how does our heart muscle move without manual control from ourselves?
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The basic idea is there’s always a pacemaker, some clump of cells that fire on a predictable repeating cycle that equals a typical heart rate. The signal is passed on through a series of what are basically electrical wires ensuring the different parts of the heart squeeze and relax in the right order.
Normally, the SA node high in the heart is the pacemaker, with your brain sending signals based on fight/flight and exercise demand to ramp heart rate up or down. Having a heart beat is really important, though, so even if the brain says nothing or those nerves are severed, the SA node will still keep a steady beat. If the SA node is damaged, other groups of cells further down will take over and keep the beat at a different rate. This is a great series of backups, but it can go awry if the wires from the SA node get damaged as you may have two different sets of pacemakers in different parts of the heart, and therefore an uncoordinated heart that doesn’t beat as effectively.
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