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

Cardiac muscle is an involuntary muscle that is unique to the heart. It has distinct characteristics that set it apart from other muscle types. Cardiac muscle fibers are striated, similar to skeletal muscle, but they function involuntarily, meaning they operate without conscious control. This involuntary nature is essential for the continuous and rhythmic contractions necessary for pumping blood throughout the body.

In contrast, skeletal muscle is voluntary and requires conscious effort to contract, which is not suitable for the heart's automatic and rhythmic function. Smooth muscle, while also involuntary and found in other systems (like the digestive tract and blood vessels), is not specific to the heart. Striated muscle refers to a category that includes both skeletal and cardiac muscle, but it does not identify the involuntary nature specific to cardiac muscle. Thus, the defining attributes of cardiac muscle as an involuntary muscle located in the heart's walls confirm it as the correct choice.