L5.3.2.1: What Causes Filaments to Slide?

• ● Calcium Ions (Ca2+)

  ○ Not only for the mobilization of ACh within the motor neuron.

  ○ Also plays a crucial role in the actual contraction by binding regulatory proteins on thin filaments and exposing myosin-binding sites, allowing myosin heads on the thick filaments to attach.

  ■ Regulatory proteins are the ones that block the myosin-binding sites.

  ○ DUMBED DOWN VER. Calcium binds to the troponin which helps the tropomyosin move off the myosin-binding sites.
• Formation of Cross Bridges (Myosin-Actin Attachment)

  ○ This is facilitated by the myosin heads.

  ○ ATP on the myosin head is broken down releasing energy which allows the myosin to pull the actin filament inwards, contracting the muscle.

  ○ ATP molecule binds to the myosin head detaching the myosin from the actin and breaking the cross bridge.
• Role of Sarcoplasmic Reticulum (SR)

  ○ The action potential stimulates the sarcoplasmic reticulum to release calcium ions into the cytoplasm.

  ○ The calcium ions trigger the binding of myosin to actin, initiating filament sliding.

  ○ When the action potential ends, calcium ions are returned to the SR storage areas, the regulatory proteins return to their resting shape and block myosin-binding sites, and the muscle fiber relaxes and settles back to its original length.
• ● Calcium ions (Ca2+) bind regulatory proteins on thin filaments and expose myosin-binding sites, allowing the myosin heads on the thick filaments to attach.

  ● Each cross bridge pivots, causing the thin filaments to slide toward the center of the sarcomere.

  ● Contraction occurs, and the cell shortens.

  ● During a contraction, a cross bridge attaches and detaches several times.

  ● ATP provides the energy for the sliding process, which continues as long as calcium ions are present.