Threshold potential
A neuron's resting membrane potential (–70 mV) can be altered to either increase or decrease likelihood of reaching threshold via sodium and potassium ions.
The German physical chemist Walther Nernst applied this concept in experiments to discover nervous excitability, and concluded that the local excitatory process through a semi-permeable membrane depends upon the ionic concentration.
Along with reconstructing the action potential in the 1950s, Alan Lloyd Hodgkin and Andrew Huxley were also able to experimentally determine the mechanism behind the threshold for excitation.
Since the experiment yielded results through the observation of ionic conductance changes, Hodgkin and Huxley used these terms to discuss the threshold potential.
[4] At resting level, on the other hand, the potassium and sodium currents are equal and opposite in a stable manner, where a sudden, continuous flow of ions should not result.
The basis is that at a certain level of depolarization, when the currents are equal and opposite in an unstable manner, any further entry of positive charge generates an action potential.
Normal functioning of the central nervous system entails a summation of synaptic inputs made largely onto a neuron's dendritic tree.
The loss of positive(+) charges of the potassium(K+) ions from the inside of the cell results in a negative potential there compared to the extracellular surface of the membrane.
If successful, the sudden influx of positive charge depolarizes the membrane, and potassium is delayed in re-establishing, or hyperpolarizing, 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.
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.
Threshold tracking techniques test nerve excitability, and depend on the properties of axonal membranes and sites of stimulation.
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.
The stimulus is automatically decreased in steps of a set percentage until the response falls below the target (generation of an action potential).
This technique can track threshold changes within a dynamic range of 200% and in general give more insight into axonal properties than other tests.
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".
A febrile seizure, or "fever fit", is a convulsion associated with a significant rise in body temperature, occurring most commonly in early childhood.
[15] With patch clamp recording, an analogous state was replicated in vitro in rat cortical neurons after induction of febrile body temperatures; a notable decrease in threshold potential was observed.
The mechanism for this decrease possibly involves suppression of inhibition mediated by the GABAB receptor with excessive heat exposure.
[20] Clinically therapeutic use of these extracts remains a subject of research, but a strong correlation is established between regular consumption of fish oil and lower frequency of hospitalization for atrial fibrillation, a severe and increasingly common arrhythmia.
