Editing Threshold potential (section)

==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.{{sfn|Bostock|Cikurel|Burke|1998|p=137}} 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.{{sfn|Bostock|Cikurel|Burke|1998|p=138}} 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.{{sfn|Burke|Kiernan|Bostock|2001|p=1576}}

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.{{sfn|Bostock|Cikurel|Burke|1998|p=141}} 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.{{sfn|Bostock|Cikurel|Burke|1998|p=141}}

==="Threshold electrotonus"===
A specific threshold tracking technique is ''threshold electrotonus'', which uses the threshold tracking set-up to produce long-lasting subthreshold depolarizing or hyperpolarizing currents within a membrane. Changes in cell excitability can be observed and recorded by creating these long-lasting currents. Threshold decrease is evident during extensive depolarization, and threshold increase is evident with extensive hyperpolarization. With hyperpolarization, there is an increase in the resistance of the internodal membrane due to closure of potassium channels, and the resulting plot "fans out". Depolarization produces has the opposite effect, activating potassium channels, producing a plot that "fans in".{{sfn|Burke|Kiernan|Bostock|2001|p=1581}}

The most important factor determining threshold electrotonus is membrane potential, so threshold electrotonus can also be used as an index of membrane potential. Furthermore, it can be used to identify characteristics of significant medical conditions through comparing the effects of those conditions on threshold potential with the effects viewed experimentally. For example, [[ischemia]] and depolarization cause the same "fanning in" effect of the electrotonus waveforms. This observation leads to the conclusion that ischemia may result from over-activation of potassium channels.{{sfn|Bostock|Cikurel|Burke|1998|p=150}}