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

Smooth muscle is involuntary and is primarily located in the walls of internal organs, such as those in the digestive tract, blood vessels, and the bladder. This type of muscle functions without conscious control, allowing for processes such as digestion and circulation to continue autonomously.

Smooth muscle fibers are non-striated, which distinguishes them from skeletal and cardiac muscles. Skeletal muscle, the type that is under voluntary control, is responsible for movement of the skeleton. Cardiac muscle, while also involuntary and specific to the heart, is distinct in its structural characteristics and function, which focus primarily on the rhythmic contraction needed to pump blood.

Additionally, the term "striated muscle" encompasses both skeletal and cardiac muscle due to their striped appearance under a microscope, but it does not include smooth muscle. Therefore, smooth muscle is the only type identified here that is both involuntary and found within the walls of internal organs.