Quantal length changes in single contracting sarcomeres

The time course of shortening was investigated in the single sarcomere, the smallest contractile unit that retains natural structure. We projected the striation patterns of single bumblebee flight-muscle myofibrils onto a linear photodiode array, which was scanned periodically to produce repetitive traces of intensity vs. position along the array. Sarcomere length was taken as the span between adjacent A-band or Z-line centroids. When myofibrils were ramp-released by a motor, individual sarcomeres shortened in steps punctuated by pauses. The single sarcomere-shortening trace was consistently stepwise both in activated and relaxed specimens. Although step size was variable, the size distribution showed a signature-like feature: the histogram comprised distinct peaks that were spaced quasi-regularly. In the activated myofibrils the interpeak separation corresponded to 2.71 nm per half-sarcomere. This value is equal to the linear advance of actin subunits along the thin filament. Thus, actin filaments translate over thick filaments by steps that may be integer multiples of the actin-subunit spacing.