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Neuroscience. 2015 Jan 22;284:470-82. doi: 10.1016/j.neuroscience.2014.09.059. Epub 2014 Oct 12.

The pre- and post-somatic segments of the human type I spiral ganglion neurons--structural and functional considerations related to cochlear implantation.

Author information

1
Department of Surgical Sciences, Head and Neck Surgery, Section of Otolaryngology, Uppsala University Hospital, SE-751 85 Uppsala, Sweden; Department of Otolaryngology, Uppsala University Hospital, SE-751 85 Uppsala, Sweden. Electronic address: lwoo24@gmail.com.
2
Department of Surgical Sciences, Head and Neck Surgery, Section of Otolaryngology, Uppsala University Hospital, SE-751 85 Uppsala, Sweden; Department of Otolaryngology, Uppsala University Hospital, SE-751 85 Uppsala, Sweden. Electronic address: Fredrik.edin@surgsci.uu.se.
3
Department of Surgical Sciences, Head and Neck Surgery, Section of Otolaryngology, Uppsala University Hospital, SE-751 85 Uppsala, Sweden; Department of Neurology, Mental Health and Sensory Organs, Otorhinolaryngologic Unit, Medicine and Psychology, Sapienza, Rome, Italy. Electronic address: atturo@libero.it.
4
Department of Otolaryngology, Medical University of Innsbruck, Anichstrasse 35, A-6020 Innsbruck, Austria. Electronic address: gunde.rieger@uibk.ac.at.
5
Department of Otorhinolaryngology-Head & Neck Surgery, European Medical School, University of Oldenburg, Steinweg 13-17, 26122 Oldenburg, Germany. Electronic address: hubert.loewenheim@uni-oldenburg.de.
6
University Department of ORL, Head & Neck Surgery, Inselspital and Department of Clinical Research, University of Bern, Switzerland; University Department of ORL, Head & Neck Surgery, HUG, Geneva, Switzerland. Electronic address: pascal_senn@me.com.
7
Department of Anatomy, Histology and Embryology, Division of Clinical and Functional Anatomy, Medical University of Innsbruck, Müllerstrasse 59, 6020 Innsbruck, Austria. Electronic address: michael.blumer@i-med.ac.at.
8
Department of Otolaryngology, Medical University of Innsbruck, Anichstrasse 35, A-6020 Innsbruck, Austria. Electronic address: annelies.schrott@i-med.ac.at.
9
Department of Surgical Sciences, Head and Neck Surgery, Section of Otolaryngology, Uppsala University Hospital, SE-751 85 Uppsala, Sweden; Department of Otolaryngology, Uppsala University Hospital, SE-751 85 Uppsala, Sweden. Electronic address: helge.rask-andersen@akademiska.se.
10
Department of Otolaryngology, Medical University of Innsbruck, Anichstrasse 35, A-6020 Innsbruck, Austria. Electronic address: rudolf.glueckert@i-med.ac.at.

Abstract

Human auditory nerve afferents consist of two separate systems; one is represented by the large type I cells innervating the inner hair cells and the other one by the small type II cells innervating the outer hair cells. Type I spiral ganglion neurons (SGNs) constitute 96% of the afferent nerve population and, in contrast to other mammals, their soma and pre- and post-somatic segments are unmyelinated. Type II nerve soma and fibers are unmyelinated. Histopathology and clinical experience imply that human SGNs can persist electrically excitable without dendrites, thus lacking connection to the organ of Corti. The biological background to this phenomenon remains elusive. We analyzed the pre- and post-somatic segments of the type I human SGNs using immunohistochemistry and transmission electron microscopy (TEM) in normal and pathological conditions. These segments were found surrounded by non-myelinated Schwann cells (NMSCs) showing strong intracellular expression of laminin-β2/collagen IV. These cells also bordered the perikaryal entry zone and disclosed surface rugosities outlined by a folded basement membrane (BM) expressing laminin-β2 and collagen IV. It is presumed that human large SGNs are demarcated by three cell categories: (a) myelinated Schwann cells, (b) NMSCs and (c) satellite glial cells (SGCs). Their BMs express laminin-β2/collagen IV and reaches the BM of the sensory epithelium at the habenula perforata. We speculate that the NMSCs protect SGNs from further degeneration following dendrite loss. It may give further explanation why SGNs can persist as electrically excitable monopolar cells even after long-time deafness, a blessing for the deaf treated with cochlear implantation.

KEYWORDS:

collagen IV; human cochlea; immunohistochemistry; laminin-β2; non-myelinated Schwann cells; spiral ganglion neurons

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