Which type of muscle is responsible for involuntary movements such as heartbeat?

Cardiac muscle is specifically designed for the involuntary actions that keep the heart functioning, such as the rhythmic contraction and relaxation necessary for pumping blood throughout the body. This type of muscle is unique because it contains striations similar to skeletal muscle, but unlike skeletal muscle, which is under voluntary control, cardiac muscle operates automatically without conscious thought. The autonomic nervous system regulates its activity, allowing it to respond to the physiological needs of the body, such as increasing heart rate during exercise or stress.

In contrast, striated muscle refers generally to both skeletal and cardiac muscles, but it does not specifically identify the involuntary aspect of cardiac muscle. Skeletal muscle is responsible for voluntary movements and is consciously controlled. Lastly, voluntary muscle refers to muscles that you can directly control, which does not include the heart. Therefore, the most accurate answer for involuntary movements such as heartbeat is indeed cardiac muscle.