Structure
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You are watching: What causes the myosin head to disconnect from actin?

Actin
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The thin filaments consist of globular actin molecules in two long chains wound around each other and each actin molecule has a binding site for myosin. The protein tropomyosin winds around the thin filaments and covers the myosin binding sites. At regular intervals along the tropomyosin cable sit troponin molecules.MyosinThe thick filaments consist of bundles of myosin molecules. Each myosin molecule is composed of two long protein chains with a globular head at one end. The myosin head attaches to the binding site on the actin filament. In addition, it
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binds ATP, acting as an enzyme to transfer energy from ATP. The energy transfer changes the shape of the myosin head ("cocks" the apparatus).
Setup
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Muscle contraction and relaxation occurs in a cycle that uses and reuses the same components. Initially, the myosin head combines with ATP cocking its head with the energy from ATP. Before the nerve sends a signal to the muscle to contract, the cocked head cannot bind to the thin actin filament since the binding site is blocked by the troponin-tropomyosin complex.ShorteningThe action potential traveling along the nerve cell releases a chemical messenger (acetylcholine) which crosses the neuromuscular junction and depolarizes the muscle fiber membrane in the same way as depolarization occurs in a nerve cell. Upon depolarization, the sarcoplasmic reticulum releases calcium ions. The calcium ions bind to the troponin, changing the shape of the troponin-tropomyosin complex such that the actin binding sites are uncovered. As soon as myosin binds to actin, the cocked head of myosin releases sliding the actin fiber. Once the action potential occurs in the muscle fiber, it takes a few milliseconds for the processes of excitation and coupling to result in an increase in tension. Peak contraction takes place in about 10 ms in fast muscle fibers and about 100 ms in slow fibers.Relaxation
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