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The Cardiac Myofilament - The Engine of the Heart

   
 Within the myofilament, there are only a handful of proteins that are directly responsible for generating force. The thick filament is composed of myosin, with myosin heads extending towards the actin thin filaments. When the myosin heads attach to their binding spots on the actin thin filament, they can perform a POWERSTROKE which pulls the filaments past each other, generating force. However, when the heart is relaxed and filling with blood (a necessary component of the cardiac cycle, if the heart can’t relax to fill with blood, it doesn’t matter how strong it is), the myosin binding spots on actin are blocked.  Specifically, a protein called tropomyosin wraps around actin to block the myosin heads from attaching, while the troponin complex (consisting of troponin I, troponin C, and troponin T) make up the “switch” that moves tropomyosin out of myosin’s binding spot when the heart needs to contract. The signal for troponin to move tropomyosin is a huge increase in intracellular calcium ions that signals the beginning of cardiac contraction.