Final answer:
Smooth muscles are characterized by their ability to sustain contractions and maintain function when stretched, absence of sarcomeres, resistance to fatigue, and involuntary control. They are found in the walls of organs and can contract as a single unit or as independent cells.
Step-by-step explanation:
The characteristics of smooth muscles include their ability to sustain very long-term contractions and maintain contractile function when stretched. Unlike skeletal and cardiac muscles, smooth muscles do not contain sarcomeres, which are the structural units of muscle fibers responsible for striation patterns in other muscle types. Instead, smooth muscles have myofibrils with actin and myosin contractile proteins, arranged in a manner that allows these muscles to contract without being arranged in sarcomeres. They are known for being resistant to fatigue, as they can maintain low-level contractions for long periods due to their unique structural properties.
Smooth muscle fibers are involuntary and found within the walls of organs such as the digestive tract and blood vessels, aiding in moving substances through the body. Single-unit smooth muscle, the more common type, has fibers joined by gap junctions so that the muscle contracts as one unit. Multiunit smooth muscle, on the other hand, doesn't possess gap junctions, and therefore, each muscle cell contracts independently.