Anerve signal is transmitted through a neuron when an excess of na+ ions suddenly enters the axon, a long cylindrical part of the neuron. axons are approximately 10.0 μm in diameter, and measurements show that about 5.60×1011 na+ ions per meter (each of charge +e) enter during this process. although the axon is a long cylinder, the charge does not all enter everywhere at the same time. a plausible model would be a series of nearly point charges moving along the axon. let us look at a 0.100 mm length of the axon and model it as a point charge.