muscle excitation – contraction coupling chapter 16 kine 3301 biomechanics of human movement
TRANSCRIPT
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Muscle Excitation – Contraction Coupling
Chapter 16
KINE 3301Biomechanics of Human Movement
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A single muscle fiber contains hundreds of myofilaments (Actin & Myosin) which are bundled together and surrounded by the muscle membrane (sarcolemma).
A muscle fiber has a diameter of 10 – 60 μm and a length of 10 – 300 mm.
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• Troponin and tropomyosin are regulatory proteins.• Under resting conditions they a crossbridge from forming
between actin and myosin.• In the presence of Ca++ stops inhibiting muscular contraction by
rotating away from it’s resting position where it covers the actin binding site.
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Sarcomere: The Functional Unit of the Muscle Cell
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Concentric Cross-bridge Cycle
• Ca++ released from SR• Ca++ binds with troponin• Tropomyosin moves away
from binding site.• An Actin-Myosin cross-
bridge is formed.• ATP downgraded to ADP +
Pi.• Myosin does mechanical
work on Actin, Myosin arm rotates shortening the muscle fiber.
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Eccentric Cross-bridge Cycle
• Ca++ released from SR• Ca++ binds with troponin• Tropomyosin moves away from binding
site.• An Actin-Myosin cross-bridge is formed.• ATP downgraded to ADP + Pi.• Myosin attempts to rotate and shorten
fiber.• External force causes the fiber to
lengthen, storing elastic energy in myosin arm.
• The cross-bridge is broken (1-4 pN) while it is still in the actively charged state, it immediately forms another cross-bridge at the next available Actin site without the need for any additional ATP.
• This “break & make” cross-bridges will continue until the length of the active state is exceeded.
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Isometric Cross-bridge Cycle
• Ca++ released from SR• Ca++ binds with troponin• Tropomyosin moves away from binding site.• An Actin-Myosin cross-bridge is formed.• ATP downgraded to ADP + Pi.• Myosin attempts to rotate and shorten fiber.• External force causes the fiber to lengthen,
storing elastic energy in myosin arm.• From this lengthened position the myosin arm
rotates and shortens the fiber.
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Contraction Type
Energy Utilized
Force Generated
Change in Fiber Length
Effect of Velocity on Force
Concentric 1 ATP 1 – 3 pN Shortened ~5 nm
Force decreases with increasing velocity:1 pN for fast velocity3 pN for slow velocity
Eccentric 1 ATP n × 1-4 pN,n is # of make & break A-M bonds
Lengthened ~5-20 nm
Force increases with increasing velocity: 3 pN for slow velocity4 pN for fast velocity
Isometric 1 ATP 3 pN Relatively Constant
none
Summary of Actin-Myosin Crossbridge Cycle