Nerve excitability has been studied for a longer time than has nerve conduction. Prof. Joseph Bergmans pioneered the use of threshold current measurement for studying axonal excitability. However nerve excitability testing has not been used because of technical difficulties. Prof. Hugh Bostock developed a semiautomatic program called QTRAC. This program enables the examiner to perform various nerve excitability tests within 15 min. Nerve excitability testing can provide information regarding physiological conditions, ion channels and the functions of energy-dependent pumps in normal controls and in individuals with diseases such as amyotrophic lateral sclerosis (ALS) and multifocal motor neuropathy (MMN). For understanding threshold tracking, information regarding basic physiological principles including the function of ion channels is required. This review provides important insights on the function of axonal ion channels and investigational methods.