section 3, chapter 9 cross-bridge cycling
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cross-bridge cyclingTRANSCRIPT
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Sliding Filament Theory of Contraction
Chapter 9, Section 3
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The Sliding Filament Model of Muscle Contraction
Figure 9.11a. Individual sarcomeres shorten as thick and thin filaments slide past one another.
During a muscle contractionThick (myosin) filaments and thin (actin) filaments slide across one another
The filaments do not change lengths
Z-bands move closer together causing the sarcomere to shorten.
I bands appear narrow
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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 myosin from the previous contraction.
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2. During a contraction, Calcium binds to troponin.
Tropomyosin is repositioned, exposing the myosin binding sites on actin filaments
Cross Bridge Cycling
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3. Myosin heads bind to actin filaments.
The phosphate is released.
Cross Bridge Cycling
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4. Myosin heads spring forward “Power Stroke” pulling the actin filaments.
ADP is released from Myosin
Cross Bridge Cycling
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5. Myosin is released from actin.
A new molecule of ATP binds to myosin, causing it to be released from the actin filament.
• ATP is not yet broken down, but it is essential to release the cross-bridges.
Cross Bridge Cycling
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6. ATP is broken down, providing the energy to “cock” the myosin filaments (recovery stroke).
7. Steps 1-6 are repeated several times.
Cross Bridge Cycling
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Figure 9.10. The cross-bridge cycle. The cycle continues as long as ATP is present, and nerve impulses release Acetylcholoine.
Watch the You-Tube video “Sliding Filament” to view cross-bridge cycling in action.
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Relaxation
When a nerve impulse ceases, two events relax muscle fibers.
1. Acetylcholinesterase breaks down Ach in the synapse.• Prevents continuous stimulation of a muscle fiber.
2. Calcium Pumps (Ca2+ATPase) remove Ca2+ from the sarcoplasm and returns it to the SR.• Without calcium, tropomyosin covers the binding sites on actin
filaments.
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RelaxationRigor Mortis is a partial contraction of skeletal muscles that occurs a few hours after death.
• After death calcium leaks into sarcoplasm, triggering the muscle contractions.
• But ATP supplies are diminished after death, so ATP is not available to remove the cross-bridge linkages between actin and myosin.
• muscles do not relax*.
• Contraction is sustained until muscles begin to decompose.
* Notice that ATP is required for muscle relaxation!
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End of Chapter 9, Section 3