sliding filament theory

The Sliding Filament Theory

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It all begins with the Myofibril

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Within the myofibril, there are sarcomeres. The sarcomeres are units that contract. 

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Caption: : There are many sarcomeres with the length of the myofibril

The thick and thin protein filaments are what caused the striated appearance of the skeletal muscle. 

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Caption: : When the filaments slide past each other the muscle is contracting.

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Caption: : The thick filaments are myosin

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Caption: : The thin filaments are actin

Even though the action between the filaments is referred to as "sliding", the myosin filament is actually pulling the actin filament.

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The pulling/sliding of the filaments happens when the crossbridges of the myosin filaments attach to the actin filaments. 

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Caption: :

The muscle contraction begins when the ATP is broken down.

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Caption: : The ATP is broken down when the myosin head reacts with Potassium

While a lot of things have happened, they are all controlled by the actions of calcium.

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Before you can understand calcium's role in muscle contraction, you have to understand the way actin filaments are structured. Actin filaments have the proteins troponin and tropomyosin. When the muscle is relaxed the tropomyosin blocks the binding sites on the actin. This prevents a crossbridge. 

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But, when the calcium levels are high enough and ATP is present, calcium ions bind to the troponin. This causes the tropomyosin to move. 

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Caption: : Here you can see the calcium ions binded to the tropomin

When the tropomyosin moved, the binding site on the actin was exposed. This enabled myosin to attatch to the actin, and form a crossbridge. 

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This is why calcium ions flow to the myofibrils and how they initiate muscle contraction.

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When the actin and myosin slide along each other the entire sacromere shortens. When the sacromere and myofibrils contract the entire muscle shortens. 

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