Which type of muscle is involuntary and found in the walls of internal organs?

Smooth muscle is the type of muscle that is involuntary and is primarily found in the walls of internal organs, such as the gastrointestinal tract, blood vessels, and the bladder. This type of muscle operates automatically without conscious control, which is essential for the functioning of internal processes.

Smooth muscle fibers are non-striated, which differentiates them from skeletal and cardiac muscles. This lack of striations is a key characteristic that helps in identifying smooth muscle under a microscope. Additionally, smooth muscle can contract slowly and sustain prolonged contractions, making it ideal for regulating functions like peristalsis in the digestive system and controlling blood flow in arteries.

In contrast, cardiac muscle, while also involuntary, is specifically found in the heart and has its unique properties, like striations and a rhythmic contraction pattern. Skeletal muscle is voluntary and is attached to bones, allowing for movement under conscious control. Striated muscle typically refers to both skeletal and cardiac muscle, emphasizing the presence of stripes in their structure, which further excludes them from being classified as the involuntary muscle in question.