Muscle Anatomy and Troponin Complexes

Organization of Muscle Fibers

• Skeletal muscle fibers show cross-striations consisting of alternating light (I bands) and dark (A bands) bands.
• A bands are anisotropic (birefringent under polarized light), while I bands are isotropic (do not alter polarized light).
• Each I band is bisected by a Z disc, visible in Transmission Electron Microscopy (TEM).

Sarcomere Structure

• The basic functional unit of muscle contraction is the sarcomere, spanning from Z disc to Z disc, measuring approximately 2.5 μm in length in resting muscle.
• The arrangement of thick (myosin) and thin (actin) myofilaments produces the distinct A and I banding patterns.

Myosin Filaments

• Thick myosin filaments are 1.6 μm long and 15 nm wide, located in the A band at the sarcomere's center.
• Myosin is a complex protein (500 kDa) comprised of two heavy chains and two pairs of light chains; heavy chains form long, rod-like tails.
• Myosin heads are globular projections that bind to actin and ATP, facilitating energy transfer during contraction.

Actin Filaments

• Thin actin filaments are 1.0 μm long and 8 nm wide, situated between thick filaments.
• Each G-actin monomer has a binding site for myosin and is anchored to the Z disc by α-actinin.

Regulatory Proteins

• Thin filaments interact with two key regulatory proteins:
  • Tropomyosin: A 40-nm-long coil of two polypeptide chains that resides in the grooves of twisted actin strands.
  • Troponin: A complex with three subunits—TnT (attaches to tropomyosin), TnC (binds Ca²⁺), and Tnl (regulates actin-myosin interaction). Troponin complexes are regularly spaced along tropomyosin.

A Band and H Zone

• A bands consist of thick filaments and overlapping thin filaments, with a central lighter zone termed the H zone, showcasing only the myosin rod portions.
• The M line, found at the center of the H zone, comprises myosin-binding protein myomesin which stabilizes thick filaments and contains creatine kinase for ATP supply during contraction.

Protein Composition

• Myosin and actin collectively account for over half of the total protein content in striated muscle.
• I bands, bisected by Z discs, consist of thin filament portions that do not overlap thick filaments, causing lighter staining.
• Titin: The largest protein in the body (3700 kDa), exhibits elastic properties and connects thick filaments to Z discs.
• Nebulin: A large accessory protein (600-900 kDa), binds thin myofilaments laterally to help anchor them to α-actinin.