Myosin (Thick Filaments)
Each thick filament contains ~300 myosin molecules arranged in a bipolar fashion.
The myosin heads (motor domains) project outward in a spiral pattern and perform
the power stroke that generates force. The bare zone at the center has no heads.
Actin (Thin Filaments)
Thin filaments are double-helical polymers of actin monomers. They're decorated
with regulatory proteins: tropomyosin winds around the helix, and troponin
complexes control calcium-dependent binding of myosin heads.
The Power Stroke Cycle
1. Attachment
Myosin head binds tightly to actin (rigor state)
2. Power Stroke
Head rotates ~70°, pulling actin ~5-10nm toward M-line
3. Release
ATP binds, myosin detaches from actin
4. Recovery
ATP hydrolysis re-cocks the head for next cycle
Titin - The Molecular Spring
Titin is a giant protein (~3.5 MDa) spanning from Z-disc to M-line. It acts as
a molecular spring, providing passive tension and preventing over-stretching.
Its elastic PEVK region unfolds progressively under force.
Structural Landmarks
Z-Disc
α-actinin lattice anchoring actin plus-ends and titin N-terminus
M-Line
Myomesin crosslinks holding myosin and titin C-terminus