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

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.