The contractile unit in a skeletal muscle cell is called the sarcomere.
A sarcomere is the region of a myofibril between two Z lines, and it is the basic functional unit of muscle contraction in skeletal and cardiac muscle. Each sarcomere is composed of interdigitating thick (myosin) and thin (actin) filaments that slide past each other during muscle contraction. When a muscle fiber contracts, the sarcomeres shorten, causing the entire muscle to contract and produce movement. Understanding the structure and function of sarcomeres is crucial for comprehending how muscles work in the human body.