system
Given the following post, respond as if you're commenting on Reddit:
human
What causes the first heartbeat?

What causes the heart to beat it's first pulse? Is there an electric pulse from the mother to jumpstart the heart when it is ready?
gpt
Heart muscle cells are unique in that they are the only muscle cells that can self-excite. They will beat on their own in an intrinsic rhythm that does not require outside stimulus. Basically, as soon as the first heart cells are formed in the embryo, they will start beating immediately even if the heart isn't formed yet.  
You may have gotten the impression that hearts need to be jump started by depictions in TV or other media, where a patient 'flatlines' and then an electric shock is used to 'jumpstart' their heart.  
In reality, that doesn't happen. The electric shock cannot start a heart - it stops it. It is used to 'stun' an heart that is fibrillating, that is a heart whose cells want to beat, but they are all out of sync so that there is no contraction. In that case an electric shock can stop them all briefly, giving an opportunity for the main signal source of the heart - the sinus node - to take over again and get an organized rhythm. In a 'flatline', called asystole, there is nothing to shock. There is no electrical activity in the heart and zapping it does nothing.