The correct answer is option 1, the sinoatrial node. The mammalian heart is indeed myogenic, which means it has the ability to generate its own electrical impulses and initiate its own heartbeat. These electrical impulses are originated from a specialized region within the heart called the sinoatrial (SA) node.

The SA node, often referred to as the natural pacemaker of the heart, is located in the right atrium. It generates electrical signals at regular intervals, causing the atria to contract. These signals are then conducted to the atrioventricular (AV) node, which serves as a relay point for the signals to reach the ventricles. From the AV node, the signals are transmitted to the muscles of the ventricles, resulting in their contraction.

While the mammalian heart is regulated by nerves, the heartbeat itself originates from the SA node. The nerves control the rate and force of the heartbeat, modifying the signals generated by the SA node. However, the initiation of the heartbeat primarily relies on the myogenic nature of the heart and the activity of the SA node.

Alert - correct answer should be option 1, the sinoatrial node.