Contractile unit of muscle fibres `:-`

Options

A. Muscle fibres distiributed througout the heart walls
B. Sarcomere
C. Myofibril
D. Sarcomere

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Structure**: Muscle fibers are composed of smaller units called myofibrils, which in turn are made up of thick and thin filaments. The thick filaments are primarily made of myosin, while the thin filaments are primarily made of actin.

2. **Identifying Bands and Zones**: Within the myofibrils, there are distinct areas known as bands and zones:
- **A Band**: This is the region where thick filaments (myosin) are present. It appears dark under a microscope.
- **H Zone**: This is the central part of the A band where there are only thick filaments and no overlapping thin filaments.
- **M Line**: This is the center of the H zone, where thick filaments are anchored.
- **I Band**: This area contains only thin filaments (actin) and appears lighter under a microscope.

3. **Defining the Sarcomere**: The sarcomere is defined as the segment of a myofibril that lies between two Z lines (or Z discs). It is the basic structural and functional unit of muscle contraction.

4. **Function of the Sarcomere**: When a muscle contracts, the sarcomeres shorten, which pulls the Z lines closer together, resulting in the contraction of the entire muscle fiber.

5. **Conclusion**: Therefore, the contractile unit of muscle fibers is the sarcomere.

**Final Answer**: The contractile unit of muscle fibers is the **sarcomere**.

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