9.3 - Skeletal Muscle Fibers

Each skeletal muscle fiber is a long cylindrical cell with multiple nuclei beneath its plasma membrane.

Sarcolemma - plasma membrane of a muscle fiber.

Muscle fibers are long (up to 30 cm) and are wide (up to ten times a body cell).

Sarcoplasm - cytoplasm of a muscle cell

  1. Sarcoplasm contains lots of glycosomes and myoglobin.

Myoglobin - red pigment that stores oxygen.

glycosomes - granules of stored glycogen. They make glucose during muscle cell activity for ATP production.

Muscle cell contains 3 specialized structures: myofibrils, sarcoplasmic reticulum, and T tubules.

Myofibrils

  • Myofibrils are made up of chains of sarcomeres.
  • Sarcomeres are made up of myofilaments.
  • Striations
    • The striations of myofibrils are made up of dark A bands and light I bands in alignment.
    • Each A band has a lighter midsection called the H zone.
    • Each H zone is bisected by the M line.
    • Each I band has a darker midsection called the Z disk/line
  • Sarcomeres
    • A sarcomere is the region between two successive Z disks, and is the smallest contractile unit of the muscle fiber.
  • Myofilaments.
    • Two types in a sarcomere:
    • Central thick filaments containing myosin (red) extend the length of A band, and are connected at the M line of sarcomere.
    • Lateral thin filaments containing actin (blue) extend across I band and partly into A band. The Z disk anchors these filaments.
    • The H zone of A band is made of thick filaments, the M line is made of thick filaments with accessory proteins, and the I band is made of thin ligaments.
  • Myofilament Molecular Comp.
    • Muscle contraction depends on actin/myosin-containing myofilaments.
    • Myosin
    • Each myosin molecule contains 6 polypeptide chains, in which the 4 heavy ones twist to form its tail. The heavy chains end in a globular head attached to tail via flexible hinge.
    • Those globular heads link thin and thick filaments together during contraction which form cross bridges. They then swivel around their point of attachment to generate force.
    • Myosin itself splits ATP and uses the energy to drive movement.
    • Each thick filament has about 300 myosin molecules, with a smooth central portion and its ends studded with myosin heads.
    • Actin
    • The subunits of actin are globular actin, which have myosin-binding sites for contraction. These G action polymerize into filamentous (or F actin). there are 2 in each thin filament.
    • Tropomyosin stiffens and stabilize the actin cores. When the muscle is relaxed they block the binding sites of actin.
    • Troponin is a globular protein with 3 subunits. One attached troponin to actin, one binds tropomyosin, and one binds calcium ions.
    • Other Proteins
    • Titin, an elastic filament, extends from the Z disk to the thick filament, and then runs within the filament to the M line.
      • It holds thick filaments together, and helps muscle cell spring into shape after stretching. It also helps muscle resist excessive stretching so sarcomeres don’t pull apart.
    • Dystrophin, a structural protein, links filaments to the sarcolemma.
    • Nebulin, myomesin, and C proteins bind filaments and sarcomeres to maintain alignment.

Sarcoplasmic Reticulum

  • Sarcoplasmic reticulum regulates intracellular levels of calcium ions. It releases calcium once muscle fiber contracts.
  • Its tubules run along the myofibril. Its terminal cisterns are larger and are at the A band-I band junctions and appear in pairs.

T Tubules

  • T tubules (T for transverse) is an extension of the sarcolemma that protrudes deeply into the muscle cell at the A band-I band junction. They increase muscle fiber’s surface area.
  • Triads - successful groupings of terminal cistern, T tubule, and terminal cistern.
  • T tubules conducts impulses which trigger calcium release.
  • T tubules and SR links
    • Integral proteins of T tubules act as voltage sensors
    • Integral proteins of SR form gated channels, through which cisterns release Calcium.

Sliding Filament Model of Contraction

  • The sliding filament model of contraction states that during contraction, thin filaments slide past thick ones so the actin and myosin filaments overlap to a greater degree.
  • As a result, the I bands shorten, the distance between Z discs shortens and move towards M line, H zones disappear, and A bands move closer together without change in length.