[Neuroscience] Fitzhugh-Nagumo Model Query

[Harry Erwin]

Where does the cubic term in the Fitzhugh-Nagumo model come from? I know
it's necessary to get the van der Pol oscillator, but how do they
justify it as a simplification of the Hodgkin-Huxley model?

-- 
Harry Erwin <http://www.theworld.com/~herwin>
My neuroscience wikiwiki is at 
<http://scat-he-g4.sunderland.ac.uk/~harryerw/phpwiki/index.php>