Which term describes a contractile unit within a myofibril that gives a striated appearance?

Which term describes a contractile unit within a myofibril that gives a striated appearance?

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

Answer: (A)

The contractile unit that shortens to produce muscle movement and gives the striped appearance is the sarcomere. Each myofibril is built from a repeating series of sarcomeres, extending from one Z-disc to the next. The striations come from alternating regions: dark bands (A bands) where thick filaments reside and light bands (I bands) where only thin filaments are present, with zones of overlap at the edges. When a muscle contracts, sarcomeres shorten as actin and myosin slide past each other, pulling the Z-discs closer and shortening the entire myofibril. The other terms describe different things: a myofilament is a single actin or myosin filament, a fascicle is a bundle of muscle fibers, and a myofibril is made up of many sarcomeres but is not the single contractile unit itself.

The contractile unit that shortens to produce muscle movement and gives the striped appearance is the sarcomere. Each myofibril is built from a repeating series of sarcomeres, extending from one Z-disc to the next. The striations come from alternating regions: dark bands (A bands) where thick filaments reside and light bands (I bands) where only thin filaments are present, with zones of overlap at the edges. When a muscle contracts, sarcomeres shorten as actin and myosin slide past each other, pulling the Z-discs closer and shortening the entire myofibril. The other terms describe different things: a myofilament is a single actin or myosin filament, a fascicle is a bundle of muscle fibers, and a myofibril is made up of many sarcomeres but is not the single contractile unit itself.