What type of muscle responds automatically to brain signals?

The correct answer is involuntary muscle. This type of muscle operates without conscious control, responding automatically to signals from the brain and the autonomic nervous system. Involuntary muscles are crucial for numerous essential functions in the body, such as digestion and circulation. Unlike skeletal muscle, which requires deliberate thought for movement, involuntary muscles contract automatically to perform vital functions, such as the movement of food through the digestive tract or the beating of the heart.

While cardiac muscle, which is specifically found in the heart, is also considered involuntary, the term "involuntary muscle" encompasses this category along with smooth muscle found in various organs. Thus, the distinction lies in recognizing that involuntary muscles, including both smooth and cardiac, respond automatically to stimuli without requiring conscious thought, making this answer the most accurate in the context of the question.

Skeletal and voluntary muscles, on the other hand, rely on conscious brain signals for their movement, which sets them apart from involuntary ones.