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A cytoskeletal periodicity in striated muscle
... Even under optimal instrumental conditions, i.e., using oiled lenses with numerical apertures of at least 1.25, green light, and Koehler illumination or normal incidence, resolution of the sarcomere repeat through diffracted intensity is limited to -400 nm, or three to four orders of the sarcomere spacing (Fig. 16). In Fourier transforms, calculated from electrondensity distribution, the scaled intensities of the 6th through the 11th orders of the sarcomere repeat are extremely low, indicadng that opdcal resolution is effectively limited by the distribution of material in the sarcomere, to -400 nm (Cooke and Meek, 1983). This circumstance could explain the absence of ...
The microscopic structure of striated muscle has been examined for more than three centuries. Within this time span, progress in understanding the structural basis of contraction has been marked by at least two major developments: one was the deduction of a fundamental repeating unit, the sarcomere, from a complex series of microscopic bands, and another was the recognition that active shortening of the sarcomere repeat occurs through the action of mechanical crossbridges, operating between two constituent sets of sliding filaments. These two ideas traced macroscopic muscular movement through shortening of a microscopic sarcomere to some as yet unresolved motion taking place within a molecular complex between the major structural proteins, myosin and actin. During the course of this millionfold increase in spatial resolution of the contractile event, resulting in a sharp contemporary focus on a cross-bridge mechanism, some unique structural properties of striated muscle have been overlooked. One outstanding example is the very uniform long-range spatial regulation of the contractile apparatus that provides the actual physical framework for contraction.
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