It all begins when the gates open.

Maria has an excellent little vignette about nerve firing (which is very very similar to cardiac cells firing) over at intueri. Here is an excerpt (whose prose is typical of her excellent writing):

Sodium ions flood into the single brain cell through the gates of channels linking the exterior and interior of the neuron. Other channels along the length of the cell follow suit, opening more gates to allow more sodium ions in. A deluge of positively-charged atoms overtakes the single cell.

Other channels follow the precedence, though they selectively permit calcium, not sodium, ions to join the influx. The excess positive charge soon beckons the potassium channels to open and, with gusto, potassium ions flee from the interior of the cell.