Divide and conquer acoustic diversity

Abstract

Humans can recognize differences in sound intensity of up to 6 orders of magnitude. However, it is not clear how this is achieved and what enables our auditory systems to encode such a gradient. Özçete & Moser (2021) report in this issue that the key to this lies in the synaptic heterogeneity within individual sensory cells in the inner ear.

Figure 1. Individual IHCs are innervated by multiple SGNs forming single synaptic contacts

In the study by Özçete and Moser, it is shown that synapses differ in multiple functional properties and can be classified into two main subtypes: low threshold synapses (typically on the Pillar side) and high threshold synapses (on the Modiolar side). Low threshold synapses are typically activated at lower IHC membrane potential (lower V_1/2), showing larger Ca²⁺ influx and more glutamate released. High threshold synapses present higher membrane potential activation (higher V_1/2). Low threshold synapses also presented a tight nanodomain coupling between Ca²⁺ channels and vesicles, whereas high threshold synapses tended to have a looser microdomain coupling.