This applet shows the V vs t and phase space trajectory for a given initial condition. Try different initial conditions by clicking in different places of the phase space.
Some things to do:
Blue lines/arrows are positive derivatives while pink are negative.
Chage epsilon to .1 or .5 and observe the slower motion around the nullclines.
Change delta from 0 to .1 to convert the system from excitable to auto-oscillatory.