Stepwise shortening: evidence and implications.

The observation that sarcomeres shorten in steps has proved controversial. On the one hand, the phenomenon implies that the contractile process cannot be based on a molecular mechanism that behaves in a random manner: The fact that the steps and pauses characterize the kinetics of large volumes of tissue implies that the elements comprising such volumes must stop and pause synchronously. On the other hand, since current contractile models do not anticipate synchronized behavior, there has been considerable speculation that the phenomenon might not be a genuine feature of contraction, but an instrument-based artifact. We present here a review of observations made with four methods that have been brought to bear on the question. All four show discrete, synchronized contractile behavior. The observation of steps with multiple independent methods implies either that each technique harbors its own " gremlin " that generates spurious steps and pauses of a similar nature, or that the phenomenon is genuine. Finally, some consistent properties of the distribution of step size are considered with respect to possible molecular models.