Pacemaker cells: why your heart beats by itself, without your brain

Who tells your heart to beat? Most people would say the brain. A heart transplant says no.

Doctors can move a heart into a new body. Every nerve between it and the brain is cut. Still, the heart beats there, all by itself.

The beat starts inside the heart, in a tiny patch of the right atrium. Its cells are called pacemaker cells, and the patch is called the SA node.

A pacemaker cell never rests. Electric charge slowly builds up inside it, all by itself. When the charge reaches a certain level, the cell fires.

The spark jumps from cell to cell, across the whole heart. Each muscle cell it reaches squeezes. That is one heartbeat.

Then the charge falls back and starts to build again. Fire, reset, fire, again and again, with nobody asking.

So what does your brain do? It does not start the beat. It turns the speed up or down. Left alone, the SA node fires about 100 times a minute. At rest, a nerve called the vagus nerve holds it back, like a brake.

On a fast song, the brake comes off and other nerves push: your heart races. A heart that works this hard needs a lot of food and oxygen. Next: how it gets them.