Striation Patterns in Active and Passive Shortening of Muscle

Abstract

IT is now widely agreed that changes in length in vertebrate striated muscle, whether due to passive stretch and release or to active contraction, take place with little or no alteration in the length of the A bands (about 1.5µ), so long as the repeat distance of the striation pattern does not fall much below 2µ, about the minimum value in a relaxed isolated fibre. On further shortening, however, discordant results have been reported. The sequence of formation of contraction bands described by Jordan¹ in fixed and stained preparations was seen by A. F. Huxley and Niedergerke^2,3 in living isolated fibres and by H. E. Huxley and Hanson⁴ in separated myofibrils, and was taken by these authors as evidence that contraction takes place by a relative sliding motion of two sets of filaments. On the other hand, later experiments, also on isolated fibres, by A. F. Huxley and Niedergerke^2,3 and several electron microscope investigations^5,6 have shown progressive shortening of the A bands without disappearance of the I band.