Figure 1

Schematic drawing of a neuron showing the position of the axon initial segment (AIS). The AIS plays two crucial roles in neurons: it forms a barrier between the somatodendritic and axonal compartments maintaining the neuron's polarity, and the AIS is the site where electrical outputs are initiated: aggregation of specific voltage-gated ion channels allows spikes to be generated in response to inputs coming from the somatodendritic compartment. Once initiated, spikes are both propagated along the axon and retropropagated towards the soma and dendrites. When axons are myelinated, spikes are propagated in a saltatory fashion, from a node of Ranvier to the next one. What is the precise AIS ion channel composition? And does the myelin sheath start immediately after the AIS or, as a corollary, is the axon organized at the first myelin-anchoring site as a hemi-node of Ranvier, with a paranode-like and a juxtaparanode-like compartment? These questions are addressed in this study.