Tracking Techniques
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.
Threshold tracking allows for the strength of a test stimulus to be adjusted by a computer in order to activate a defined fraction of the maximal nerve or muscle potential. A threshold tracking experiment consists of a 1-ms stimulus being applied to a nerve in regular intervals. The action potential is recorded downstream from the triggering impulse.The stimulus is automatically decreased in steps of a set percentage until the response falls below the target (generation of an action potential). Thereafter, the stimulus is stepped up or down depending on whether the previous response was lesser or greater than the target response until a resting (or control) threshold has been established. Nerve excitability can then be changed by altering the nerve environment or applying additional currents. Since the value of a single threshold current provides little valuable information because it varies within and between subjects, pairs of threshold measurements, comparing the control threshold to thresholds produced by refractoriness, supernormality, strength-duration time constant or “threshold electrotonus” are more useful in scientific and clinical study.
Tracking threshold has advantages over other electrophysiological techniques, like the constant stimulus method. This technique can track threshold changes within a dynamic range of 200% and in general give more insight into axonal properties than other tests. Also, this technique allows for changes in threshold to be given a quantitative value, which when mathematically converted into a percentage, can be used to compare single fiber and multifiber preparations, different neuronal sites, and nerve excitability in different species.
Read more about this topic: Threshold Potential
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