During muscle contraction

Options

A. Thin myofilaments shorten
B. Thick myofilaments shorten
C. A band reduces in width
D. H zone reduces in width

Answer

### Step-by-Step Solution:

1. **Understanding Muscle Structure**:
- Muscle fibers are made up of units called sarcomeres. Each sarcomere is defined by the area between two Z lines (Z bands).
- The Z line is where the thin filaments (actin) are anchored, while the thick filaments (myosin) are located in the center.

2. **Components of the Sarcomere**:
- The key components of a sarcomere include:
- **Z Line**: The boundary of the sarcomere.
- **M Line**: The center of the sarcomere where thick filaments are anchored.
- **H Zone**: The area in the center of the sarcomere that contains only thick filaments.
- **I Band**: The area that contains only thin filaments, which is located on either side of the Z line.

3. **Process of Muscle Contraction**:
- During muscle contraction, the sarcomere shortens. This does not mean that the filaments themselves shorten, but rather that they slide past each other.
- The thin filaments (actin) slide over the thick filaments (myosin), causing the Z lines to move closer together.

4. **Changes in Sarcomere Dimensions**:
- As the muscle contracts, the distance between the Z lines decreases, resulting in a shorter sarcomere.
- The H zone becomes narrower as the overlapping of thick and thin filaments increases during contraction.

5. **Conclusion**:
- The overall effect of muscle contraction is that the sarcomere shortens due to the sliding filament mechanism, where the thick and thin filaments overlap more, reducing the width of the H zone and bringing the Z lines closer together.