Myofibrils are

Options

A. microfilaments
B. microtubules
C. myoglobin
D. ER

Answer

### Step-by-Step Solution:

1. **Definition of Myofibrils**: Myofibrils are cylindrical structures found in muscle cells (muscle fibers). They are composed of bundles of protein filaments, primarily actin and myosin, which are essential for muscle contraction.

2. **Function of Myofibrils**: The primary function of myofibrils is to facilitate muscle contraction. When myofibrils contract, they shorten the muscle fiber, leading to movement. This is crucial for various bodily functions, including locomotion and the functioning of the heart.

3. **Composition of Myofibrils**: Myofibrils are made up of two main types of proteins:
- **Actin**: A thin filament that plays a key role in muscle contraction.
- **Myosin**: A thick filament that interacts with actin to produce contraction.

4. **Relation to Muscle Fibers**: Myofibrils combine to form muscle fibers. Each muscle fiber contains numerous myofibrils, which work together to enable the muscle to contract effectively.

5. **Importance in Different Muscles**: Myofibrils are particularly important in cardiac muscle (heart) and skeletal muscle, where they enable the muscle to perform its function of contraction and relaxation.

### Conclusion:
Myofibrils are essential components of muscle fibers that play a critical role in muscle contraction and overall movement.

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