Myofibrils
Summary
myofibril histology: OVERVIEW
Thick filaments.
- Form from myosin
- The A band refers to the length of the thick filaments, "think "A" for d-a-rk – they are aniosotropic (or birefringent) in polarized light.
- H Zone is a zone of only thick filaments.
- M line bisects the A band.
Thin filaments
- Form from actin
- The I band is the region along the thin filaments (between the thick filaments).
- Think "I" for L-i-ght – they are "isotropic" (do not alter polarized light).
Z disks
- Transverse bands at the ends of the thin filaments.
Sarcomere
- The contractile unit of the myofibril.
- Comprises the area between the Z-disks.
Thin myofilament: Details
- The thin filament slides towards the H zone.
Actin
- Spherical molecules joined in pairs of strands (like beads on a string). It is referred to as F-actin for filamentous actin, and comprises a polymer of G-actin monomers that are arranged in a double helix.
Tropomyosin
- Threadlike strands
Troponin
- Protein complexes that bind tropomyosin, actin, and also calcium (show their calcium-binding sites).
Cap Z
- Binds the Z disk to the thin filaments.
- This is (+) end stabilizer.
Tropomodulin
- The (-) end stabilizer.
Troponin subunits
- TnT binds to tropomyosin
- TnI, which inhibits actin interaction with myosin.
- TnC, which binds calcium ions.
Nebulin
- Attaches to the Z disk, passes along the thin filament and binds to tropomodulin to help stabilize the thin filament and determines its length.
- It is commonly stated that two nebulin molecules wrap around each thin filament to anchor it to the Z disk.
thick filaments: details
- Comprise myosin molecules (technically myson II), which form a golfclub shape, and comprise two heavy chains and two light chains.
- The head forms from the heavy chain.
- Unlike the thin filaments, the thick filaments are not directly connected to the Z disc. Instead, show that they are anchored to the ends of the sarcomere via titin molecules, which extend from the Z disks to the M line, and regulates the sarcomere's elasticity, in addition to managing assembly of myosin.
muscle cell in a cylindrical, 3-dimensional view
Here, we draw a muscle cell in a cylindrical, 3-dimensional view.
Sarcoplasmic reticulum (SR)
- Form web-like rows.
- Store calcium.
- Are a key component to coupling muscle cell excitation to myofibril contraction.
Terminal cisternae (aka lateral cisternae) of the sarcoplasmic reticulum.
- Flank the transverse tubules (T-Tubules)
Transverse tubules (T-Tubules)
- Are tubular invaginations of sarcolemma.
- Elsewhere, we see that T-tubules and terminal cisternae connect the terminal synapse firing (its depolarization) to the sarcoplasmic reticulum, again, ultimately coupling muscle cell excitation and myofibril contraction.
Triads of terminal cisternae
- Triads of terminal cisternae with intervening T-tubules occur at the A-I junctions.
Desmin
- Filaments encircle the Z disks.
- Desmin-related myopathy (DRM) is an inherited disease in which, and results in disorganized and weak skeletal muscle fibers. DRM can be fatal, as it also affects cardiac and smooth muscles.
Plectin
- Links the desmin filaments.
Alpha-beta-crystallin
- A heat shock protein, which protects desmin from stress-induced damange.
Thus, together, desmin, plectin, and alpha-beta-crystallin constitute a Z-disk protection network.
Alpha-actinin
- A Z disk component that binds to actin.
Costameres
- Specialized regions of the sarcolemma, which bind to key protein complexes.
Dystrophin-associated glycoprotein complex (DAGC), which comprises:
- The dystroglycan subcomplex, which links dystrophin (discussed soon) to laminin, a key external lamina protein (called laminin-2 in skeletal muscle).
- The sarcoglycan subcomplex, which, when defective can cause sarcoglycanopathies – a similar manifestation of weakness as those from dystrophinopathies – and are a common cause of limb-girdle muscular dystrophy.
- Dystrophin, which stabilizes the sarcolemma during muscle contraction.
Duchenne muscular dystrophy (DMD)
Pathological slide
- Muscle from an individual with Duchenne muscular dystrophy (DMD).
- X-linked form of muscular dystrophy affects boys and occurs from a genetic mutation that prevents the synthesis of dystrophin.
- We see a sparse group of muscle cells and see that much of the muscle is replaced with fatty and fibrous connective tissue, which presents with pseudohypertrophic muscles: muscles that are enlarged from fat and connect tissue (not muscle).
- Eventually patients with this illness succumb to their weakness.
Syntrophins
- Are recruited to the sarcolemma and manage the assembly of other proteins.
Dystrobrevins
- Link desmin to dystrophin and syntrophin.
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