Final answer:
Smooth muscle is spindle-shaped, has a single nucleus and contracts in a twisting way. It is characterized by non-striated fibers, making it different from striated muscle, and can sustain long-term contractions essential for the function of hollow organs.
Step-by-step explanation:
Smooth Muscle Characteristics
Among the options provided regarding smooth muscle, the correct statement is that smooth muscle contracts in a twisting way. Smooth muscle cells lack transverse tubules, which is a characteristic of striated muscles such as skeletal and cardiac muscles. These striated muscles are typically larger and more powerful than smooth muscles and have their fibers arranged in sarcomeres, which gives them a striated appearance.
Smooth muscle fibers are unique in that they are spindle-shaped with a single nucleus and can maintain long-term contractions and muscle tone—even when stretched. This is crucial for the function of hollow organs like the stomach or bladder that continuously fill and empty. Furthermore, they do not have the striated appearance of skeletal and cardiac muscle due to the different organization of their contractile proteins, actin, and myosin.
The function of smooth muscle is largely involuntary, controlled by the autonomic nervous system and by the local conditions within the tissues. They are important in the movement of substances through the body, such as food in the digestive tract or blood in the vessels.