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The cross-bridge of skeletal muscle is not synchronized either by length or force step.

Grazi E - Int J Mol Sci (2015)

Bottom Line: A rapid force step does not synchronize attached cross-bridges.The change of X-ray interference during the second phase does not measure the stroke size.These conclusions significantly change the picture of the mechanism of skeletal muscle contraction.

View Article: PubMed Central - PubMed

Affiliation: Department of Scienze Biomediche e Chirurgiche Specialistiche, Ferrara University, Via Borsari 46, 44121 Ferrara, Italy. enrico.grazi@unife.it.

ABSTRACT
Force and length steps, applied to a muscle fiber in the isometric state, are believed to synchronize attached cross-bridges. This alleged synchronization facilitates the interpretation of the experiments. A rapid force step elicits an elastic response of the attached cross-bridges, followed by an isotonic phase. The decay of this second isotonic phase is of the first order. This excludes that the attached cross-bridges may decay all at the same time. The change of the X-ray interference distance during the second phase measures the stroke size only in the unrealistic case that the cross-bridges are and remain all attached. A rapid force step does not synchronize attached cross-bridges. The change of X-ray interference during the second phase does not measure the stroke size. These conclusions significantly change the picture of the mechanism of skeletal muscle contraction.

No MeSH data available.


Related in: MedlinePlus

Length responses following a step change in force. The numbers indicate the phases. Adapted from figure 1B of [2].
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ijms-16-12064-f001: Length responses following a step change in force. The numbers indicate the phases. Adapted from figure 1B of [2].

Mentions: Piazzesi et al. [2], with 150 µs force steps, separated the elastic response of the attached cross-bridges and filaments (phase 1) from the subsequent isotonic phases of the velocity transient. Of these, the rapid phase 2 was claimed to represent the synchronous execution of the working stroke. The slower phases 3 and 4 were assigned to detachment and attachment of the myosin head with the subsequent filament sliding (Figure 1). Piazzesi et al. [2] showed that the speed and the amplitude of phase 2 increases with the amplitude of the force step. They concluded that the step size of the power stroke increases from 4 nm at 0.8 T0 to ~7 nm at T = 0 (where T0 is the isometric tension and T is the actual tension of the fiber).


The cross-bridge of skeletal muscle is not synchronized either by length or force step.

Grazi E - Int J Mol Sci (2015)

Length responses following a step change in force. The numbers indicate the phases. Adapted from figure 1B of [2].
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4490429&req=5

ijms-16-12064-f001: Length responses following a step change in force. The numbers indicate the phases. Adapted from figure 1B of [2].
Mentions: Piazzesi et al. [2], with 150 µs force steps, separated the elastic response of the attached cross-bridges and filaments (phase 1) from the subsequent isotonic phases of the velocity transient. Of these, the rapid phase 2 was claimed to represent the synchronous execution of the working stroke. The slower phases 3 and 4 were assigned to detachment and attachment of the myosin head with the subsequent filament sliding (Figure 1). Piazzesi et al. [2] showed that the speed and the amplitude of phase 2 increases with the amplitude of the force step. They concluded that the step size of the power stroke increases from 4 nm at 0.8 T0 to ~7 nm at T = 0 (where T0 is the isometric tension and T is the actual tension of the fiber).

Bottom Line: A rapid force step does not synchronize attached cross-bridges.The change of X-ray interference during the second phase does not measure the stroke size.These conclusions significantly change the picture of the mechanism of skeletal muscle contraction.

View Article: PubMed Central - PubMed

Affiliation: Department of Scienze Biomediche e Chirurgiche Specialistiche, Ferrara University, Via Borsari 46, 44121 Ferrara, Italy. enrico.grazi@unife.it.

ABSTRACT
Force and length steps, applied to a muscle fiber in the isometric state, are believed to synchronize attached cross-bridges. This alleged synchronization facilitates the interpretation of the experiments. A rapid force step elicits an elastic response of the attached cross-bridges, followed by an isotonic phase. The decay of this second isotonic phase is of the first order. This excludes that the attached cross-bridges may decay all at the same time. The change of the X-ray interference distance during the second phase measures the stroke size only in the unrealistic case that the cross-bridges are and remain all attached. A rapid force step does not synchronize attached cross-bridges. The change of X-ray interference during the second phase does not measure the stroke size. These conclusions significantly change the picture of the mechanism of skeletal muscle contraction.

No MeSH data available.


Related in: MedlinePlus