BIO 304 · Human Anatomy & Physiology
Sliding Filament & the Cross-Bridge Cycle
Muscular System · Module 6
A reference for the Cross-Bridge Cycle video. Muscle does not get shorter because filaments shrink. The filaments slide past each other. Each cross-bridge needs ATP, calcium, and an open binding site on actin.
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- Trace excitation-contraction coupling from action potential to calcium release.
- Describe the four steps of the cross-bridge cycle in order.
- Explain rigor mortis using calcium and ATP availability.
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Excitation-Contraction Coupling
From nerve to calcium
- Action potential arrives at axon terminaldepolarizes the motor neuron ending
- ACh releasevoltage-gated Ca²⁺ opens; vesicles fuse; acetylcholine into synapse
- ACh binds nicotinic receptoron the motor end plate (sarcolemma)
- End-plate potentialNa+ enters; sarcolemma depolarizes to threshold
- Action potential along sarcolemmaand down the T-tubules
- DHP → ryanodine receptorvoltage sensor in T-tubule pulls open Ca²⁺ channel on SR
- Ca²⁺ releaseSR floods cytoplasm with Ca²⁺
Calcium does what?
- Binds troponintroponin changes shape
- Tropomyosin shiftsunblocks the myosin-binding site on actin
- Cross-bridge can now formmyosin head reaches actin
Cross-Bridge Cycle & Stop
Four-step cycle (repeats)
- 1. Cross-bridge formationenergized myosin head binds actin
- 2. Power strokehead pivots; thin filament slides toward M line; ADP + Pi released
- 3. Cross-bridge detachmentnew ATP binds myosin head; head releases actin
- 4. Re-cockingATP → ADP + Pi; head energizes back to start; ready to bind again
Sliding filament result
- Thin filaments slidetoward the M line on both sides
- Z discs pulled inwardsarcomere shortens; I band and H zone narrow
- A band stays the samethick filament length does not change
Relaxation
- Action potential stopsno more depolarization
- SERCA pumps Ca²⁺ back into SRcytoplasm Ca²⁺ drops; requires ATP
- Tropomyosin re-covers actinno more binding sites
- Sarcomere returns to restthin filaments slide back
Rigor mortis
- No ATP after deathmyosin cannot release from actin
- Permanent cross-bridgesmuscles stay locked
- Resolves with proteolysisenzymes break down the proteins themselves
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