Neurons in the brain have extensive dendritic arbours that receive thousands of synaptic inputs all along them. The transformation of all these inputs to an output in a single neuron occurs through the integration of synaptic events and the generation of an action potential (AP) at the axon initial segment (AIS). The AIS, therefore, is the site that controls neuronal output by gating the generation of APs. It has been recently shown that this neuronal compartment can be reorganized following a change in neuronal activity and that this structural plasticity is associated with a change in neuronal excitability. In addition, the AIS of pyramidal neurons is innervated by a specific type of inhibitory interneuron, named the chandelier cell, that forms axo-axonic connections specifically with it. At classical excitatory and inhibitory synapses, the nanoscale molecular organization of synaptic proteins has been shown to be a key factor in modulating the efficiency of synaptic transmission between neurons. However, the precise molecular organization of axo-axonic synapses is still poorly understood, as is its role in regulating neuronal output. Here we use super-resolution microscopy to decipher the precise molecular organization of axo-axonic synapses and its modifications during activity-dependent forms of plasticity.

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