Humans can consciously control many behaviors, such as blinking, walking, and lifting things, but there are some physiological activities that are generally beyond the control of consciousness, such as heartbeat. Why aren't such basic physiological activities controlled by higher-level neural activities?
Muscles are the tissues that enable behavioral movements. Human muscles are divided into three types: skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscles are directly controlled by the motor cortex of the brain and can be moved voluntarily, also known as voluntary muscles. Cardiac and smooth muscles are not controlled by the motor cortex of the brain and cannot be moved voluntarily, also known as involuntary muscles. Cardiac muscle controls heartbeat. In the upper part of the right atrium of the heart, there is a small structure called the sinoatrial node. The myofibers in the sinoatrial node can automatically generate rhythmic excitations to produce action potentials, which are transmitted to other parts of the heart muscle and cause the heart to contract. Therefore, the sinoatrial node is called the pacemaker of the heart. Although the heart is not directly controlled by the motor cortex of the brain, it is controlled by the sympathetic and parasympathetic nerves. The sympathetic and parasympathetic nerves are collectively called the autonomic nervous system. Sympathetic nerve endings secrete adrenaline, which speeds up heartbeat and raises blood pressure; parasympathetic nerve endings secrete acetylcholine, which slows down heartbeat and lowers blood pressure. Together, the sympathetic and parasympathetic nerves work to maintain normal and stable heartbeat and blood pressure.
Therefore, basic physiological activities such as heartbeat are regulated by the autonomic nervous system and are generally beyond the control of human consciousness.