Editing Threshold potential (section)

===Variations===
The value of threshold can vary according to numerous factors.  Changes in the ion conductances of sodium or potassium can lead to either a raised or lowered value of threshold. Additionally, the diameter of the axon, density of voltage activated sodium channels, and properties of sodium channels within the axon all affect the threshold value.{{sfn|Trautwein|1963|p=281}} Typically in the axon or dendrite, there are small depolarizing or hyperpolarizing signals resulting from a prior stimulus. The passive spread of these signals depend on the passive electrical properties of the cell. The signals can only continue along the neuron to cause an action potential further down if they are strong enough to make it past the cell's membrane resistance and capacitance. For example, a neuron with a large diameter has more ionic channels in its membrane than a smaller cell, resulting in a lower resistance to the flow of ionic current. The current spreads quicker in a cell with less resistance, and is more likely to reach the threshold at other portions of the neuron.{{sfn|Nicholls|Martin|Fuchs|Brown|2012|p=121}}

The threshold potential has also been shown experimentally to adapt to slow changes in input characteristics by regulating sodium channel density as well as inactivating these sodium channels overall. [[Hyperpolarization (biology)|Hyperpolarization]] by the delayed-rectifier potassium channels causes a [[Refractory period (physiology)#Neuronal refractory period|relative refractory period]] that makes it much more difficult to reach threshold. The delayed-rectifier potassium channels are responsible for the late outward phase of the action potential, where they open at a different voltage stimulus compared to the quickly activated sodium channels. They rectify, or repair, the balance of ions across the membrane by opening and letting potassium flow down its concentration gradient from inside to outside the cell. They close slowly as well, resulting in an outward flow of positive charge that exceeds the balance necessary. It results in excess negativity in the cell, requiring an extremely large stimulus and resulting depolarization to cause a response.