Plasticity at the axon initial segment.

Experience dependent alterations in neural activity are mediated by diverse forms of plasticity, which are conventionally thought to occur at either synaptic terminals and/or postsynaptic membrane, such as dendrites and cell soma. However, our recent study has revealed that plasticity is not limited to synaptic sites, but it also takes place at the site where neural activity arises, the axon initial segment (AIS), which is a highly specialized region in the axon concentrated with voltage-gated Na+ channels. We observed in an avian brainstem auditory neuron that the AIS reorganized itself to elongate after deprivation of sensory inputs, which augmented the excitability of the neuron. Notably, this elongation of AIS caused spontaneous firing in some neurons, suggesting its compensatory role to restore neural activity in the circuit. Given that the AIS is the source of neural activity, this plasticity should be a most efficient mechanism for neurons to control their activity. This finding will provide a new insight into development, maintenance and refinement of neural circuits.