An axon initial segment is required for temporal precision in action potential encoding by neuronal populations

TL;DR

This study demonstrates that the axon initial segment's high sodium channel density is essential for precise timing and high-bandwidth encoding of action potentials, rather than for their initiation.

Contribution

The paper provides the first experimental evidence that high NaV density in the AIS is crucial for temporal precision and information encoding, not just AP initiation.

Findings

High NaV density is not required for AP initiation.

High NaV density is crucial for temporal precision.

AIS architecture affects information encoding bandwidth.

Abstract

Central neurons initiate action potentials (APs) in the axon initial segment (AIS), a compartment characterized by a high concentration of voltage-dependent ion channels and specialized cytoskeletal anchoring proteins arranged in a regular nanoscale pattern. Although the AIS was a key evolutionary innovation in neurons, the functional benefits it confers are not clear. Using a mutation of the AIS cytoskeletal protein \beta IV-spectrin, we here establish an in vitro model of neurons with a perturbed AIS architecture that retains nanoscale order but loses the ability to maintain a high NaV density. Combining experiments and simulations we show that a high NaV density in the AIS is not required for axonal AP initiation; it is however crucial for a high bandwidth of information encoding and AP timing precision. Our results provide the first experimental demonstration of axonal AP initiation without high axonal channel density and suggest that increasing the bandwidth of the neuronal code and hence the computational efficiency of network function was a major benefit of the evolution of the AIS.