Sarcomeres Shorten as Actin and Myosin Slide Past Each Other
Definition
Sarcomere
A sarcomere is the basic contractile unit of muscle fiber. Each sarcomere consists of two main protein filaments, actin and myosin,which are the active structures responsible for muscular contraction.
- The Sarcomere consists of:
- Z-Discs (Z-Lines): Define sarcomere boundaries, anchor thin filaments (actin).
- Thin Filaments (Actin): Extend from the Z-discs toward the sarcomere center.
- Thick Filaments (Myosin): Occupy the central region, interlock with actin filaments.
- I-Bands (Light Bands): Contain only actin.
- A-Bands (Dark Bands): Overlap region of actin and myosin.
- H-Zone: Central part of the A-band where only myosin is present when muscle is relaxed.
- Z-Discs are pulled closer together to shortening the sarcomere.
- During muscle contraction, the sarcomeres shorten as the Z-discs are pulled closer together.
- This shortening occurs because the actin and myosin filaments slide past each other; the filaments themselves do not change length.
- Think of the sarcomere like a spring that compresses during contraction and stretches back out during relaxation.
- This elasticity is critical for muscle function and recovery.
The Sliding Filament Mechanism
- Cross-Bridge Formation
- Myosin heads attach to binding sites on actin, forming cross-bridges.
- Initiated by Ca²⁺ release (triggered by a nerve impulse).
- Power Stroke
- Myosin heads pivot, pulling actin toward the sarcomere center.
- ATP hydrolysis provides the energy for this movement.
- Detachment
- A new ATP binds to the myosin head, causing it to release actin.
- Without ATP, the myosin head would remain locked in place (rigor).
- Resetting the Myosin Head
- The myosin head hydrolyzes ATP, returning to its cocked position.
- Ready to form another cross-bridge for continuous sliding.
- Imagine pulling a rope hand-over-hand.
- Each time you grip and pull the rope, it’s like a myosin head attaching and pulling on an actin filament.
- Letting go to re-grip the rope mimics the detachment and resetting of the myosin head.
