initial experiments revolved around the concept that any electrical change that is brought about in neurons must occur through the action of ions. the german physical chemist walther nernst applied this concept in experiments to discover nervous excitability, and concluded that the local excitatory process through a semi-permeable membrane depends upon the ionic concentration. also, ion concentration was shown to be the limiting factor in excitation. if the proper concentration of ions was attained, excitation would certainly occur. this was the basis for discovering the threshold value.