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