Superresolution imaging reveals activity-dependent plasticity of axon morphology linked to changes in action potential conduction velocity

Significance Recent work has called into question the classic view of axons as electroanatomical cables that faithfully transmit nerve impulses in an all-or-none fashion over variable distances. Because of their small size, below the diffraction barrier of light microscopy, it has not been possible to resolve their dynamic morphology in living brain tissue. Enabled by a combination of live-cell superresolution stimulated emission depletion (STED) microscopy, electrophysiology, and mathematical modeling, we show here that high-frequency action potential firing induces structural enlargement of boutons and axon shafts in hippocampal brain slices, which in turn drives changes in the conduction velocity of nerve impulses (action potentials). The findings reveal a mechanism for tuning the timing of communication between nerve cells. Axons convey information to nearby and distant cells, and the time it takes for action potentials (APs) to reach their targets governs the timing of information transfer in neural circuits. In the unmyelinated axons of hippocampus, the conduction speed of APs depends crucially on axon diameters, which vary widely. However, it is not known whether axon diameters are dynamic and regulated by activity-dependent mechanisms. Using time-lapse superresolution microscopy in brain slices, we report that axons grow wider after high-frequency AP firing: synaptic boutons undergo a rapid enlargement, which is mostly transient, whereas axon shafts show a more delayed and progressive increase in diameter. Simulations of AP propagation incorporating these morphological dynamics predicted bidirectional effects on AP conduction speed. The predictions were confirmed by electrophysiological experiments, revealing a phase of slowed down AP conduction, which is linked to the transient enlargement of the synaptic boutons, followed by a sustained increase in conduction speed that accompanies the axon shaft widening induced by high-frequency AP firing. Taken together, our study outlines a morphological plasticity mechanism for dynamically fine-tuning AP conduction velocity, which potentially has wide implications for the temporal transfer of information in the brain.