How the electrons from electrode play a role in stimulating the neuron?

Question

I am trying to understand how a neuron fires action potential after being stimulated via electrode (extracellular stimulation). GHK equation gives the membrane potential,which is -70 mV when neuron is at rest. When the membrane potential increases to -55 mV, it fires AP. But then an electrode gives electrons, which do not appear in the equation. Then how the electrons from electrode play a role in stimulating the neuron? Also, is there a way to calculate how many electrons or how much current would be required to stimulate a neuron?