Nonsteady motion in unloaded contractions of single frog cardiac cells.

We studied the mode of shortening of enzymatically isolated single frog cardiac cells with a high-speed videosystem to see whether or not shortening is smooth. The segmental shortening of the cell in response to electrical stimulation exhibited a clear pause following the initial shortening over a distance of approximately 11 nm/half-sarcomere. Several preparations showed a second pause following the initial one. Nonsteady motion with a pause lasted usually a few tens of milliseconds. The duration of nonsteady motion was shorter in cells with large velocities of steady shortening following the pause than those with smaller velocities

Stepwise dynamics of connecting filaments measured in single myofibrillar sarcomeres.

Single relaxed myofibrils of bumblebee flight muscle were subjected to motor-imposed ramp-length changes. The image of the striations was projected onto a linear photodiode array, and sarcomere length was computed as the spacing between centroids of contiguous A-bands. Centroid position was determined by integrating the respective A-band intensity peak and computing the location at which the area on one side was equal to the other. The resulting trace of centroid to centroid span versus time was stepwise, with periods of rapid shortening alternating with periods of pause. An alternative nondiscrete sensor gave similar steps. If thick filament length remains constant, stepwise sarcomere length changes imply that length changes in the connecting filament must be stepwise. Thus, shortening of the connecting filament occurs as a sequence of discrete events rather than as a continuous event

Discrete sarcomere length distribution in skeletal muscle.

We analyzed the microstructure in the first-order laser diffraction line from both resting and tetanically contracting single twitch fibers from frog anterior tibial muscle to see if the distribution of sarcomere lengths is continuous or discrete. Measuring the distance between adjacent microstructural elements lying parallel, we plotted a histogram of the corresponding differences of sarcomere length. The histograms obtained both from resting and contracting fibers had a prominent peak at approximately 12-14 nm. The result suggests that the sarcomere length distribution may be discrete with unit separation of approximately 12-14-nm sarcomere length