Charged batteries, ready to fire
Every heart cell sits like a tiny charged battery. At rest, the inside is slightly negative compared with the outside — this baseline is the resting membrane potential. The charge difference exists because charged particles called ions (mainly potassium, sodium, and calcium) are unevenly spread across the cell membrane, with the cell working constantly to keep them that way.
When the wave of excitation arrives, tiny gates in the membrane snap open. Sodium rushes in and the inside of the cell flips from negative toward positive. That rapid upswing is depolarization — the “fire” of the cell. Then, more slowly, other gates open to let positive charge back out and the cell returns to its resting state; this recovery is repolarization. The whole up-and-back cycle is one cardiac action potential.
A plateau that makes the heart special
Here is what makes heart muscle unusual. In a nerve cell the spike goes up and comes straight back down in a couple of milliseconds. In a working heart muscle cell, after the fast rise the voltage stays high for a while on a long plateau, held up by calcium flowing in. This plateau stretches the action potential out to a couple of hundred milliseconds — long enough to drive a sustained squeeze rather than a flicker.
That incoming calcium is also the bridge between electricity and movement. It triggers the cell’s contractile machinery, a hand-off called excitation–contraction coupling. In other words, the voltage spike you’ve been following is what actually tells the muscle to shorten and pump.
Not all heart cells look the same on the inside. Pacemaker cells in the SA node have no stable resting level at all — they slowly drift upward on their own until they reach threshold and fire again. That self-charging drift is the cellular face of the automaticity you met in the first guide.