Sarcomere, actin filaments, myosin fillaments, sarcoplasmic reticulum.

Answer

### Step-by-Step Text Solution

1. **Understanding Sarcomere**:
- The sarcomere is the basic structural and functional unit of striated muscles (skeletal and cardiac muscles). It is defined as the segment between two Z lines (or Z discs). Each sarcomere contains various components essential for muscle contraction.

2. **Components of Sarcomere**:
- Within a sarcomere, there are several important structures:
- **Z Line**: The boundary of each sarcomere.
- **M Line**: The center of the sarcomere where thick filaments (myosin) are anchored.
- **H Zone**: The region in the center of the A band where only thick filaments are present.
- **A Band**: The area that contains both thick (myosin) and thin (actin) filaments.

3. **Actin Filaments**:
- Actin filaments are thin filaments composed of globular actin molecules (G-actin). They play a crucial role in muscle contraction by providing mechanical support and determining the shape of the muscle cell. Actin filaments are involved in the interaction with myosin filaments during contraction.

4. **Myosin Filaments**:
- Myosin filaments are thick filaments that interact with actin filaments to facilitate muscle contraction. They are referred to as the basic unit of contraction in muscles. The myosin heads bind to actin to form cross-bridges, which is essential for the contraction process.

5. **Sarcoplasmic Reticulum**:
- The sarcoplasmic reticulum is a specialized form of the endoplasmic reticulum found in muscle cells. Unlike the typical endoplasmic reticulum, the sarcoplasmic reticulum lacks ribosomes and is primarily involved in the storage and release of calcium ions (Ca²⁺). Calcium ions are crucial for muscle contraction as they facilitate the formation of cross-bridges between actin and myosin filaments.

### Summary
- The sarcomere is the fundamental unit of striated muscle, consisting of actin (thin) and myosin (thick) filaments, with the sarcoplasmic reticulum playing a key role in calcium storage and release, essential for muscle contraction.