Anatomy and Physiology / Unit 5: The Muscular System
5.3 The Sliding Filament Model
Notes
Muscles shorten when thin filaments slide over thick filaments, shortening each sarcomere. The filaments themselves do not shorten.
- Excitation-contraction coupling: (1) a nerve impulse releases ACh; (2) an action potential spreads along the sarcolemma; (3) the SR releases calcium ions; (4) calcium binds troponin, moving tropomyosin and exposing myosin-binding sites on actin.
- Cross-bridge cycle: myosin heads bind actin, pivot (the power stroke) and pull the thin filament inward, then release when ATP binds. ATP is split to re-cock the head.
- When stimulation stops, calcium is pumped back into the SR and the muscle relaxes.
- Rigor mortis occurs after death because no ATP is available to release the cross-bridges.
- In a sarcomere contraction the H zone and I band get narrower while the A band stays the same length.
Practice
What ion binds to troponin to begin muscle contraction?
Calcium from the SR binds troponin, exposing the actin binding sites.
What is the role of ATP in the cross-bridge cycle?
ATP binding releases myosin from actin; its breakdown energizes the head.
A person dies, and after a few hours the muscles become stiff. The main reason is that:
ATP is needed to break cross-bridges, so stiffness (rigor mortis) develops when ATP runs out.
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