Correct Option
The correct option is Sarcomere.
Explanation
Skeletal muscle fibers are composed of parallelly arranged filaments known as myofibrils or myofilaments. These myofibrils exhibit a striated appearance due to the alternate arrangement of two contractile proteins: Actin (thin filament) and Myosin (thick filament).Detailed Analysis:
- Sarcomere (Correct): In a myofibril, the actin filaments are firmly attached to a dense, dark elastic fiber called the 'Z' line (or Z disc). The portion of the myofibril situated between two successive 'Z' lines is defined as the sarcomere. It is considered the functional unit of muscle contraction.
- Sarcolemma (Incorrect): The sarcolemma is the plasma membrane that encloses a muscle fiber. It is analogous to the cell membrane in other cells and surrounds the sarcoplasm (cytoplasm of the muscle fiber).
- Myofibril (Incorrect): Myofibrils are the rod-like units within a muscle cell that contain the contractile filaments. The sarcomere is a segment within a myofibril, not the myofibril itself.
- H zone (Incorrect): The H zone is the central region of the thick filament (myosin) in the 'A' band that is not overlapped by thin filaments (actin) when the muscle is in a resting state.
Key Takeaway:
The sarcomere is the fundamental functional and contractile unit of a muscle fiber, structurally delimited by two adjacent Z-lines.