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

Abstract

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, 2018. © 2018 Wiley Periodicals, Inc.

Vertebrate Sensory Ganglia: Common and Divergent Features of the Transcriptional Programs Generating Their Functional Specialization

“…SSN are typically pseudo-unipolar neurons with a single axon that bifurcates into two branches: a distal branch innervating target tissues and a proximal branch innervating the CNS. However, cochlear ganglia also contain some bipolar neurons while all vestibular ganglia neurons are bipolar (Carricondo and Romero-Gómez, 2019). VSN are found in the geniculate, petrose and nodose ganglia.…”

Cranial Nerves: Mind Your Head

“…This Special Issue of The Anatomical Record compiles works in the above mentioned Ontogeny/Phylogeny and Morphology/Clinical Significance topics. The organizational/developmental point of view (Volume 1 of this Special Issue) is populated with the long debated cranial par 0 or nervus terminalis (Peña‐Melián et al ., ), the very atypical and interesting features of the olfactory nerve (Crespo et al ., ), the development and pathologies of optic nerves (Herrera et al ., ), the organization and function of the oculomotor complex in different vertebrate brains, and the complex evolutionary relationships derived from the comparative analysis of eye muscles innervation (Company et al ., ; Ferran and Puelles, ), the anatomy and physiology of the auditory portion of the VIIIth nerve and the segmental organization of vestibular complex afferents and efferents (Carricondo and Romero‐Gómez, ; Diaz and Puelles, ), the atypical features of the spinal accesory motor neurons and the present neuromeric brainstem conception (prosomeric model) after new data from the visceral cranial nerve efferents (Puelles et al ., ; Watson and Tvrdik, ), and characterization of the cranial nerves in lampreys (Pombal and Megías, ). This volume 1 Ontogeny/Phylogeny of the Cranial Nerves Special Issue is completed with a historical overview of the cranial nerves (Porras et al ., ).…”

Cranial Nerves: Phylogeny and Ontogeny

“…The phylogeny and ontogeny volume includes an historical review (Porras‐Gallo et al, ), a review from a segmental perspective (Puelles et al, ), two comparative contributions on amphioxi (Ferran and Puelles, ) and lampreys (Pombal and Megías, ), and articles devoted to cranial nerves zero (Peña‐Melián et al, ), I (Crespo et al, ), II (Herrera et al, ), III, IV and VI (Company et al, ), VIII cochlear (Carricondo and Romero‐Gómez, ) and vestibular (Diaz and Puelles, ), and XI (Watson and Tvrdik, ).…”

“…The subject of the eighth cranial pair is explored in two contributions. The first examines the cochlear nerve, focusing on the cochlear spiral ganglion (Carricondo and Romero‐Gómez, ). Two neuron types can be distinguished: afferent type I (95%; bipolar neurons with myelinated axons innervating cochlear inner hair cells) and afferent type II (5%; pseudounipolar neurons with unmyelinated axons that innervate the cochlear outer hair cells).…”