In a muscle fibre `Ca^(++)` is stored in __________.

Answer

**Step-by-Step Solution:**

1. **Understanding the Role of Calcium Ions in Muscle Contraction:**
- Calcium ions (Ca²⁺) play a crucial role in muscle contraction. They are essential for the activation of the proteins involved in the contraction process.

2. **Identifying the Key Proteins:**
- The primary proteins that calcium ions activate during muscle contraction are troponin and tropomyosin. These proteins regulate the interaction between actin and myosin, which are the contractile proteins in muscle fibers.

3. **Locating Calcium Ion Storage in Muscle Fibers:**
- In muscle fibers, calcium ions are stored in a specialized form of endoplasmic reticulum known as the sarcoplasmic reticulum. This structure is specifically adapted for the storage and release of calcium ions.

4. **Understanding the Release Mechanism:**
- When a muscle fiber is stimulated to contract, a series of biochemical reactions occur that lead to the release of calcium ions from the sarcoplasmic reticulum. This release is what triggers the muscle contraction process.

5. **Final Answer:**
- Therefore, the calcium ions in a muscle fiber are stored in the **sarcoplasmic reticulum**.

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