Muscle Contraction and Relaxation

Cross Bridge Cycle in Muscle Contraction

  • The cross bridge must detach either to:

    • Continue muscle contraction (by forming another cross bridge)
    • Relax the muscles (no further cross bridges are made)

  • Detachment Process:

    • To detach the cross bridge, ATP must attach to the myosin head, initiating the following:
    • ATP Cross Bridge Detachment:
      • After ATP attaches to myosin:
      • The link between myosin and actin weakens.
      • The myosin head detaches, resulting in the cross bridge "breaking."

  • Energy Transfer:

    • The ATP will then lose its terminal phosphate, transferring energy to the myosin molecule which:
    • Returns to its original non-flexed position.

  • Calcium's Role:

    • If calcium remains bound to TnC and tropomyosin remains in the groove of the thin filament:
    • Myosin is ready to form another cross bridge and power stroke.
    • This can:
      • Further contract (shorten) the muscle.
      • Maintain muscle contraction without further shortening.
    • The myosin head can attach and detach multiple times during contraction.

  • Myosin Head Behavior:

    • All myosin heads do not attach and detach simultaneously.
    • At any point, some myosin heads are bound to actin, maintaining tension in the sarcomere while others have detached.

  • ATP Hydrolysis:

    • During contraction, myosin hydrolyzes ATP to ADP and P, facilitating the following:
    • The myosin head returns to its prestroke, high-energy, or "cocked," position.

Relaxation of Skeletal Muscle

  • For muscle relaxation:
    • Tropomyosin must return to its original position, blocking the myosin binding site.
    • Calcium must be pumped back into the terminal cisternae:
    • This creates a diffusion gradient that draws the bound calcium off the TnC.

Steps in Excitation-Contraction Coupling (E-C Coupling)

  • Action Potential (AP) Propagation:

    • The AP propagates along the sarcolemma and down the T tubules:
    • Voltage-sensitive tubule protein plays a key role in this process.
    • Components Involved:
      • Sarcolemma: Membrane surrounding muscle fibers.
      • T tubule: Extensions of the sarcolemma that penetrate into the muscle cell, facilitating AP transmission to deeper regions.

  • Calcium Release Mechanism:

    • Voltage-sensitive tubule proteins undergo a conformational change as a result of the AP:

    • This change opens the calcium release channels in the terminal cisternae of the sarcoplasmic reticulum (SR).

    • As a result, calcium ions (Ca2+) are released into the cytosol.

    • Calcium Pumps: Perform primary active transport of calcium, involving ATP to facilitate muscle contraction and relaxation.