Conformations of crossbridges in contracting skeletal muscle

Conformations of crossbridges during isometric contraction were observed and compared with those in rigor state. Rabbit skeletal muscle fibers were chemically skinned, rapidly frozen during isometric contraction or in the rigor state, then freeze-substituted. Longitudinal and cross sections were observed. Crossbridges in contracting fibers are arranged on the thin filament, retaining a periodicity of approximately 14.3 nm with some variability, while crossbridges in rigor fibers tend to attach to actin target zones which have a periodicity of approximately 36 nm. Rigor crossbridges are wide near the thin filament; crossbridges in contracting fibers are thinner and have more uniform width from the proximal region to the distal region. The majority of the mass of the crossbridges during contraction is not as close to the thin filament as in rigor fibers. Shapes of crossbridges during contraction are variable, especially when viewed along the filament axis.