Final answer:
The sarcomere is the functional contractile unit of the myofibril where muscle tension is produced, composed of Z-discs, actin, and myosin filaments that slide to cause contraction.
Step-by-step explanation:
The functional contractile unit of the myofibril where muscle tension is produced is called the sarcomere. Sarcomeres are the structural building blocks of a muscle and are responsible for its ability to contract. Each sarcomere is bordered by Z-discs, to which the actin filaments are anchored. These sarcomeres are organized within myofibrils, which run the length of the muscle fiber and attach to the sarcolemma at each end. Muscle contractions occur when myosin heads form cross-bridges with actin filaments, resulting in the sliding of thin filaments over thick filaments, thereby shortening the sarcomere and generating tension. This process is regulated by calcium ions and ATP and is fundamental to muscle movement and stability.