The basic contractile unit within a skeletal muscle fiber is called which?

The basic contractile unit within a skeletal muscle fiber is called which?

• A. Sarcomere
• B. Myofibril
• C. Myofilament
• D. Nucleus

Answer: (A)

A sarcomere is the basic contractile unit because it’s the smallest repeating segment of a myofibril that actually shortens to produce force. It runs from one Z disc to the next and contains interlacing thin (actin) and thick (myosin) filaments. When a contraction is triggered, myosin heads latch onto actin and pull, causing the filaments to slide past each other. This sliding shortens the distance between Z discs, so the sarcomere contracts. A muscle fiber contains many sarcomeres lined up end-to-end in each myofibril, and many myofibrils come together in a fiber, so the entire muscle shortens as its sarcomeres contract in unison. The myofilaments are the actual protein structures that do the pulling, but they operate within the sarcomere, not as the contractile unit itself. The nucleus is just a cell organelle and does not participate in contraction.

A sarcomere is the basic contractile unit because it’s the smallest repeating segment of a myofibril that actually shortens to produce force. It runs from one Z disc to the next and contains interlacing thin (actin) and thick (myosin) filaments. When a contraction is triggered, myosin heads latch onto actin and pull, causing the filaments to slide past each other. This sliding shortens the distance between Z discs, so the sarcomere contracts. A muscle fiber contains many sarcomeres lined up end-to-end in each myofibril, and many myofibrils come together in a fiber, so the entire muscle shortens as its sarcomeres contract in unison. The myofilaments are the actual protein structures that do the pulling, but they operate within the sarcomere, not as the contractile unit itself. The nucleus is just a cell organelle and does not participate in contraction.