Excitation-Contraction Coupling

Nerve Impulses and Calcium Release

  • As the nerve impulse travels down the T tubules, it initiates a cascade of events in muscle contraction.
  • The process begins with the opening of voltage-sensitive calcium channels located in the terminal cisterna where calcium ions are stored.
  • Calcium ions (Ca²⁺) flow out of the terminal cisterna into the cytosol of the muscle cell.
  • The released calcium ions diffuse towards Troponin C (TnC), which is a crucial regulatory protein within the muscle fibers.

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

  • Calcium Binding to TnC

    • Calcium binds to TnC.
    • This binding induces a conformational change in the arrangement of the troponin complex, which consists of three proteins: TnI, TnT, and TnC.

  • Troponin and Tropomyosin Dynamics

    • As troponin proteins undergo conformational changes, they cause movement in the associated tropomyosin molecules, allowing for the myosin-binding sites on actin filaments to become exposed.
    • Troponin blocks myosin-binding sites: The initial arrangement of troponin prevents myosin from binding to actin.
    • Once the tropomyosin is slid away, the myosin-binding sites are exposed and prepared for the binding of myosin.

  • Cross-Bridge Formation

    • The energized myosin head is now free to attach to the exposed myosin binding site on the actin filament, forming what is known as a cross-bridge.
    • Visual Representation:
    • Actin filament (thin filament) facilitates cross-bridge interaction.
    • Myosin filament (thick filament) interacts by its head with actin during contraction.
    • The cross-bridge formation step crucially relies on the presence of adenosine triphosphate (ATP) molecules, which energize the myosin heads.

Power Stroke Mechanism

  • Once the cross-bridge is established, several events occur:

    • The myosin head flexes inward.
    • This inward flexing results in pulling the thin filament (actin) towards the M line of the sarcomere, the structural unit of a muscle cell.

  • Release of ADP

    • During this power stroke, adenosine diphosphate (ADP) is released from the myosin head, indicating the energy exchange happening in the rotary movement of the head.
    • At this stage, myosin transitions to a low-energy configuration, which aids in the further pulling action of actin.

  • H-Zone and Muscle Contraction Dynamics

    • The H zone is the region of the sarcomere that becomes narrower during muscle contraction as actin and myosin slide over each other during the power stroke.
    • The overall contraction results from continuous cycles of cross-bridge formation and power strokes, powered by ATP hydrolysis, ultimately leading to muscle shortening and force generation.