J Muscle Res Cell Motil 1986 Dec 01;76:568-78. doi: 10.1007/bf01753572.

An unusual Z-system in the obliquely striated muscles of crinoids: three-dimensional structure and computer simulations.

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The peculiar functional structure of the Z-line in the obliquely striated muscles of some feather stars is described. It is known that cross-striated muscles are characterized by linear and continuous Z-bands, and obliquely striated muscles by disconnected, obliquely aligned Z-elements. Owing to this discontinuous organization, the sarcomere can perform wide active lengthenings, shortenings, and even ''super-elongations'' in the helical fibres. In contrast, the obliquely striated fibres of crinoids show markedly continuous and homogeneous oblique Z-lines; such a structure is not compatible with ''super-performances'' like sliding and shearing of the sarcomere elements, but instead could allow functions comparable to those characteristic of a cross-striated muscle (quick, short movements, mechanically amplifiable by bone levers). This odd situation, only interpretable in terms of evolutionary constraint, could be considered opposite and symmetrical to that of cross-striated ''super-contracting'' muscles, where the Z-line is exceptionally fragmented to allow the sarcomere to super-contract. The possible architecture of a significant parameter such as the Z-line, which determines muscle fibre potential capacities, is analysed in detail: through qualitative-quantitative evaluation of electron micrographs, supported by statistical analysis of the data; and by computer simulations. The data obtained suggest that the most realistic conformation of the whole Z-complex in these muscles consists of a multiple system of continuous, ribbon-like helical planes running in parallel along the fibre from end to end and regularly cutting it with a constant thickness. The proposed model seems morphologically compatible with the experimentally verified situations and functionally compatible with the mechanical requirements for a normal contraction and for a balanced distribution of the involved strengths.

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Genes referenced: LOC100893907

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