Chapter 9, Section 3 Sliding Filament Theory of Contraction
The Sliding Filament Model of Muscle Contraction During a muscle contraction • Thick (myosin) filaments and thin (actin) filaments slide across one another • The filaments do not change lengths • Z-bands move closer togetherSarcomere shortens • I bands appear narrow Figure 9.11a. Individual sarcomeres shorten as thick and thin filaments slide past one another.
Cross Bridge Cycling
Cross-Bridge Cycling 1. When a muscle is relaxed, tropmyosin covers the binding sites on actin. •
A molecule of ADP and Phosphate remains attached to cross-bridges from the previous contraction.
Cross Bridge Cycling
Cross-Bridge Cycling 2. During a contraction, Calcium binds to troponin. • Tropomyosin is repositioned, exposing the binding sites on actin
Cross Bridge Cycling
Cross-Bridge Cycling 3. Cross-Bridges on myosin bind to exposed binding sites on actin filaments.
Cross Bridge Cycling
Cross-Bridge Cycling 4. Cross-bridge springs forward “Power Stroke” pulling the thin filaments. • ADP and P are released from cross-bridges
Cross Bridge Cycling
Cross-Bridge Cycling 5. A new ATP binds to myosin, which releases the cross-bridges from actin. • ATP is not yet broken down, but it is essential to release the cross-bridges.
Cross Bridge Cycling Cross-Bridge Cycling 6. ATP is broken down, providing the energy to “cock� the myosin cross-bridges (recovery stroke). 7. Steps 1-6 are repeated several times.
Figure 9.10. The cross-bridge cycle. The cycle continues as long as ATP is present, and nerve impulses release Acetylcholoine.
Watch the attached You-Tube video “Sliding Filament� to view cross-bridge cycling in action.
Relaxation When a nerve impulse ceases, two events relax muscle fibers. 1. Acetylcholinesterase breaks down ACh • Enzyme within synapse of NMJ and on motor endplate. • Rapidly decomposes Acetylcholine • Prevents continuous stimulation of a muscle fiber.
2.
Calcium Pumps remove Ca2+ from sarcoplasm • Actively returns calcium to Sarcoplasmic reticulum (requires ATP) • Without calcium, tropomyosin covers binding sites on actin
Relaxation Rigor Mortis • Partial contraction of skeletal muscles a few hours after death. • Calcium leaks into sarcoplasm, triggering muscle contractions. • But ATP supplies quickly diminish, so muscles do not relax*. • Contraction is sustained until muscles begin to decompose.
* Notice that ATP is required for muscle relaxation!
End of Chapter 9, Section 3