Excitation–contraction coupling in skeletal muscle

Question 1

Summary of events leading to release of calcium

Answer 1

Action potentials in sarcolemmal membrane

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Depolarization of the T tubules

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Causes a conformational change in its dihydropyridine receptor

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Opens Ca²⁺ release channels (ryanodine receptors) in the nearby SR

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Release of Ca²⁺ from the SR into the ICF

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Intracellular Ca²⁺ increases

Memory aid:

• AT-DR-CTCC
  
  • AP in sarcolemmal membrane
  • T tubule depolarization
  • Dihydropyridine receptor conformational change
  • Ryanodine receptor opens
  • Calcium release from SR
  • Trop C binding
  • Conformational change
  • Cross-bridge cycle

Question 2

Summary of events leading to cross-bridge cycle

Answer 2

Ca²⁺ binds to troponin C on the thin filaments

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Conformational change in troponin that moves tropomyosin out of the way

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Cross-bridge cycle begins

Memory aid:

• AT-DR-CTCC
  
  • AP in sarcolemmal membrane
  • T tubule depolarization
  • Dihydropyridine receptor conformational change
  • Ryanodine receptor opens
  • Calcium release from SR
  • Trop C binding
  • Conformational change
  • Cross-bridge cycle

Question 3

Summary of the cross-bridge cycle

Answer 3

1. At first, no ATP is bound to myosin (A), and myosin is tightly attached to actin. In rapidly contracting muscle, this stage is brief. In the absence of ATP, this state is permanent (i.e., rigor)
2. ATP then binds to myosin (B), producing a conformational change in myosin that causes myosin to be released from actin
3. Myosin is displaced toward the plus end of actin. There is hydrolysis of ATP to ADP and inorganic phosphate (Pi). ADP remains attached to myosin (C).
4. Myosin attaches to a new site on actin, which constitutes the power (force-generating) stroke (D). ADP is then released, returning myosin to its rigor state
5. The cycle repeats as long as Ca2+ is bound to troponin C. Each cross-bridge cycle “walks” myosin further along the actin filament.

Question 4

Sequence of events that lead to relaxation

Answer 4

Ca²⁺ is reaccumulated by the SR Ca²⁺-ATPase (SERCA)

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Intracellular Ca²⁺ concentration decreases

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Ca²⁺ is released from troponin C

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Tropomyosin again blocks the myosin-binding site on actin

Note: As long as intracellular Ca2+ concentration is low, cross-bridge cycling cannot occur

Question 5

Rigor (state of permanent contraction) occurs if

Answer 5

Adenosine triphosphate (ATP) levels are depleted

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No ATP bound

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Myosin remains attached to actin and the cross-bridge cycle cannot continue