ABSTRACT
Cardiac cells respond characteristically to applied electrical currents. If an electrical stimulus has an intensity that is above a certain threshold, ionic channels are activated and the cell generates an active response called an action potential, and the cell is excited. If the electrical stimulus is below threshold, cells respond passively because no ionic channels are activated. The action potential is a measurement of the variation over time of the electrical potential across the cell membrane after the cell has been excited. Changes in membrane potential are a result of the flow of ions through proteins that are embedded in the cell membrane and form ionic channels with conductivity that is, in general, voltage and time dependent.
