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

Smooth muscle is the correct answer because it is characterized as involuntary muscle tissue that is found within the walls of internal organs such as the intestines, blood vessels, and bladder. It operates autonomously, meaning it functions without conscious control, which is essential for the automatic regulation of physiological processes like digestion and blood flow.

This type of muscle tissue is non-striated under the microscope, allowing for sustained, rhythmic contractions that are necessary for organ function. Smooth muscle fibers have a spindle shape, and when they contract, they help to propel substances through the organs they encase.

In contrast, cardiac muscle, while also involuntary, is specifically found only in the heart and is striated, making it functionally distinct from smooth muscle. Skeletal muscle is voluntary and striated, allowing for conscious control over body movements. While striated muscle refers to both skeletal and cardiac muscle types, smooth muscle is clearly differentiated from these due to its unique structure and involuntary functioning.