What are myofibrils and what structures do they contain?

Myofibrils are highly organized structures found within muscle cells, specifically within skeletal and cardiac muscle fibers. They are the contractile elements of muscle tissue and are composed of long chains of sarcomeres, which are the basic functional units of muscle contraction. Each sarcomere contains overlapping thick and thin filaments made primarily of the proteins actin and myosin, enabling muscle contraction through sliding filament theory.

By understanding that myofibrils consist of sarcomeres, it clarifies their essential role in muscle function. Sarcomeres shorten during muscle contraction, which ultimately results in the movement of muscles. The structural organization of myofibrils is critical for effective contraction and overall muscle function, highlighting their significance in the muscular system. The other options do not pertain to myofibrils or muscle-related structures; therefore, they do not align with the correct identification or function of myofibrils.