Cardiac cells maintain concentrations of ions that are different
than the concentrations of the same ions outside the cells,
creating both electrical and chemical gradients across the membrane.
In the absence of electrical stimulation, the difference in
ion concentrations across the membrane leaves the membrane in a
polarized state. Upon electrical stimulation, specialized
proteins called ion channels permit the passage of ions
across the cell membrane in response to the membrane potential and
the movement of ions. The resulting time evolution of the membrane potential
is called an action potential. The following applets illustrate how ions diffuse,
how the resting membrane potential is set, and how action potentials
are generated.