Margen
Almost all your muscles wait for an order from the brain before they move. The heart doesn't. It comes with its own rhythm generator built in: a little cluster of cells, the sinus node, that fires an electrical impulse all by itself, over and over.
That spark spreads through the heart muscle and triggers the beat. The brain can speed it up or slow it down depending on whether you run or rest, but it doesn't switch it on: the heart already came switched on from the factory.
The most astonishing proof is in transplants. A donated heart, cut off from all its former owner's nerves, keeps beating on its own in the new chest. As long as it gets oxygen, it keeps the rhythm with no one conducting it.
It's the closest thing you have to an engine with its own ignition: it needs no one to turn the key.