The sliding filament theory explains the mechanism of muscle contraction based on the movement and interaction of actin and myosin filaments. Here's a step-by-step explanation:

1. **Resting Phase**: In a relaxed muscle fiber, the myosin (thick) and actin (thin) filaments overlap only slightly.

2. **Action Potential Initiation**: When the muscle cell is stimulated by a nerve, an action potential (electrical signal) is generated which travels along the muscle fiber.

3. **Calcium Ion Release**: This action potential triggers the release of calcium ions from the sarcoplasmic reticulum (a specialized endoplasmic reticulum) into the cytoplasm of the muscle cell.

4. **Cross-Bridge Formation**: The calcium ions bind to a protein called troponin on the actin filaments. This causes the tropomyosin to move, exposing the binding sites for myosin on the actin filament. The myosin heads then attach to these exposed binding sites, forming a cross-bridge.

5. **Power Stroke**: Once attached, the myosin heads pivot, pulling the actin filaments towards the center of the sarcomere (the functional unit of muscle contraction). This is known as the power stroke and it causes the muscle to contract.

6. **ATP Binding and Cross-Bridge Detachment**: An ATP molecule then binds to the myosin head, causing it to detach from the actin filament.

7. **ATP Hydrolysis and Recovery Stroke**: The ATP is hydrolyzed into ADP and phosphate, which provides the energy for the myosin head to return to its original position. The cycle can then repeat, leading to further muscle contraction.

The structure of muscle is crucial for this process. Muscles are composed of bundles of muscle fibers, each of which contains numerous myofibrils. These myofibrils are made up of repeating units called sarcomeres, which contain the actin and myosin filaments. The precise arrangement and interaction of these components allow for the coordinated contraction and relaxation of muscles, enabling movement.

Question

The sliding filament theory explains the mechanism of muscle contraction based on the movement and interaction of actin and myosin filaments. Here's a step-by-step explanation:

1. **Resting Phase**: In a relaxed muscle fiber, the myosin (thick) and actin (thin) filaments overlap only slightly.

2. **Action Potential Initiation**: When the muscle cell is stimulated by a nerve, an action potential (electrical signal) is generated which travels along the muscle fiber.

3. **Calcium Ion Release**: This action potential triggers the release of calcium ions from the sarcoplasmic reticulum (a specialized endoplasmic reticulum) into the cytoplasm of the muscle cell.

4. **Cross-Bridge Formation**: The calcium ions bind to a protein called troponin on the actin filaments. This causes the tropomyosin to move, exposing the binding sites for myosin on the actin filament. The myosin heads then attach to these exposed binding sites, forming a cross-bridge.

5. **Power Stroke**: Once attached, the myosin heads pivot, pulling the actin filaments towards the center of the sarcomere (the functional unit of muscle contraction). This is known as the power stroke and it causes the muscle to contract.

6. **ATP Binding and Cross-Bridge Detachment**: An ATP molecule then binds to the myosin head, causing it to detach from the actin filament.

7. **ATP Hydrolysis and Recovery Stroke**: The ATP is hydrolyzed into ADP and phosphate, which provides the energy for the myosin head to return to its original position. The cycle can then repeat, leading to further muscle contraction.

The structure of muscle is crucial for this process. Muscles are composed of bundles of muscle fibers, each of which contains numerous myofibrils. These myofibrils are made up of repeating units called sarcomeres, which contain the actin and myosin filaments. The precise arrangement and interaction of these components allow for the coordinated contraction and relaxation of muscles, enabling movement.

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