Sarcomere is the area between
**Core Concept**
The sarcomere is the functional unit of striated muscle tissue, composed of repeating units of actin and myosin filaments. It is the smallest segment of muscle fiber that contracts and relaxes during muscle movement.
**Why the Correct Answer is Right**
The sarcomere is bounded by two Z-disks, also known as Z-lines, which are structures composed of alpha-actinin that anchor the actin filaments. The sarcomere is the region between these two Z-disks, where the sliding filament theory of muscle contraction occurs. The actin filaments are attached to the Z-disks at their ends, while the myosin filaments are free to slide along the actin filaments, generating force and movement.
**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the sarcomere is not bounded by the M-line, which is a structure that separates the A-bands of adjacent sarcomeres.
* **Option B:** Incorrect because the sarcomere is not bounded by the mitochondria, which are organelles found throughout the muscle fiber, but not specifically at the boundaries of the sarcomere.
* **Option C:** Incorrect because the sarcomere is not bounded by the plasma membrane, which surrounds the entire muscle fiber.
**Clinical Pearl / High-Yield Fact**
Remember that the sarcomere is the smallest unit of muscle contraction, and understanding its structure and function is crucial for diagnosing and treating muscle disorders, such as muscular dystrophy.
**Correct Answer: D. Z-disks**