Muscle Contraction

Muscle Contraction Mechanism

  • Role of Calcium Ions

    • During muscle contraction, calcium ions (Ca2+) play a crucial role.
    • Calcium ions bind to the protein troponin.
    • This binding causes a conformational change in troponin, leading to the movement of tropomyosin.
    • Tropomyosin is moved out of the way, allowing exposure of binding sites for myosin on actin myofilaments.

  • Myosin Head and Cross Bridge Formation

    • In the previous cycle of movement, ADP and inorganic phosphate (Pi) are attached to the myosin head.
    • The myosin heads then attach to the newly exposed binding sites on the actin myofilaments, forming what is known as "cross bridges."
    • The phosphate group is released when the cross-bridge forms.
    • Energy that was stored in the myosin head is utilized to pivot the head, causing the actin myofilament to slide past the myosin myofilament.

  • Role of ADP Release

    • As the myosin head pivots and performs the power stroke, ADP is released from the myosin head.

  • ATP Binding and Cycle Continuation

    • The bond between actin and the myosin head is broken when a molecule of ATP binds to the myosin head.
    • This ATP is subsequently hydrolyzed into ADP and phosphate, releasing energy that is stored in the myosin head for future movement.
    • The myosin head returns to its upright, cocked position and becomes ready to bind to actin again if calcium ions are still available.

  • Repetition of Cycle

    • If calcium ions are still present in the muscle cell, the entire contraction cycle can repeat.
    • This cycle continues as long as calcium is available to keep the binding sites on actin exposed and ATP is present to power the movements of the myosin heads.