What stimulates the heartbeat in the human body?

The sinoatrial (SA) node is known as the heart's natural pacemaker. It is located in the right atrium of the heart and is responsible for initiating the electrical impulses that stimulate the heartbeat. These impulses cause the heart muscles to contract and pump blood throughout the body. The SA node generates signals at regular intervals, resulting in a consistent heart rhythm.

While other components of the heart's conduction system, like the atrioventricular (AV) node, play important roles in conducting the heartbeat signal, they do not initiate it. The vagus nerve influences the heart rate by slowing it down during rest, but it does not stimulate the heartbeat directly. The hypothalamus is involved in numerous bodily functions, including temperature regulation and hormonal control, but it is not directly responsible for stimulating heartbeats. Hence, the SA node is the correct answer as it is the primary pacemaker of the heart.