How does neuron stimulation work?
The electric field of an external electrode provokes the opening of sodium channels, which in turn has the effect that the positive charged sodium ions travel into the cell body and invoke the wanted change to the Membran potential…

reference: Simulation Neuronaler Netze [("Simulation of Neural Networks")] / Andreas Zell. - Bonn; Paris; Reading, Mass [u.a.]: Addison-Wesley, 1994, first edition