Which of the following changes occurs when a skeletal muscle contracts?

Options

A. A bands shorten
B. I bands shorten
C. Z lines slide farther apart
D. Actin filaments contract

Explanation

To answer the question "Which of the following changes occurs when a skeletal muscle contracts?", we need to understand the process of muscle contraction at the molecular level. Here’s a step-by-step solution:

### Step-by-Step Solution:

1. **Understanding Muscle Structure**: Skeletal muscles are made up of myofibrils, which in turn are composed of sarcomeres. Sarcomeres are the basic contractile units of muscle fibers.

2. **Muscle Contraction Trigger**: Muscle contraction is initiated by the release of calcium ions (Ca²+) from the sarcoplasmic reticulum. These calcium ions bind to a protein complex called troponin.

3. **Role of Troponin and Tropomyosin**: When calcium binds to troponin, it causes a conformational change that moves tropomyosin away from the binding sites on actin filaments, allowing myosin heads to attach to actin.

4. **ATP Binding**: Myosin heads have ATP binding sites. When ATP binds to myosin, it is hydrolyzed to ADP and inorganic phosphate, which energizes the myosin head and moves it to a high-energy state.

5. **Cross-Bridge Formation**: The energized myosin head binds to the exposed binding sites on the actin filament, forming a cross-bridge.

6. **Power Stroke**: The myosin head pivots, pulling the actin filament toward the center of the sarcomere. This movement is known as the power stroke.

7. **Changes in Sarcomere Structure**: During contraction:
- The **I-band** (the region containing only actin filaments) shortens.
- The **A-band** (the length of the myosin filaments) remains the same.
- The **Z-lines** move closer together.

8. **Conclusion**: The specific change that occurs during skeletal muscle contraction is that the **I-band shortens**.

### Final Answer:
The correct answer is that the **I-band shortens** when a skeletal muscle contracts.

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