The axon initial segment (AIS) that separates axonal and somato-dendritic compartments is a highly specialised neuronal structure enriched with voltage-gated Na + channels and functions as the site of spike initiation in neurons. The AIS was once thought to be uniform and static in structure, but has been found to be organised in a manner specific to the function of individual neurons and to exhibit plasticity with changes in synaptic inputs. Such structural specialisations are found in the avian auditory system. In the nucleus magnocellularis (NM), which is involved in a precise relay of timing information, the length of the AIS differs depending on sound frequency and increases with decreasing frequencies to accommodate frequency-specific variations in synaptic inputs. In the nucleus laminaris, which integrates the timing information from both NMs for sound localisation, the length and the location of the AIS vary depending on sound frequency: AISs are shorter and more remote for higher frequency. Furthermore, the AISs of NM neurons elongate to increase their excitability when synaptic inputs are removed by cochlea ablation, suggesting their contribution to the homeostatic control of neural activity. These structural tunings and plasticities of the AIS are thus indispensable for the function of the auditory circuits in both normal and pathological conditions.
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