When doing traditional electro-acupuncture the negative lead will typically be felt more than the positive lead. This is because negative charge stimulates firing on the nerve. You can change this by decreasing the current density at the negative lead. The easiest way to do this is by using an electrode pad instead of a needle at the negative lead. I used this method in my anti-inflammation electro-acupuncture (AIEAC) treatment to get the sensation to the inflamed area where the pain is. That way I would get additional pain releaf from the electrical sensation at the inflamed area. Also it turns out it also dumps the negative charge where it does the least harm (on the skin, especially at the lower arm or leg). The electrons come from two places.
Now when you stick a needle in past the skin, you will cause electrons to accumulate under the skin where the needle is placed. First, they come from the surface of the skin where they naturally reside due to the capacitance of the skin. Earthing will increase the charge accumulation. The ions will go through the needle (and even through the hole after the needle is removed). Second, the electrons also come from the formation of structured water around the needle that naturally occurs. From the charge separation that occurs with EZ formation. the needle will become negatively charged. All this negative charge will cause the nerve endings near where the needle is placed to increase its firing rate. So, here the autonomic effects of acupuncture join with the bioelectric effects.