Recent X–ray diffraction studies of muscle contraction and their implications

Abstract

Recent studies on the interference fringes in the myosin meridional reflections provide a new source of structural information on cross–bridge movement during mechanical transients and steady shortening. Many observations can be interpreted satisfactorily by the tilting lever–arm model, with some assumptions, including the presence of fixed repeating structures contributing to the M3 and higher–order meridional reflections. In isometric contraction, the lever arms are oriented near the start of the working stroke, with a dispersion of ca +−20–25°. Upon a rapid release of 10–12 nm, they move to the end of the stroke, with a well–known T2 delay of 1–2 ms. This delay must represent additional processes, which have to occur even in tension–generating heads, or activation of attached heads, which initially do not develop force. Surprisingly, in muscles shortening at moderate loads (0.5–0.6 P 0 ), the mean position of the heads moves only 2–3 nm closer to the M–line than in the isometric case, reminiscent of the Piazzesi–Lombardi model.