Spacing Changes in the Actin and Myosin Filaments during Activation, and Their Implications
Small but important axial spacing changes have been found to occur in the X-ray reflections from both the actin and myosin filaments when a muscle changes from the resting state to a state of isometric contraction1,2. The overall spacing change in the actin is an increase of approximately 0.25–0.30%, measured on the first actin meridional reflection at 27.3 Å. This implies a length increase of approximately 2.5–3.0 nm in the actin filaments in each half-sarcomere. This is an appreciable fraction of the total compliance measured in quick release experiments3, and therefore has a considerable effect on their interpretation.
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