In a muscle undergoes rapid contraction and relaxation, the sarcoplasmic reticulum extension

Options

A. requires constant plugging in and out of `Ca^^(2+)`
B. rapidly synthesise myosin
C. does not require energy
D. all of these

Explanation

To solve the question regarding the sarcoplasmic reticulum during rapid muscle contraction and relaxation, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Role of Sarcoplasmic Reticulum (SR)**:
- The sarcoplasmic reticulum is a specialized type of smooth endoplasmic reticulum found in muscle cells. Its primary function is to store calcium ions (Ca²⁺).

2. **Identify the Process of Muscle Contraction**:
- During muscle contraction, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm (the cytoplasm of muscle cells). This release of Ca²⁺ is crucial for muscle contraction.

3. **Identify the Process of Muscle Relaxation**:
- When the muscle relaxes, calcium ions are pumped back from the sarcoplasm into the sarcoplasmic reticulum. This process is essential for the muscle to return to its resting state.

4. **Consider the Implications of Rapid Contraction and Relaxation**:
- In the case of rapid contraction and relaxation, the muscle requires a continuous and efficient mechanism to release and reabsorb calcium ions. This means that there must be a constant "plugging in and out" of Ca²⁺ ions between the sarcoplasmic reticulum and the sarcoplasm.

5. **Evaluate the Options**:
- Option 1: Requires constant plugging in and out of Ca²⁺ - This is true as explained.
- Option 2: Rapid synthesis of myosin - This is not directly related to the rapid contraction and relaxation process.
- Option 3: Does not require energy - This is incorrect because pumping Ca²⁺ back into the SR requires energy (ATP).
- Option 4: All of these - This is incorrect as not all statements are true.

6. **Conclusion**:
- The correct answer is **Option 1: Requires constant plugging in and out of Ca²⁺ ions**.