This applet shows the membrane potential produced by a cell using the FHN model as a function of time after it is stimulated above its threshold for excitation in the upper plot. The lower plot shows the membrane potential plotted in phase space (state space), where the axes are the two variables u and v. The red curve is the function du/dt=0 and the green line is the function dv/dt=0. Note that the membrane potential closely follows portions of the red curve during the action potential plateau and during rest, while it rapidly jumps from one branch to the other during the action potential upstroke and during repolarization.
To use the applet, click the Start button to initiate the simulation and the Reset button to return the parameters to their initial values.
Things to try:
The FHN model can be found in
FitzHugh R. (1961) Biophys J 1, 445-466.
or more information can be found, for example, in
Courtemanche M, Skaggs W, Winfree AT. (1990) Physica D 41, 173.