Figure 2.

The monomer spacing of an actin filament stretched during fully developed isometric tension. (a) Electron micrograph (H. E. Huxley, unpublished) showing interdigitated actin and myosin filaments in frog (skeletal) sarcomere lattice. (b) Detail from part (a), showing the positions of myosin heads attached to actin filaments (crossbridges), denoted by red dashed lines. (c) The piecewise change in spacing between neighboring actin monomers coincides with the locations of myosin molecules bound to actin sites. Here, interactions are shown between only one myosin filament with one actin filament [red dashed lines between panels (b) and (c)], but in the hexagonal lattice shown in the band of part (a), three myosin filaments will interact with each actin filament so the piecewise changes in spacing will be about three times more frequent than shown. Note that the spacing typically increases from the free end of an actin filament towards the z axis, mimicking the increase in the tensile force in actin filaments. On some rare occasions, however, the spacing decreases reflect the effect of compressive forces in bound crossbridges. (d) The displacements of actin monomers relative to the position of the actin free end.