding sites, stopping the myosin to form new cross-bridges.

References: OpenStax College, Biology. (OpenStax CNX. Mar 13, 2015)