The propagation of an action potential along an axon can be compared to a lit firecracker fuse. Just as the fuse burns with consistent intensity from one end to the other without fading, an action potential travels down the axon at its full strength, ensuring the signal does not weaken over distance.

Firecracker Fuse Analogy for Action Potential Propagation

The all-or-none property of action potentials ensures that nerve signals do not lose strength as they travel. This is why the brain perceives an injury to a distant body part, like a toe, as being just as painful as an injury to a closer part, like the nose. The action potential signal arrives at the brain with its original, full intensity regardless of the distance it has traveled.

Pain Perception and the All-or-None Property

A specific neuron fires an electrical signal when it receives stimulation that just barely exceeds its minimum threshold. If this same neuron is later stimulated with an input that is twice as strong as the minimum threshold, how will the resulting electrical signal compare to the first one?

The all-or-none property dictates that an action potential is either triggered completely or not at all. Once the threshold of excitation is met, the action potential is generated and propagated at its full, consistent strength along the entire axon, without diminishing over distance. This principle explains why the brain perceives a signal from a distant body part, such as a toe, with the same intensity as one from a closer part, like the nose, as the signal's strength is maintained throughout its journey.

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California Polytechnic State University, San Luis Obispo

Large shift in membrane potential that lasts a short time, which causes a neuron to fire. These large shifts in membrane potential (i.e., electrical signals) are how information is transmitted within a neuron. 

Action Potential

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

The reduction of the electrical potential of the membrane. During depolarization, 
voltage-gated Na+ ion channels begin to open, thus letting some Na+ ions pass through the membrane. The action potential is sparked when the rushing in of the positive Na+ ions into the neuron reduces the electrical potential to around -40 mV.

Depolarization

Hyperpolarization is a phase of a neuron's action potential where the membrane potential becomes more negative than its resting state, reaching approximately -80mV. This increased negativity occurs after repolarization, as potassium (K+) ion channels remain open, allowing positively charged K+ ions to continue leaving the cell. This outflow of positive ions is driven by repulsion from the positively-charged sodium ions that had previously entered the cell.

Hyperpolarization

All-or-None Property

A change in the electrical potential of the membrane due to the binding of neurotransmitters in the synapses. These potentials determine whether a neuron fires or not.

Postsynaptic Potentials

Saltatory conduction is the process by which an action potential propagates along a myelinated axon, appearing to 'jump' in a leapfrog fashion between the Nodes of Ranvier. At each node, an influx of sodium ions generates the action potential. These ions then diffuse to the adjacent section of the axon, raising its charge past the threshold of excitation and triggering a new influx of sodium ions at the next node. This mechanism allows the electrical signal to travel rapidly down the axon.

Saltatory Conduction

The passing of information received from an action potential through the synapse. It occurs in the following steps:
- Exocytosis of neurotransmitter-containing vesicles occurs as the incoming action potential depolarizes the presynaptic membrane, leading to a rush of Ca 2+ entering the terminal
- As a result of exocytosis, transmitters are released into the synaptic cleft and bind with receptor molecules in the postsynaptic membrane
- These bindings cause ion channels in the postsynaptic membrane to open, creating a local EPSP or IPSP in the postsynaptic neuron
- The effects of the released transmitters are halted through either degradation or reuptake
- Some transmitters bind to autoreceptors, which further regulates the release of transmitters

Synaptic Transmission

The speed of an action potential, also known as the conduction velocity,  generally depends on two things: 1.) The diameter of the axon, and 2.) The myelination of the axon. Axons that have larger diameters conduct impulses faster because depolarization is allowed to more rapidly spread throughout the interior of the cell. Axons that are myelinated also conduct impulses faster than axons that are unmyelinated. This is because myelin resists the flow of ionic currents, and so it channels the depolarization down the axon interior until it reaches a small gap in the cellular membrane that lacks myelin, where it can be regenerated. Therefore, myelination allows action potentials to "jump" between nodes along the axon (see "Saltatory Conduction").  

Conduction Velocity

The firing of the action potential only occurs at a particular point. In sound, this means the potential only fires at a certain frequency. Having an accurate phase-locked representation generally needs averaging across several cells rather than a single cell. This helps with telling where you are in a cycle, because the neuron will only fire at a certain point in that cycle. 

phase-locked action potentials

The period of time immediately following an action potential during which a neuron's responsiveness to subsequent stimulation is either completely absent or significantly reduced.

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