The mammalian heart is myogenic and it is regulated by nerves. The heartbeat originates from

The mammalian heart is myogenic, meaning the impulse for contraction is generated within the heart muscle itself rather than by external nerve stimulation. The heartbeat originates from the sinoatrial (SA) node, which is known as the heart's natural pacemaker [1]. Located in the upper part of the right atrium, the SA node consists of specialized myocardial cells with the intrinsic ability to depolarize rhythmically and initiate action potentials [2]. These electrical impulses then spread through the atria and travel to the atrioventricular (AV) node, eventually reaching the ventricles to trigger contraction [1]. While the heart is myogenic, its rate can be regulated by the nervous and endocrine systems to meet the body's oxygen demands [1]. In contrast, the QRS wave and T wave are components of an ECG representing ventricular depolarization and repolarization, respectively, rather than the origin of the beat.

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK537194/
2. [2] https://www.sciencedirect.com/topics/neuroscience/heart-ventricle-conduction