Which muscle structure is responsible for the contractile unit known as the sarcomere?

The myofibrils are indeed the muscle structures responsible for the contractile unit known as the sarcomere. Within each myofibril, numerous sarcomeres are arranged in series, and each sarcomere is bounded by Z lines. The arrangement of thick (myosin) and thin (actin) filaments within the sarcomere allows for the sliding mechanism that leads to muscle contraction. When a muscle contracts, the myofilaments slide past one another, decreasing the distance between Z lines, which results in shortening of the sarcomere and overall muscle contraction.

Other muscle structures, such as the sarcolemma, sarcoplasmic reticulum, and muscle fibers, play important roles in muscle function but do not constitute the contractile unit itself. The sarcolemma is the membrane surrounding the muscle fiber, while the sarcoplasmic reticulum is involved in calcium storage and release during muscle contraction. Muscle fibers refer to the larger muscle cells that contain many myofibrils. Thus, myofibrils are the specific structures containing sarcomeres that facilitate contraction.