To maintain the potential accross the neurons, there are three ion gateway channels that are used namely - sodium, potassium, calcium. However, calcium channels are very few. The probable reason of having only sodium and potassium channels is that Na and K are Group 1 elements, so it is easy to remove an electron from them and make them positively charged, due to their low ionisation potential. Since less energy is expended in this procedure, we have evolved in this manner of having only Na and K channels in axon.