Answer
Striations or stripes in muscle tissues are due to overlapping arrangements of thin actin (light) filaments and thick myosin (dark) filaments.. These proteins are not unique to muscle cells and they aid in many functions for example, mitosis, cellular motility and transport of extracellular materials.
Each myofiber ( muscle cell) is filled with a number of long protein cord or myofibrils. Each myofibril is a bundle of parallel protein myofilaments of three kinds --thick filaments, thin filaments and elastic filaments
The dark bands of myosin are called A bands and the light bands of actin are called I bands because of their behavior in polarized lighe: A stands for anistropic and I stands for isotropic. Isotropic material have one refractve index and show the same characteristics when viewed from different durections. Anisotropic substances have more than one refractive index(RI) and evince different properties when viewed from different directions.
Work Step by Step
In the A band of striated muscles dark myosin filaments lie side by side. Where thin and thick filaments overlap the area is very dark because here the thick filament is surrounded by hexagonal pattern of thin filaments.
H region
This is a lighter region of the A band where there are no thin filaments
The M Line:
In the middle of the H region, the thick filaments are joined to one another through a dark transverse protein complex called the M line
Z disc
Each light I band is bisected by a narrow Z line (disc). The Z disc provides anchorage for thin( and elastic) filaments
Sarcomere
Each segment of a myofibril from Z-disc to Z-disc constitutes a sarcomere. The sarcomere is the unit
of contraction of the myofiber.
In the process of muscle shortening( contraction) individual sarcomeres shorten and pull Z discs together, which causes dystrophin and linked peripheral proteins to pull on associated extracellular proteins
As Z discs come closer they tug on the sarcolemma and this results in shortening of the myofiber.
Cardiomyocytes also have striations but they are fainter than those of skeletal muscles. Heart muscle cells have single nuclei; they are branched and separated by intercalated discs with gap junctions and desmosomes which hold the cells together. These cells also have light I bands of actin and dark A bands of myosin. They also have T-tubules that run from the surface of a cell to the interior . These T-tubules are bigger and wider than those found in skeletal muscles. Instead of forming triads, cardiac T-tubules form dyads with terminal cisterna
