Talking back: Development of the olivocochlear efferent system

Abstract

<p class="first" id="P1">Developing sensory systems must coordinate the growth of neural circuitry spanning from receptors in the peripheral nervous system (PNS) to multi-layered networks within the central nervous system (CNS). This breadth presents particular challenges, as nascent processes must navigate across the CNS-PNS boundary and coalesce into a tightly intermingled wiring pattern, thereby enabling reliable integration from the PNS to the CNS and back. In the auditory system, feedforward spiral ganglion neurons (SGNs) from the periphery collect sound information via tonotopically organized connections in the cochlea and transmit this information to the brainstem for processing via the VIII ^th cranial nerve. In turn, feedback olivocochlear neurons (OCNs) housed in the auditory brainstem send projections into the periphery, also through the VIII ^th nerve. OCNs are motor neuron-like efferent cells that influence auditory processing within the cochlea and protect against noise damage in adult animals. These aligned feedforward and feedback systems develop in parallel, with SGN central axons reaching the developing auditory brainstem around the same time that the OCN axons extend out towards the developing inner ear. Recent findings have begun to unravel the genetic and molecular mechanisms that guide OCN development, from their origins in a generic pool of motor neuron precursors to their specialized roles as modulators of cochlear activity. One recurrent theme is the importance of efferent-afferent interactions, as afferent SGNs guide OCNs to their final locations within the sensory epithelium, and efferent OCNs shape the activity of the developing auditory system. </p><p id="P2">Inner ear efferents (IEEs) provide feedback to the inner ear. Originating with facial branchial motor neurons (FBMNs) from a pool of MNv progenitors in the brainstem, IEE precursors produce olivocochlear neurons (OCNs) and vestibular efferent neurons (VENs). In the cochlea, OCN axons project along spiral ganglion neuron (SGN) afferent processes. </p><p id="P3"> <div class="figure-container so-text-align-c"> <img alt="" class="figure" src="/document_file/cf02f5b3-65dc-4dad-84df-ebdb4f0d6bf1/PubMedCentral/image/nihms971444u1.jpg"/> </div> </p>