Final answer:
Contractile proteins like actin and myosin are aligned in skeletal and cardiac muscles, causing striations, but not in smooth muscles which lack the organized sarcomere structure.
Step-by-step explanation:
The contractile proteins are aligned in skeletal and cardiac striated muscles but not in smooth muscles. This difference is due to the microscopic organization of their contractile proteins, actin, and myosin. In both skeletal and cardiac muscles, these proteins are organized into a regular pattern in the cytoplasm of muscle cells, which are called fibers. This organization creates a striped pattern known as striations, which are visible under high magnification.
In contrast, smooth muscle cells do not have striations. This is because they lack the sarcomeres that account for the banded appearance of striated muscle fibers. Instead, in smooth muscles, the actin and myosin filaments are arranged alternatively and staggered, allowing for sustained, long-term contractions and maintaining contractile function even when stretched.