Sarcomere is the functional unit of contraction in a muscle fibre. Indentify the portion of myofibril that constitutes a sarcomere

Options

A. Portion of myofibril between two successive Z-lines
B. Portion of myofibril between two successive I-band
C. Portion of myofibril between two successive A-bands
D. Portion of myofibril between two successive M-lines.

Answer

To identify the portion of myofibril that constitutes a sarcomere, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of Myofibrils**: Myofibrils are long, thread-like structures found in muscle fibers, composed of repeating units called sarcomeres.

2. **Identify the Components of a Sarcomere**: A sarcomere consists of:
- **Myosin Filaments**: Thick filaments located in the center of the sarcomere.
- **Actin Filaments**: Thin filaments that are present on either side of the myosin filaments.

3. **Define the Bands and Zones**:
- **A-band**: The region where myosin filaments are present, which includes the overlapping area with actin.
- **I-band**: The region containing only actin filaments, not overlapping with myosin.
- **H-zone**: The central region of the A-band where there are only myosin filaments (no overlap with actin).
- **M-line**: The center of the H-zone, where myosin filaments are anchored.

4. **Identify the Z-line**: The Z-line (or Z-disc) is a structure that marks the boundary between adjacent sarcomeres. It is located at the center of the I-band.

5. **Define a Sarcomere**: A sarcomere is defined as the segment of myofibril that extends from one Z-line to the next Z-line. This is the functional unit of muscle contraction.

6. **Conclusion**: Therefore, the portion of the myofibril that constitutes a sarcomere is the segment between two successive Z-lines.

### Final Answer:
The portion of myofibril that constitutes a sarcomere is the area between two successive Z-lines.

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