Morphology, Development, and Neurotrophic Regulation of Cochlear Afferent Innervation
Spiral ganglion neurons (SGNs) are primary sensory neurons of the auditory system that send auditory information encoded by the inner ear to the central nervous system. The success of cochlear implant therapy is totally dependent on the status of SGN func
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Morphology, Development, and Neurotrophic Regulation of Cochlear Afferent Innervation Kenji Kondo, Yulian Jin, Makoto Kinoshita, Tatsuya Yamasoba, and Kimitaka Kaga Abstract Spiral ganglion neurons (SGNs) are primary sensory neurons of the auditory system that send auditory information encoded by the inner ear to the central nervous system. The success of cochlear implant therapy is totally dependent on the status of SGN function. Therefore, information regarding the neurogenesis, survival, and neurite growth of SGNs is important not only to understand the pathophysiology of sensorineural hearing loss but also to improve cochlear implant therapy. SGNs are anatomically and functionally divided into two subtypes, type I and type II. Type I SGNs connecting inner hair cells contribute to the transmission of sound information into the central auditory pathway, while type II SGNs connecting outer hair cells are involved in active tuning of frequency in the cochlea. In the developing cochlea, the survival and neurite formation of SGNs are strongly regulated by neurotrophic factors, especially neurotrophin 3 (NT-3) and brain- derived neurotrophic factor (BDNF). Also, in the adult cochlea, the loss of hair cells induces secondary loss of SGNs presumably because of a loss of neurotrophic support. When the deafened ear is treated with exogenous BDNF or NT3, there is a significant enhancement of SGN survival and resprouting of neurites. Therefore, chronic application of neurotrophic factors in the cochlea may improve the efficacy of cochlear implants. Keywords Spiral ganglion neuron • Survival • NT-3 • BDNF K. Kondo (*) • M. Kinoshita • T. Yamasoba Department of Otorhinolaryngology and Head and Neck Surgery, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan e-mail: [email protected] Y. Jin Department of Otorhinolaryngology, Yanbian University Hospital, No.1327, Juzi Street, Yanji 133 000, Republic of China K. Kaga National Institute of Sensory Organs, National Tokyo Medical Center, 2-5-1 Higashigaoka, Meguro-Ku, Tokyo 152-8902, Japan Center for Speech and Hearing Disorders, International University of Health and Welfare Clinic, 2600-6 Kitakanemaru, Ohtawara, Tochigi 324-0011, Japan © Springer Science+Business Media Singapore 2017 K. Kaga (ed.), Cochlear Implantation in Children with Inner Ear Malformation and Cochlear Nerve Deficiency, Modern Otology and Neurotology, DOI 10.1007/978-981-10-1400-0_4
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4.1 Introduction Spiral ganglion neurons (SGNs) are primary sensory neurons of the auditory system that send auditory information encoded by the inner ear to the central nervous system. SGNs are bipolar, with a peripheral neurite that forms a synapse with hair cells in the organ of Corti and a central neurite that projects to the cochlear nucleus of the medulla. Corresponding to the precise structural organization of the organ of Corti, the SGNs and their neurites form specific neural circuits along the longitudinal and radial axis of the cochlea t
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