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Structure of Skeletal Muscle



Gross Structure of the Skeletal Muscle

  • Skeletal muscle is made up of muscle fascicles.

  • Muscle fascicles are made up of muscle cells.

  • Muscle cells:

    • are also called muscle fiber or myofiber.

    • are the contractile unit of the muscle.

    • are elongated and most of the muscle fibers extend the entire length of the muscle.

Muscle Fiber (Muscle Cell)

  • contains the following structures:

Myofibrils

  • are cylindrical.

  • arranged parallel to the length of the muscle fiber.

  • Each muscle fiber contains 100s to 1000s of myofibrils.

  • Each myofibril is a chain of regularly repeating units called a sarcomere.

  • Adjacent sarcomeres are joined at Z disks.

  • Each sarcomere consists of myofilaments.

Myofilaments

  • are responsible for muscle contraction.

  • are of two types: thin filaments and thick filaments.

Thin Filaments are made up of:

  • Actin

    • Each actin molecule has a myosin-binding site.

    • Many actin molecules polymerize to form a double-stranded helix called filamentous actin (F-actin).

    • F-actin is attached to the Z disk at one end. And it makes the backbone of thin filament

  • Tropomyosin

    • is a threadlike structure.

    • Two tropomyosin molecules wind around actin helix.

    • Cover the myosin binding site on the actin molecules.

  • Troponin

    • Attaches tropomyosin to actin.

    • Made up of 3 subunits:

      • Troponin C: Binds with calcium

      • Troponin T: Binds with Tropomyosin

      • Troponin I: Binds with actin → inhibits contraction.

    • There are many such troponin molecules on one thin filament.

Thick Filaments:

  • Lie in between and partially interdigitate with thin filaments.

  • Structurally they are assembly of hundreds of myosin II molecules.

  • Myosin II Molecule is made up of two identical heterotrimers.

  • Each trimer is made up of:

    • One myosin heavy chain, and

    • Two light chains, namely:

      • Essential light chain

      • Regulatory light chain

  • Myosin heavy chain has:

    • Head: Head has binding site for actin at tip, and binding site for ATP at middle.

    • Neck: Both light chains are bound to myosin heavy chain at neck region.

    • Rod: Here heavy chains of both trimer wind around each other

  • Light Chains:

    • Mechanically stabilizes the neck region of myosin.

    • Play regulatory role in cross-bridge interaction.

  • At the middle of thick filament, myosin molecules on both sides are cross-linked. This midline is called M line. There are no heads near this central region of thick filament.

  • Myosin is responsible for force generation.

  • Each thick filament is surrounded by 6 thin filaments.

Bands

  • Partial interdigitation of thick and thin filaments produces light and dark bands alternately.

Light Band (I band)

  • is the region of thin filaments which does not overlap with thick filaments.

  • During contraction, thick filament overlaps with more and more portion of thin filament → this band shortens during contraction.

  • Z disk appears as a dark perpendicular line at the center of this band.

Dark Band (A Band)

  • is region of thick filaments.

  • Its length remains constant during contraction.

  • The part of A band where thin filaments are not there is called H band.

  • M line is seen at the middle of the A band.

Z disks

  • is made up of α actin proteins.

  • Cross-links thin filaments from adjacent sarcomeres in the same myofibril.

  • Z disks of adjacent myofibrils are also connected → This connects neighboring myofibrils and aligns their sarcomeres in one plane → this highly organized structure...

    • allows skeletal muscle to generate considerable mechanical force.

    • gives the muscle a striated appearance when seen under a microscope.

Transverse Tubules

  • Run transverse to the myofibrils.

  • are formed by invagination of cell membrane →

    • Branches to surround all myofibrils → passes near the junction of A and I band of myofibril → each sarcomere is surrounded by two T tubules.

    • Contains extracellular fluid.

    • Carries action potential travelling along the surface of cell membrane deep inside the cell.

Sarcoplasmic Reticulum

  • is a type of endoplasmic reticulum.

  • Stores calcium.

  • Made up of two major parts:

    • Longitudinal tubules: Surround most of the surface of the myofibril.

    • Terminal cisternae: Associate with T tubules. Each T tubule has one terminal cisterna on each side.

Triad Junction

  • Combination of T tubule and two neighboring terminal cisternae is called a triad junction or triad.

  • Here L type of Ca channel on T tubules, and Ca release channels (Ryanodine receptors) on terminal cisternae are in physical contact with each other → Activation of L type of Ca channel by action potential traveling along T tubule physically opens Ca release channels on terminal cisternae also.

T tubule-Sarcoplasmic reticulum system plays important role in excitation-contraction coupling.


Sarcoplasm

  • is intracellular fluid in between myofibrils.

Mitochondria

  • are numerous in myofiber.

  • lie parallel to myofibrils.

  • Provide ATPs required for muscle contraction.

Nucleus

  • One muscle cell contains multiple nuclei.

Sarcolemma

  • is made up of plasma membrane and outer coat.

  • Outer coat is made up thin layer of polysaccharide material that contains numerous thin collagen fibers.

  • At the end of the fiber the outer layer fuses with the muscle tendon

 

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