Found out that they just add 6 nS or 67 nS to ge, gi when a neuron spikes. Checked that this yields Ie, Ii on the order of 10^-7 mA when it spikes, and 0 mA otherwise. Ileak is on the order of 10^-4 mA and Ina is on 10^-1 mA. So this is too small to make it spike when there are only a few inputs, but with 4000 cells apparently there is enough. I confirmed this by running the simulation with only 40 cells. There was almost no spontaneous firing except at initializaion.