Structure of skeletal muscle

A skeletal muscle is made of many bundles, or fascicles, held together by a common layer of collagenous connective tissue called fascia; each bundle contains many muscle fibres.

Each fibre is covered by a plasma membrane, the sarcolemma, around the sarcoplasm. The fibre is a syncytium, because its sarcoplasm holds many nuclei.

The sarcoplasmic reticulum, the fibre's endoplasmic reticulum, stores calcium ions.

The sarcoplasm is packed with many parallel myofibrils (myofilaments), each showing alternate dark and light bands. The stripes come from the arrangement of two proteins, actin and myosin.

The light I band (isotropic) holds actin, the thin filaments; the dark A band (anisotropic) holds myosin, the thick filaments. Both run parallel to each other and to the fibre's long axis.

An elastic Z line bisects each I band, and the thin filaments are anchored to it. A thin fibrous M line holds the thick filaments together at the middle of the A band.

The stretch of myofibril between two successive Z lines is the sarcomere, the functional unit of contraction.

At rest the ends of the thin filaments partly overlap the free ends of the thick filaments; the central part of the thick filaments left without overlap is the H zone.