<P> The functions of T - tubules include rapidly transmitting electrical impulses known as action potentials from the cell surface to the cell's core, and helping to regulate the concentration of calcium within the cell in a process known as excitation - contraction coupling . </P> <P> The cardiac syncytium is a network of cardiomyocytes connected to each other by intercalated discs that enable the rapid transmission of electrical impulses through the network, enabling the syncytium to act in a coordinated contraction of the myocardium . There is an atrial syncytium and a ventricular syncytium that are connected by cardiac connection fibres . Electrical resistance through intercalated discs is very low, thus allowing free diffusion of ions . The ease of ion movement along cardiac muscle fibers axes is such that action potentials are able to travel from one cardiac muscle cell to the next, facing only slight resistance . Each syncytium obeys the all or none law . </P> <P> Intercalated discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium (in contrast to the skeletal muscle, which becomes a multicellular syncytium during mammalian embryonic development). The discs are responsible mainly for force transmission during muscle contraction . Intercalated discs consist of three different types of cell - cell junctions: the actin filament anchoring adherens junctions, the intermediate filament anchoring desmosomes, and gap junctions . They allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle . However, novel molecular biological and comprehensive studies unequivocally showed that intercalated discs consist for the most part of mixed - type adhering junctions named area composita (pl . areae compositae) representing an amalgamation of typical desmosomal and fascia adhaerens proteins (in contrast to various epithelia). The authors discuss the high importance of these findings for the understanding of inherited cardiomyopathies (such as arrhythmogenic right ventricular cardiomyopathy). </P> <P> Under light microscopy, intercalated discs appear as thin, typically dark - staining lines dividing adjacent cardiac muscle cells . The intercalated discs run perpendicular to the direction of muscle fibers . Under electron microscopy, an intercalated disc's path appears more complex . At low magnification, this may appear as a convoluted electron dense structure overlying the location of the obscured Z - line . At high magnification, the intercalated disc's path appears even more convoluted, with both longitudinal and transverse areas appearing in longitudinal section . </P>

What are the characteristics of the cardiac muscle