Which type of muscle responds automatically to signals from the brain and cannot be controlled consciously?

The correct answer is involuntary muscle. This type of muscle operates automatically without conscious control, meaning it responds to signals from the brain and autonomously regulates functions within the body. Involuntary muscles are essential for processes such as digestion and heart function, and they include smooth muscle found in the walls of organs and blood vessels, as well as cardiac muscle, which makes up the heart.

While cardiac muscle is indeed a form of involuntary muscle, the broader category referred to in the question is involuntary muscle itself, which encompasses both cardiac and smooth muscles. This distinction is crucial; though cardiac muscle is specific to the heart, the term "involuntary muscle" includes all muscle types that function without conscious thought.

Skeletal muscle, on the other hand, is the type of muscle that is under voluntary control, allowing for conscious movements such as walking or lifting objects. Striated muscle refers to a structural characteristic found in both skeletal and cardiac muscles, emphasizing their striped appearance under a microscope, but it does not determine the control mechanism of the muscle. Thus, involuntary muscle is the most accurate choice when considering the definition related to non-conscious control.