In the fibre there are densely packed myofibrils that are not always aligned. There are several obstacles to the t-system forming the required network for this function. This arrangement effectively solves a surface area to volume problem for the fibre, allowing the fast and uniform propagation of excitation to these sites virtually simultaneously, reducing the delay to Ca 2+ release activation to the briefest possible time ( Melzer et al., 1995). This membrane system reaches the Ca 2+ release sites on the repeating contractile unit of the fibre, the sarcomere. The tubular (t-) system of skeletal muscle is an internalization of the plasma membrane and remains continuous with the extracellular space. Under osmotic stress the distribution of tubular diameters shifted significantly in toad muscle only, with change specifically occurring in the transverse but not longitudinal tubules. Mean diameters of tubules were 85.9☖.6 and 91.2☘.2 nm, from rat and toad muscle under isotonic conditions, respectively. Furthermore local fluorescence intensity variations from sub-resolution tubules were converted to tubule diameters. There was a high density of tubules close to the nuclear envelope because of the close and parallel alignment of the long axes of the myofibrils and the nuclei. These images showed complex arrangements of tubules that have not been described previously and also allowed the association of the t-system with cellular organelles to be visualized. In this study we serially imaged fluorescent dye trapped in the t-system of fibres from rat and toad muscle using the confocal microscope, and deconvolved and reconstructed these images to produce the first three-dimensional reconstructions of large volumes of the vertebrate t-system.

However, we currently do not have a very detailed view of this membrane network because of limitations in previously used imaging techniques to visualize it. Hence the t-system forms a complex and dense network throughout the fibre that is responsible for excitation–contraction coupling and other signalling mechanisms.

To ensure a uniform spread of excitation throughout the fibre to all the Ca 2+ release sites, the muscle internalizes the plasma membrane, to form the tubular (t-) system. In response to excitation there must be a rapid and uniform release of Ca 2+ throughout for contraction. Skeletal muscle fibres are very large and elongated.

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