Structural arrangements and the contraction mechanism in striated muscle

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Abstract
This review will try to summarize our present conclusions concerning the contractile structure of striated muscle, based on all the evidence, old and new. We have been attempting to give as detailed and as definite a picture of the contractile mechanism as possible, and to specify what properties it must have, what properties it can have, and what properties it does not have. The more concisely we can do this, the more able we will be to design suitable experiments to distinguish between the remaining possibilities for the detailed molecular properties and events involved in contraction, which still remain almost entirely unknown. The ‘double array of filaments’ model for striated muscle, as it was originally propounded (Hanson & Huxley 1953), has remained essentially unchanged. According to this model, the A -bands contain an ordered array of 100 Å diameter filaments, spaced about 450 Å apart and containing myosin, with each filament continuous from one end of the A -band to the other. A second array of thinner filaments, containing actin, extends on either side of the Z -line, through the I -bands and into the A -bands, interdigitating with the array of thick filaments and terminating at the edges of the H -zones. Cross-bridges extend between the thick and thin filaments in the region of overlap.