Cardiac muscle, like skeletal muscle, appears striated due to the organization of muscle tissue into sarcomeres. A sarcomere is the basic unit of a muscle in both cardiac and skeletal muscle tissue. Cardiac muscles are composed of tubular cardiomyocytes, or, cardiac muscle cells. The cardiomyocyes are then in turn composed of tubular myofibrils, which are repeating sections of sarcomeres. Sarcomeres appear under the microscope as striations, with alternating dark and light bands. Individual sarcomeres are composed of long, fibrous proteins that slide past each other when the muscles contract and relax Figure 1.The two most important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament. Myosin has a long, fibrous tail and a globular head, which binds to actin. The myosin head also binds to ATP, which is the source of energy for muscle movement. Actin molecules are bound to the Z-disc, which forms the borders of the sarcomere.
The names of the various sub-regions of the sarcomere are based on their light or dark appearance when viewed through a light microscope Figure 2. Each sarcomere is bordered by two dark colored bands called Z-discs or Z-lines. These Z-discs are dense protein discs that do not easily allow the passage of light, and thus appear black under the microscope. The area between the Z-discs is further divided into two lighter colored bands of actin at either end called the I-bands, and a darker, grayish band of myosin in the middle called the A-band. Although both cardiac and skeletal muscle are composed of sacromeres and have striations, skeletal muscle is typically linear and longitudinal while cardiac muscle may be branched.
Intercalated discs are complex adhering structures which connect single cardiomyocytes to an electrochemical syncytium. They are mainly responsible for transmission of action potentials during muscle contraction. A syncytium is a multinucleated cell formed from the fusion of multiple single cells. In cardiac muscle, intercalated discs connecting cardiomyocytes to the syncytium support the rapid spread of action potentials and the synchronized contraction of the myocardium. Intercalated discs consist of three types of cell-cell junctions:
- Adherens junctions, which anchor actin filaments
- Desmosomes, which anchor intermediate filaments
- Gap junctions, which are responsible for electrochemical and metabolic coupling.
Gap junctions specifically allow action potentials to spread in the myocardium by permitting the passage of ions between cells, producing depolarization of the cardiac muscle.
Under light microscopy, intercalated discs appear as thin lines dividing adjacent cardiac muscle cells and running perpendicular to the direction of muscle fibers.