A tiny little fetal heart beating peacefully on its own!
That's a classic presentation of cardiac automaticity, which is the property of a specialized cardiac cells called pacemakers to generate spontaneous electrical impulse.
Cells normally capable of generating an impulse are the sinus node and AV node among others, which fire at regular intervals and leads to the beat.
So as you might guess, the cardiac action potential and regulatory mechanisms lies within the heart itself and depends on these intrinsic nodes rather than the CNS.
Furthermore, the fetal myocardium begins to contract rhythmically by 3 weeks after conception, initiated from these spontaneously depolarizing myocardial pacemaker cells.