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.STED microscopy | axons | synaptic boutons | action potential conduction velocity | plasticity A xons have long been considered to be static transmission cables, serving to faithfully transmit all-or-none action potentials (APs) over long distances. However, several studies have revealed during the last few years that axon physiology is highly dynamic and regulated (1). It was shown that axons are capable of analog computations (2), undergo activity-dependent structural changes affecting the axonal initial segment (3) and boutons (4), and are subject to local control by astrocytes (5). As these mechanisms can substantially influence AP signaling and synaptic transmission, the computational capability and role of axons in neural circuit function are more complex than previously thought.The conduction of AP along axons imposes widely varying delays on synaptic transmission, and hence shapes the dynamics and timing of signal processing in the brain. In unmyelinated axons, the conduction speed of AP depends crucially on axon diameters (6, 7), which typically vary along the same axon (8), as well as between axons from different cell types (9). However, it is unknown whether axon diameters are dynamic and regulated by activity-dependent mechanisms, which could affect the timing of information transfer between cells and brain areas. This knowledge gap is mostly a result of the inability of conventional light microscopy to resolve thin unmyelinated axons, which can have diameters well below 200 nm, according to electron microscopy studies (10, 11).To overcome this limitation, we applied stimulated emission depletion (STED) microscopy together with electrophysiology in brain slices, ...
