Muscle fibres

The muscle cells in skeletal muscle are known as muscle fibres that have many mitochondria and nuclei distributed throughout. Each of the fibres contain multiple myofibrils that are made up of repeating units of the sarcomere. The sarcomere is made up of two sets of filaments; actin (thin) and myosin (thick). There are distinct sections within the sarcomere:

• The A band: The centre section of the sarcomere. It contains the whole length of the myosin filaments including the overlap with the thin filaments
• The I band: Only actin filaments are present
• The M line: In the centre of the thick filaments, holding them together
• The H zone: The centre of the A band that contains only the thick filaments
• The Z line: The end of the sarcomere. It holds the thin filaments together

The myosin filaments have a long tail and a globular head which attaches to actin during contraction. The actin filaments consist of two F actin strands coiled into a double helix. Tropomyosin and troponin are wound around the double helix to give the overall structure of the thin filaments.

The muscle fibres can be arranged differently depending on their function. Sometimes muscle fibres are arranged parallel to their muscle-force axis, this architecture of muscle is called parallel or longitudinally arranged muscle fibres. As well as orientation of muscle fibres they can all be positioned in one direction, unipennate muscles , or slightly different directions, multipennate muscles. Most muscles are arranged as multipennate muscles.