Cardiac Muscle

Abstract

Cardiac muscle belongs to the striated types of muscle containing the same arrangements of myofilaments as skeletal muscle. However, unlike in skeletal muscle, no syncytia are built but cylindrical muscle cells containing one large cube-shaped nucleus in central position are arranged end to end and join with adjacent cells at special attachment sites, the intercalated disks. Intercalated disks represent the junctions at the boundaries between neighboring cells. Because cardiac muscle cells end in a step-like manner, the intercalated disks show also step-like arrangements with transverse components oriented at a right angle to the myofibrils, and lateral components oriented longitudinally in parallel position to the myofibrils. Cell-to-cell junctions located at the intercalated disks include three types. Fasciae adhaerentes (adhering junctions) are always located in the transverse parts of the intercalated disks. These are the sites where the actin filaments of the terminal sarcomeres are anchored and connected with the plasma membrane. Constituents and organization of the fasciae adhaerentes can be compared with those of the adhering zone (belt desmosome) in epithelia (cf. Fig. 98). Maculae adhaerentes (spot desmosomes, cf. Fig. 101), found in both the transverse and lateral parts of the intercalated disks, reinforce the fasciae adhaerentes and fix the adjacent cells to one another. Gap junctions (nexus, communicating junctions, cf. Fig. 99) are confined to the lateral parts of the intercalated disks. Gap junctions permit ionic traffic between the neighboring cells and provide the base for functional cell coupling. Cardiac muscle behaves as a functional syncytium, although it is composed of individual cells. At the lateral regions of the intercalated disks, gap junctions are protected from forces during contraction.