AlphaNa = 0.1 * f(0.1*(VoltageDelta-10.0))
The limit for $x\to 0$ can actually be interpreted as difference quotient, you might also appeal to l'Hopital, to find that $f(x)\to 1$. So if you want to play it safe, you can employ a 3-tier or 4-tier implementation of this function
if abs(x)<1e-12: ...
Some brief comments, since this is an old question.
Some results on theoretical examination of eigenspectra of random matrices for neural networks: (Rajan & Abbott 2006, Muir & Mrsic-Flogel 2015). Much of random matrix theory doesn't apply however, since graphs of realistic networks are non-Hermitian and can be quite constrained.
In general you can ...