Jaime Tobón LM, Moser T
Cell Rep, 2022. under review.
Synaptic heterogeneity can expand the encoding capacity of individual neurons or sensory cells. The afferent synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) in the cochlea exhibit marked heterogeneity in structure and function. Synapses at the modiolar IHC-side have larger active zones and Ca 2+ -channel clusters, yet their activation requires stronger depolarization. SGNs differ in their spontaneous activity and sound threshold. However, the connection between synaptic heterogeneity and neural response diversity has remained elusive. Here, we bridged this gap by ex-vivo paired recordings of IHCs and postsynaptic boutons with stimuli and conditions aimed to mimic those of in-vivo SGN-characterization. We find that the spontaneous rate ( SR ) of synaptic transmission depends on synapse position. High SR synapses predominantly localize to the pillar IHC-side, larger spontaneous EPSCs, lower voltage-thresholds, shorter response latencies and stronger initial release rates. This study reveals that presynaptic heterogeneity in IHCs translates into diverse SGN properties.