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not nervous - autorhythmic myocardial cells

Cool. Is there an explanation as do why it does that?

some cells on the heart depolarise naturally, the point is so that heart beat is not reliant on Central Nervous System pumping every single time. Nerves to the heart modify the rate of the heart's depolarisation instead of cause it. It also means if you have nerve damage to the primary pacemaker site - Sinoatrial node - your heart doesn't stop. What I think is pretty cool is that even after a heart attack that damages you sinoatrial node other parts of the heart become pacemaker sites - usually the atrioventricular node. This means that the heart is 1) protected from failure of the CNS, 2) has multiple failsafes within itself 3) is a pretty cool guy who doesn't afraid of anything.
The way it depolarises itself is some really cool ion channel fuckery that is unique to the heart, if that was what you were asking.

The fact that the human heart has a fail safe just sounds amazing. Does this only apply to humans?

Nah all mammals are pretty much the same and anything else with a recognisable heart will probs be similar, so lizards and fish yes spiders and ants probs not. It makes sense to me, if animals didn't evolve it they would just get heart attacks and die immediately which is not good survival haha

Future Diary then!