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. 2004 Mar 29;164(7):1045–1054. doi: 10.1083/jcb.200308158

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Copyright © 2004, The Rockefeller University Press

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Figure 6. — A working model for hypercontraction. A simplified five-stage model of contraction is depicted. The resting state of muscle (A) contains myosin unbound to the thin filament. During activation of the muscle (B), intracellular Ca²⁺ increases, allowing a limited number of troponin complexes to shift into the open state. The open state allows noncooperative binding of myosin (C) to the thin filament. When enough myosin heads bind, cooperative mechanisms facilitate binding of multiple heads (D). Release of P_i then allows contraction to occur (E), followed by ADP–ATP exchange to complete the cycle. Drosophila mutants are listed next to proposed steps at which they alter this simplified cycle.