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Muscle Proteins

• In the thin filaments actin proteins are strung
  together like a bead of pearls
• In the thick filaments myosin proteins look like
  golf clubs bound together

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Muscle Proteins

• In this first graphic, the myosin binding sites
  on the actin proteins are readily visible.
• The regulatory proteins troponin and tropomyosin
  have been added to the bottom graphic The myosin
  binding sites have been
• covered

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Muscle Proteins

• In this graphic the troponin-tropomyosin complex
  has slid down into the gutters of the actin
  molecule unblocking the myosin binding site
• The troponin-tropomyosin complex can slide back
  and forth depending on the presence of Ca2

Myosin binding site exposed
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Muscle Proteins

• Ca2 binds to troponin which changes the shape of
  the troponin-tropomyosin complex and uncovers the
  myosin binding sites on actin

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Muscle Proteins

• Besides contractile and regulatory proteins,
  muscle contains about a dozen structural proteins
  which contribute to the alignment, stability,
  elasticity, and extensibility of myofibrils
• Titan is the third most plentiful protein in
  muscle, after actin and myosin - it extends from
  the Z disc and accounts for much of the
  elasticity of myofibrils
• Dystrophin is discussed later as it relates to
  the disease of muscular dystrophy

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The Sliding-Filament Mechanism

• With exposure of the myosin binding sites on
  actin (the thin filaments)in the presence of
  Ca2 and ATPthe thick and thin filaments slide
  on one another and the sarcomere is shortened

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The Sliding-Filament Mechanism

• The sliding of actin on myosin (thick filaments
  on thin filaments) can be broken down into a 4
  step process

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• Step 1 ATP hydrolysis
• Step 2 Attachment

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• Step 3 Power Stroke
• Step 4 Detachment

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The Sliding-Filament Mechanism
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Contraction and Movement OverviewInteractions
Animation

• Contraction and Movement

You must be connected to the internet to run this
animation.
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Length-Tension Relationship

• Sarcomere shortening produces tension within a
  muscle

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Excitation-Contraction Coupling

• We will come back to the term excitation-contracti
  on coupling in a little bit
• Before we can describe the
• entire process, from
• thinking of moving a
• muscle to actual contraction
• of sarcomeres, we must
• first explore the processes
• that occur at the neuromuscular junction

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Neuromuscular Junction

• Excitation-Contraction coupling (EC coupling)
  involves events at the junction between a motor
  neuron and a skeletal muscle fiber

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Neuromuscular Junction

• An enlarged view of the neuromuscular junction
• The presynaptic membrane is on the neuron while
  the postsynaptic membrane is the motor end plate
  on the muscle cell. The two membranes are
• separated by a space,
• or cleft