To answer the question "Which one of the following takes place in a myofibril when skeletal muscle contracts?", we need to analyze the changes that occur in the sarcomere, the functional unit of a myofibril, during muscle contraction.
### Step-by-Step Solution:
1. **Understanding Sarcomere Structure**:
- The sarcomere consists of alternating light (I band) and dark (A band) bands. The I band is the light band, while the A band is the dark band. The H zone is the central part of the A band where there are no thin filaments (actin) when the muscle is relaxed.
2. **Muscle Contraction Process**:
- During muscle contraction, the myosin (thick filaments) pull the actin (thin filaments) toward the center of the sarcomere. This sliding filament theory explains how muscle fibers shorten.
3. **Changes in Bands During Contraction**:
- When the muscle contracts:
- The **I band** (light band) becomes shorter as the actin filaments are pulled inward.
- The **A band** (dark band) remains unchanged in length because it represents the length of the myosin filaments, which do not change during contraction.
- The **H zone** (the area within the A band that lacks actin) disappears as the actin filaments overlap with the myosin filaments.
4. **Identifying the Correct Option**:
- Given the options:
- Option 1: Light band shortened, dark band shortened, H zone shortened (Incorrect)
- Option 2: Light band shortened, dark band unchanged, H zone unchanged (Incorrect)
- Option 3: Light band shortened, dark band unchanged, H zone disappears (Correct)
- Option 4: Light band unchanged, dark band shortened, H zone shortened (Incorrect)
- The correct answer is **Option 3**: Light band shortened, dark band unchanged, and H zone disappears.
### Conclusion:
The correct answer to the question is **Option 3**: When skeletal muscle contracts, the light band shortens, the dark band remains unchanged, and the H zone disappears.
