Elsevier

Neuroscience

Volume 158, Issue 1, 12 January 2009, Pages 211-222
Neuroscience

Presynaptic Mechanism
Review
Action potential initiation and propagation: Upstream influences on neurotransmission

https://doi.org/10.1016/j.neuroscience.2008.03.021Get rights and content

Abstract

Axonal action potentials initiate the cycle of synaptic communication that is key to our understanding of nervous system functioning. The field has accumulated vast knowledge of the signature action potential waveform, firing patterns, and underlying channel properties of many cell types, but in most cases this information comes from somatic intracellular/whole-cell recordings, which necessarily measure a mixture of the currents compartmentalized in the soma, dendrites, and axon. Because the axon in many neuron types appears to be the site of lowest threshold for action potential initiation, the channel constellation in the axon is of particular interest. However, the axon is more experimentally inaccessible than the soma or dendrites. Recent studies have developed and applied single-fiber extracellular recording, direct intracellular recording, and optical recording techniques from axons toward understanding the behavior of the axonal action potential. We are starting to understand better how specific channels and other cellular properties shape action potential threshold, waveform, and timing: key elements contributing to downstream transmitter release. From this increased scrutiny emerges a theme of axons with more computational power than in traditional conceptualizations.

Section snippets

Sodium channels

In neurons, voltage-gated sodium (Nav) conductances play an essential role in action potential initiation and propagation (Hodgkin and Huxley, 1952). Nav channels activate and inactivate within milliseconds. As the cell membrane is depolarized, sodium channels activate, resulting in the influx of sodium ions to further depolarize the membrane. This inward current produces the upstroke of the action potential. Along with the gating of potassium channels, sodium channel inactivation participates

Potassium channels

Potassium channels are the most structurally and functionally diverse of voltage-gated ion channels and accordingly play a major role in characteristic spiking patterns and spike waveforms. Potassium channels modulate the resting membrane potential, action potential threshold, spike shape, afterhyperpolarization, and interspike interval. A diverse group of voltage-gated potassium (Kv) channel subunits has been identified (Doyle et al 1998, Dodson and Forsythe 2004, Trimmer and Rhodes 2004,

Action potential initiation and propagation

As noted above, a characteristic of nearly all neurons studied is preferential initiation in the axon, with subsequent development/back-propagation into the somatodendritic compartment. Direct recordings from single axons yield direct, quantitative information regarding the initiation and propagation of the action potential. Action potential propagation has been studied most extensively in the peripheral nervous system because these fibers are easily accessible, often large and myelinated, and

Conclusion

The action potential is essential to our understanding of nervous system function. Its shape, velocity of conduction, and propagation fidelity are essential to the timing, synchrony, and efficacy of neuronal communication. As such, action potentials have been the subject of intense scrutiny for nearly a century. Nevertheless, axonal properties, particularly those of the vertebrate CNS, remain somewhat elusive, given the limited and rather indirect experimental tools that can be applied to the

Acknowledgments

We thank laboratory members for advice and we acknowledge NIH grants MH78823 and NS54174 for support of work in our laboratory.

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