Gating of hair cell Ca ²⁺ channels governs the activity of cochlear neurons

Our sense of hearing processes sound intensities spanning six orders of magnitude. In the ear, postsynaptic spiral ganglion neurons (SGNs) tile this intensity range with their firing rate codes. Presynaptic inner hair cells (IHCs) vary Ca ²⁺ -influx among their active zones (AZs) diversifying glutamate release and likely contributing to SGN firing diversity. Here we show that low-voltage activation of IHC-Ca ²⁺ -influx of mice, modeling the human Ca _V 1.3 ^A749G mutation, increases spontaneous SGN-firing and lowers sound threshold. Altered synaptic morphology in Ca _V 1.3 ^A749G/A749G mice already at ambient sound levels of standard mouse husbandry indicates a risk for noise-induced alterations in Ca _V 1.3 ^A749G patients. We conclude that heterogeneous voltage-dependence of Ca _V 1.3 activation among IHC-AZs contributes to the diversity of SGN firing for sound intensity coding and synaptic vulnerability.