The Cochlear Spiral Ganglion Neurons: The Auditory Portion of the VIII Nerve

The VIII nerve is formed by sensory neurons that innervate the inner ear, i.e., the vestibular and the auditory receptors. Neurons of the auditory portion, the cochlear afferent fibers that innervate the sensory hair cells of the organ of Corti, have their somas in the cochlear spiral ganglion where two types of neurons can be distinguished. Afferent Type-I neurons are the 95% of the total population. Bipolar and myelinated fibers, each one innervates only one cochlear inner hair cell (IHC). In contrast, afferent Type-II neurons are only the 5% of the spiral ganglion population. They are pseudounipolar and unmyelinated fibers and innervate the cochlear outer hair cells (OHC) so that one afferent Type-II fiber contacts with multiple OHCs, but each OHC only receives one contact from one Type-II neuron. Both types of VIII nerve fibers are glutamatergic, but these asymmetric innervations of the cochlear sensory cells could suggest that the IHC codifies the truly auditory message but the OHC only informs about mechanical aspects of the state of the organ of Corti. In fact, the central nervous system (CNS) has control over the information transmitted by the Type-I neuron by means of axons from the superior olivary complex that innervate them to modulate, filter and/or inhibit the entry of auditory message to CNS. The aim of this paper is to review the current knowledge about the anatomy and physiology of the auditory portion of the VIII nerve. Anat Rec, 302:463-471, 2019. © 2018 Wiley Periodicals, Inc.