Neuroscience / electrophysiology · BEGINNER LESSON
Action potential
An action potential is a rapid depolarizing electrical event in a neuron, followed by repolarization. In these sources it is treated as a basic unit of information processing and as a measurable spike in voltage over time.
What it is and what is measured
An action potential (AP) is a brief electrical event in a neuron. It is measured as a rapid rise in membrane voltage (depolarization) followed by a fall back toward resting level (repolarization). One source defines APs as rapid depolarizing events with a steep rising rate and a steep falling rate, so researchers can measure features such as threshold, rapidity, width, and amplitude. In nervous-system signaling, APs are basic elements of information processing. [0,2]
[1] The impact of dynamic reversal potential on the evolution of ... [3] Input-Specific Organization of Intrinsic Excitability Expands Coding ...
How it happens
Neurons fire an AP when depolarization is strong enough to reach threshold. In normal neural activity, APs are usually initiated in the axon, often at the axon initial segment, because that region has a high density of voltage-gated sodium channels. Electrical stimulation can also trigger APs by changing transmembrane potential through the electric field; whether a nearby structure fires depends on factors such as distance from the stimulus, axon orientation, tissue properties, and stimulation settings. A cathode makes nearby extracellular space more negative, which can depolarize tissue and, if sufficient, trigger an AP. [1,2]
[2] The Physics and Physiology of Electrical Stimulation of the Brain [3] Input-Specific Organization of Intrinsic Excitability Expands Coding ...
Why action potential shape can vary
The sources show that APs are not always identical. During repeated current steps, later APs in a burst can change in a monotonic way, meaning they shift in one direction across the train. The change can become larger when stimulation is stronger. One study reports that local ion concentration changes alter reversal potentials, especially for sodium and potassium ions, and this can drive systematic changes in AP attributes during bursts. Adding cooperative sodium-channel gating improved the match to some observed patterns, especially for the first AP in each burst. [0]
[1] The impact of dynamic reversal potential on the evolution of ...
Limitations and what the evidence can and cannot explain
The evidence comes from public recordings and modeling, so it is strong for showing recurring patterns but limited in sample size for some neuron types. The models reproduced some trends, such as changes in rapidity and width, but underestimated threshold changes and did not fully explain all first-AP behavior. So the sources support a mixed view: ion concentration dynamics and cooperative sodium-channel gating matter, but the complete mechanism is not yet captured by the model. [0]
[1] The impact of dynamic reversal potential on the evolution of ...
SEE THE IDEA
From stimulus to action potential
Current step or electrical stimulation changes membrane polarization.
If depolarization is strong enough, the neuron reaches the firing point.
The axon, often the initial segment, is a common site where the spike begins.
Ion concentration changes and sodium-channel gating can alter threshold, rapidity, width, and amplitude.
Stimulus changes membrane potential; if threshold is crossed, an action potential starts in the axon and its shape can be modified by ion dynamics during repeated firing.
A simple path from depolarizing input to a measurable spike, with later changes in spike shape during bursts.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.