What is the basic contractile unit of a muscle, consisting of actin and myosin filaments?

What is the basic contractile unit of a muscle, consisting of actin and myosin filaments?

• A. Golgi tendon organs
• B. Muscle spindles
• C. Sarcomeres
• D. Tendons

Answer: (C)

The sarcomere is the basic contractile unit of a muscle, a repeating segment of a myofibril that shortens during contraction. It is bounded by Z-lines, with thin actin filaments extending from each Z-line toward the center and thick myosin filaments nestled between them. When a stimulus triggers contraction, calcium ions enable myosin heads to form cross-bridges with actin and pull the filaments past each other using energy from ATP. This sliding filament interaction shortens the sarcomere, and because many sarcomeres align in series within a muscle fiber, the whole muscle shortens. The other structures mentioned—Golgi tendon organs, muscle spindles, and tendons—play sensory or connective roles and do not serve as the contractile unit.

The sarcomere is the basic contractile unit of a muscle, a repeating segment of a myofibril that shortens during contraction. It is bounded by Z-lines, with thin actin filaments extending from each Z-line toward the center and thick myosin filaments nestled between them. When a stimulus triggers contraction, calcium ions enable myosin heads to form cross-bridges with actin and pull the filaments past each other using energy from ATP. This sliding filament interaction shortens the sarcomere, and because many sarcomeres align in series within a muscle fiber, the whole muscle shortens. The other structures mentioned—Golgi tendon organs, muscle spindles, and tendons—play sensory or connective roles and do not serve as the contractile unit.