Axonal spike initiation can be maintained with low axonal Na channel density, but temporal precision of spiking is lost

In central neurons, action potentials (APs) originate in the axon initial segment (AIS) presumably due to its high concentration of voltage-dependent sodium (NaV) channels. In the AIS, NaV and their cytoskeletal anchoring proteins display a highly periodical nanoscale arrangement with unknown functional relevance. In mouse hippocampal neurons, we examined a mutation of the AIS-specific protein $\beta$IV-spectrin and assessed AP initiation locus, AIS molecular organization, and the precision of in vivo-like AP encoding. In mutants, NaV concentration was strongly reduced in AIS but not soma, while AIS nanoscale organization was preserved. Surprisingly, APs still originated in the axon, even if the axonal channel density was as low as in the somatic membrane. However, the timing of those APs was less precise, therefore the neuron's encoding bandwidth was substantially decreased. Therefore, the high NaV density in the AIS is not required for axonal AP initiation, it is crucial for AP timing precision.