Motor mechanisms

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Motor mechanisms
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Keywords (reading p. 1080-1086)

• Bundle, fiber, myofibril, sarcomere
• Z-line, thick filament, thin filament
• Actin, myosin, sliding filament model
• Molecular basis for filament movement
• Troponin, tropomyosin
• Sarcoplasmic reticulum
• Integration of synaptic signals
• neurotransmitters

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Motor mechanisms

• Create movement
• Can be cilia, flagella, contractile proteins,
  muscles
• Will focus on skeletal muscle
• Muscle of vertebrates that is under voluntary
  control

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Structure of skeletal muscle

• Bundle
• Fiber
• Myofibril
• sarcomere

Part of Campbell 6th ed p. 1081
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Features of muscle cells

• of nuclei- multinucleate formed by fusion of
  embryonic cells
• length - this results in very long cells

Part of Campbell 6th ed p. 1081
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Sarcomere

• Structure gives muscle a striated appearance
• Z line, thick filaments (myosin), thin filaments
  (actin)

Part of Campbell 6th ed p. 1081
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Part of Campbell 6th ed p. 1081
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Sliding-filament model of muscle contraction
Part of Campbell 6th ed p. 1081
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Thick and thin filaments slide past each other
Part of Campbell 6th ed p. 1081
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At maximal contraction, there is no space at end
of thick filament,thin filaments overlap
Part of Campbell 6th ed p. 1081
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Molecular basis for movement of filaments against
each other
http//www.youtube.com/watch?vInIha7bCTjMfeature
related
Part of Campbell 6th ed p. 1082
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ATP bound, head retracted and unattached
Part of Campbell 6th ed p. 1082
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Hydrolysis of ATP cocks head
Part of Campbell 6th ed p. 1082
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Myosin head attaches to actin filament
Part of Campbell 6th ed p. 1082
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Release of ADP Pi causes a further
conformational change pushing against the actin
filament
Part of Campbell 6th ed p. 1082
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Binding of ATP to myosin head causes dissociation
from actin filament
Part of Campbell 6th ed p. 1082
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Cycle repeats and sarcomere shortens
Part of Campbell 6th ed p. 1082
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Control of muscle contraction by Ca

• Tropomyosin- blocks the myosin binding sites on
  the actin filament when muscle is at rest
• Troponin complex-binds calcium and controls the
  position of tropomyosin

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At rest, myosin cannot bind because sites are
covered by tropomyosin
http//www.youtube.com/watch?vWRxsOMenNQM
Part of Campbell 6th ed p. 1083
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During muscle contraction Ca levels rise. Ca
binds to troponin which then pulls tropomyosin
way from the binding sites
Part of Campbell 6th ed p. 1083
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What triggers the Ca rise that induces muscle
contraction?
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Ca regulated by the sarcoplasmic reticulum
Campbell 6th ed p. 1083
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Structure of the sarcoplasmic reticulum

• T tubules - are a network of the fiber plasma
  membrane that goes deep into the muscle fiber.
• This allows transmission of the action potential
  into the fiber

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Sequence of events leading to muscle contraction

• Motor neuron releases acetylcholine
• Depolarization of the muscle fiber membrane
  results in action potentials
• Action potentials trigger release of Ca from
  the sarcoplasmic reticulum
• Increased Ca allows actin and myosin to slide
  against each other

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Characteristics of other muscle types

• Cardiac muscle - found only in the heart,
  striated, gap junctions allow direct electrical
  signaling between cells
• Smooth muscle - involuntary muscle, meshwork of
  actin and myosin, can contract more (greater
  shortening), but with less tension.