Testing the excitability of axons can provide insights into the ionic mechanisms underlying the pathophysiology of axonal dysfunction in human neuropathies and motor neuron diseases. Threshold tracking, which was developed in the 1990s, non-invasively measures a number of axonal excitability indices, which depend on membrane potential and on the Na+ and K+ conductances. This paper reviews recent advances in ionic-pathophysiological studies in human subjects in vivo. Membrane potential of human axons can be estimated, because most of the ion channels expressed on the axolemma are voltage-dependent, and patterns of changes in multiple excitability indices can suggest whether axons are depolarized or hyperpolarized. This has been clearly demonstrated in a single patient with acute hypokalemia (hyperpolarization) and patients with chronic renal failure (depolarization due to hyperkalemia). Muscle cramps / fasciculations arise from hyperexcitability of the motor axons. The enhanced excitability can result from altered ion channel function; an increase in persistent Na+ conductance, a decrease in accommodative K+ conductance, and focal membrane depolarization, all of which increase excitability, have been demonstrated in amyotrophic lateral sclerosis or other disorders affecting lower motor neurons. Patients with demyelinating neuropathy often complain of muscle fatigue. During voluntary contraction, the activation of the electrogenic Na+-K+ pump and resulting membrane hyperpolarization can cause activity-dependent conduction block when the safety factor for impulse transmission is critically reduced. Studies of ion-channel pathophysiology in human subjects have recently begun. Investigating ionic mechanisms is of clinical relevance, because once a specific ionic conductance is identified, blocking or activating it may provide a new therapeutic option for a variety of neuromuscular diseases.