What is sarcoplasmic reticulum? What is its function?

Explanation

### Step-by-Step Solution

1. **Definition of Sarcoplasmic Reticulum**:
- The sarcoplasmic reticulum (SR) is a specialized type of endoplasmic reticulum found in muscle cells. It is a closed membrane structure that surrounds the myofibrils, which are the contractile elements of the muscle.

2. **Function of Sarcoplasmic Reticulum**:
- The primary function of the sarcoplasmic reticulum is to store calcium ions (Ca²⁺). Calcium is crucial for muscle contraction.
- When a muscle receives a stimulus (such as a nerve impulse), the sarcoplasmic reticulum releases the stored calcium ions into the cytoplasm of the muscle cell.
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