Which of the following is the contractile protein of a muscle?

Options

A. Actin
B. Myosin
C. Trpoponin
D. tropomyosin

Answer

To determine the contractile protein of a muscle, we need to understand the structure of muscle fibers and the proteins involved in muscle contraction.

### Step-by-Step Solution:

1. **Understand Muscle Structure**:
- Muscles are made up of myofibrils, which are the basic structural units of muscle fibers. These myofibrils are arranged parallel to each other and are striated.

2. **Identify the Components of Myofibrils**:
- Myofibrils contain thick and thin filaments. The thick filaments are primarily made up of the protein myosin, while the thin filaments are primarily made up of the protein actin.

3. **Differentiate Between Myosin and Actin**:
- Myosin is the major contractile protein found in the thick filaments and is responsible for muscle contraction. It is darker in color.
- Actin, on the other hand, is found in the thin filaments and is lighter in color. While it plays a crucial role in muscle contraction, it is not the primary contractile protein.

4. **Consider Other Proteins**:
- Tropinin and tropomyosin are associated with actin and play roles in muscle contraction regulation, but they are not the main contractile proteins.

5. **Conclusion**:
- Based on the above information, the contractile protein of a muscle is **myosin**.

### Final Answer:
The contractile protein of a muscle is **myosin**.

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