Intercalated discs

Skeletal and cardiac muscles also have important differences. Skeletal muscle cells are elongated and run the length of the entire muscle furthermore, these cells have no electrical communication between them. Cardiac muscle cells, on the other hand, branch and interconnect with each other. Intercellular junctions found where adjoining cells meet end-to-end are referred to as intercalated discs. Two types of cell-to-cell junctions exist within these discs. Desmosomes hold the muscle cells together and provide the structural support needed when the heart beats and exerts a mechanical... 

Spira AW The nexuses in the intercalated disc of the canine heart Quantitative data for an estimation... 

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural... 

S100A1 is the most abundant S100 protein found in striated muscle and predominates in myocardial tissue (Kato and Kimura, 1985). Besides its cytoplasmic occurrence, S100A1 was reported in these cells to associate with the sarcolemma, sarcoplasmatic reticulum (SR), contractile filaments, intercalated discs, outer mitochondrial membrane and other intracellular membrane stmctures (Arcuri et al., 2002 Donato et al., 1989 Haimoto and Kato, 1988 Sorci et al., 1999). However, the exact location of S100A1 on the contractile elements of the sarcomere is still controversial (Maco et al., 2001 Zimmer, 1991). 

Figure 10.9 Unexpected cross-reactivity with contractile filaments (cross-striations, intercalated discs) in cardiac myocytes. No staining of sarcolemmal cells, interstitial cells or endothelium. See color insert.

As a prerequisite for fine motor control, skeletal muscle fibers are electrically isolated from one another, and, accordingly, do not express either connexin-43 (the major gap junction protein) or N-cadherin (the major adherens protein in cardiac intercalated discs). Asynchronous islands of intramyocardial skeletal muscle can result in lethal arrhythmias in mice [21]. Although cell types such as fetal cardiomyocytes [22] and cardiomyocytes derived from murine or human embryonic stem cells [23] are capable of electromechanical coupling, their clinical use has unfortunately been hampered by technical, ethical, moral, social and legal hurdles. 

Figure 1. (A) Discrete cable model of cylindrical cardiac cells, each 100// in length and 16// in diameter, interconnected by an intercalated disc structure of 80A. (B) Core conductor network utilized for the intercalated disc interaction between two adjacent cells.

It should be mentioned, that the incorporation of the intercalated discs in the model completely changes classical relationships obtained from continuous ( Hodgkin-Huxley ) cable theory. For example, the inverse square root relationship between velocity and axial resistance does not hold, and an increase in accompanies slow conduction caused by high disc resistance. These deviations from classical continuous cable theory are further discussed elsewhere (Diaz et al., 1983 Rudy et al., 1983). 

Diaz P, Rudy Y, Plonsey R (1983) The intercalated discs as a cause for discontinuous propagation in cardiac muscle A theoretical simulation. Ann Biomed Eng 11 177-190 Durrer D, Van Dam R Th, Freud GE, Janse MJ, Meijler FL, Arzbaecher RC (1970) Total escitation of the isolated human heart. Circ 41 899... 

The intercalated disc is composed of three structurally distinct regions (a) the macula adherens or desmosome which is a complex where the central lamella appears to receive filamentous projections from the sarcolemma providing integrity to muscle during contraction, (b) the fascia adherentes or intermediate junction where actin myofibrils terminate, and (c) the nexus or gap junction which provides a low impedance pathway permitting the conduction of APs from cell to cell. 

Although cadherins do not exclusively cluster, the formation of adherens junctions is important for several tissue functions [18]. In epithelial tissues cadherins mediate zonula adherens junctions, which partition the cell membrane into apical and basolateral surfaces. In neuronal cells they mechanically stabilize synaptic junctions by mediating adhesion between the presynaptic and postsynaptic cells. In cardiac myocytes they establish intercalated discs, which both stabilize gap junctional coupling and transmit contractile forces. 

Sjostrand FS, Anderssoncedergren E, Dewey MM (1958) The ultrastructure of the intercalated discs of frog, mouse and guinea pig cardiac muscle. J Ultrastruct Res 1(3) 271-287... 

M. Delmar, F.X. Liang, Connexin43 and the regulation of intercalated disc function. Heart Rhythm 9 (2012) 835-838. 

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