threshold tracking techniques test nerve excitability, and depend on the properties of axonal membranes and sites of stimulation. they are extremely sensitive to the membrane potential and changes in this potential. these tests can measure and compare a control threshold (or resting threshold) to a threshold produced by a change in the environment, by a preceding single impulse, an impulse train, or a subthreshold current. measuring changes in threshold can indicate changes in membrane potential, axonal properties, and/or the integrity of the myelin sheath.