How much electrical energy is required for intracellular stimulation (firing) of a human neuron? Electrical energy is measured in joules, and equal to voltage multiplied by current. If you don't know energy, you may specify voltage applied and current. Just for example, to capture heart, a pacemaker must discharge 0.1 microjoule of electrical energy.
There might be more parameters involved like pulse width, phase etc. Energy combines all the parameters in one single value. In case you don't know energy level, please specify the defining parameters as well.