A whole skeletal muscle is a hierarchy of wrappings within wrappings. The entire muscle is enclosed by a connective-tissue sheath, the epimysium. Inside it, the muscle is divided into bundles called fascicles, each fascicle wrapped in its own sheath, the perimysium. Each fascicle is in turn made up of many individual muscle fibres (which are single, elongated, multinucleate cells), and each fibre is wrapped by a thin sheath called the endomysium.
The plasma membrane of a muscle fibre is called the sarcolemma, and the cytoplasm inside it is the sarcoplasm. The sarcoplasm is packed with mitochondria (sometimes called sarcosomes, supplying the large amount of ATP contraction needs), many nuclei lying just under the sarcolemma (since the fibre is a fused, syncytial cell), and an extensive internal membrane system, the sarcoplasmic reticulum, which stores and releases the calcium ions needed to trigger contraction. Running the length of the fibre, packed side by side within the sarcoplasm, are hundreds of thread-like myofibrils, and it is these myofibrils that actually contain the contractile machinery.
Each myofibril is not uniform along its length but is built of a repeating unit called the sarcomere, laid end to end — it is the sarcomere that gives skeletal muscle its striated (banded) appearance under the microscope. A sarcomere runs from one Z-line (Z-disc) to the next. Immediately either side of the Z-line is the I-band, a lighter zone containing only thin (actin) filaments anchored to the Z-line. Occupying the centre of the sarcomere is the A-band, a darker zone that marks the full length of the thick (myosin) filaments; toward its edges the thick and thin filaments overlap, but at its very centre lies the H-zone, a narrower, lighter region where only thick filaments are present with no overlapping thin filaments. Running through the middle of the H-zone is the M-line, which anchors the thick filaments in place and keeps them centred. During contraction the I-band and H-zone narrow as the thin filaments are pulled inward, while the A-band's length stays constant — direct evidence that the filaments slide past each other rather than shortening themselves.