Z lines: muscles, filaments and flesh
What are they? Repeating dark discs in muscle cells. The basic contractile unit of a muscle cell — a sarcomere — is defined as the segment between two Z lines. Sarcomeres are small: a muscle cell may contain 100,000 of them, and Z lines can only be seen under a microscope. How do they work? When a nerve stimulates a muscle, it contracts by shortening its fibres. It does this by constricting each sarcomere so the distance from one Z line to the next decreases. Between the Z lines are thin filaments made of a protein called actin, and thick ones, made of another protein, myosin. When muscles contract, the actin and myosin filaments slide over each other, forcing the Z lines closer together. Why the name? Z stands for Zwischenscheibe, which is German for ‘‘ between disc’’. Sarcomere comes from the Greek word sarx, meaning flesh. A related word is sarcophagus, which literally means flesh-eating.