Electron Microscopic Reconstruction of Neural Circuitry in the Cochlea

Abstract

Recent studies have revealed great diversity in the structure, function and efferent innervation of afferent synaptic connections between the cochlear inner hair cells (IHCs) and spiral ganglion neurons (SGNs), which likely enables audition to process a wide range of sound pressures. By performing an extensive electron microscopic reconstruction of the neural circuitry in the mature mouse organ of Corti, we demonstrate that afferent SGN-dendrites differ in strength and composition of efferent innervation in a manner dependent on their afferent synaptic connectivity with IHCs. SGNs that sample glutamate release from several presynaptic ribbons receive more efferent innervation from lateral olivocochlear projections than those driven by a single ribbon. Next to the prevailing unbranched SGN-dendrites, we found branched SGN-dendrites that can contact several ribbons of 1-2 IHCs. Unexpectedly, medial olivocochlear neurons provide efferent innervation of SGN-dendrites, preferring those contacting single-ribbon, pillar-side synapses. We propose a fine-tuning of afferent and efferent SGN-innervation.