Cross Bridge Cycle – Class 11 | Chapter – 20 | Biology Short Notes Series PDF

Cross Bridge Cycle: Andrew Huxley developed the cross-bridge hypothesis quantitatively for the first time in 1957. The cross-bridge cycling demonstrates the shortening of muscles due to the movement of the contractile proteins. The cross-bridge muscle contraction cycle is recognized in all types of muscles, such as cardiac, smooth, and skeletal muscles. This means that all muscle types contract by cross-bridge cycling. The cycling of cross-bridge is due to the cyclic attachment and detachment of contractile proteins.

Stages of Cross Bridge Cycle

Stages of Cross Bridge Cycle

Step 1: The Design of Cross-Bridges

  • When excitation-contraction coupling causes the sites of myosin binding on actin to be released, the cycle begins. Myosin heads, forming cross bridges, connect to these sites.
  • Myosin begins in its high-energy state, in which ADP and inorganic phosphate occupy the ATP.
  • The head of the myosin is ready for the power stroke to pivot.

Step 2: The Power Stroke

  • Inorganic phosphate release from the myosin head causes the head to pivot, pushing the thin filament toward the core of the sarcomere.
  • ADP is then also released. The head of the myosin is now in a low-energy state but is still bound to actin.
  • Eventually, the energy for the power stroke is provided by the ATP that binds to the head of the myosin.

Step 3: The Cross-Bridge detachment

  • Until an ATP molecule attaches to the empty ATP-binding site on the myosin head, myosin remains attached to actin.
  • This weakens the link between the head of the myosin and the actin and detaches the head of the myosin.
  • In its low-energy state, the disconnected myosin head remains.

Step 4: It is Myosin head Reactivation

  • This happens when the myosin head-bound ATP undergoes hydrolysis to ADP and inorganic phosphate. During hydrolysis, the energy released reactivates the head of the myosin, pivoting it back to its high energy state.
  • The cross-bridge loop begins again when the energized myosin head creates another cross-bridge.
  • Cross bridge cycles occur between the thick and thin filaments along the entire length of each sarcomere during muscle contraction.
  • But not all myosin heads are simultaneously attached or detached. They may be in various stages of the cycle of cross-bridges.
  • Note that a myofibrilcomprises thousands of sarcomeres to read the definition and thousands of myofibrils to read the definition in a muscle fibre.
  • Cross bridge cycling causes the entire muscle to contract on both of these sarcomeres

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By Team Learning Mantras