The Human Spiral Ganglion: New Insights into Ultrastructure, Survival Rate and Implications for Cochlear Implants

Abstract

This study was based on high-resolution SEM assessment of freshly fixed, normal-hearing, human inner ear tissue. In addition, semiquantitative observations were made in long-term deafened temporal bone material, focusing on the spiral ganglia and nerve projections, and a detailed study of the fine bone structure in macerated tissues was performed. Our main findings detail the presence of extensive bony fenestrae surrounding the nerve elements, permitting a relatively free flow of perilymph to modiolar structures. The clustering of the spiral ganglion cells in Rosenthal’s canal and the detailed and intricate course of postganglionic axons are described. The close proximity of fibers to cell soma is demonstrated by impression in cell surfaces, and presence of small microvilli-like structures at the contact regions, anchoring nerve fibers to the cell wall. Extensive fenestrae and the presence of a fragile network of endosteal bony structures at the surfaces guiding nerve fibers are described in detail for the first time. This unique freshly prepared human material offers the opportunity for a detailed ultrastructural study not previously possible on postmortem fixed material and more accurate information to model electrostimulation of the human auditory nerve through a cochlear implant. On the basis of this study, we suggest that the concentration and high density of spiral ganglion cells, and the close physical interaction between neural elements, may explain the slow retrograde degeneration found in humans after loss of peripheral receptors. Moreover, the fragile bony columns connecting the spiral canal with the osseous spiral lamina may be a potential site for trauma in (perimodiolar) electrode positioning.

Effects of antioxidants on auditory nerve function and survival in deafened guinea pigs

“…Decreases of EABRs in treated groups with local AOs occurred more rapidly than those of treated group with systemic antioxidants. AOs applied into the scala tympani may work rapidly on the cochlear nerve with access via the habenula perforate and fenestrae in the modiolar bone throughout the basal and first turn of the human and guinea pig cochlea (Lim and Kim, 1983;Kücük et al, 1991;Glueckert et al, 2005). However, using this pathway AOs may also reach the fluid surrounding the cells within the organ of Corti (e.g.…”

“…We assume that this in part reflected the direct effect of the ototoxic antibiotic neomycin on the afferent nerve fibers. While the primary target of neomycin is the sensory hair cells, with direct delivery of a 10% solution to the scala tympani, the drug has direct access to the auditory nerve fibers (Lim and Kim, 1983;Kücük et al, 1991;Glueckert et al, 2005) and previous work has indicated that the drug may directly damage afferent nerve fibers (Leake-Jones, et al, 1980) as well as hair cells. We suggest that the rapid and dramatic elevation in ABR threshold observed immediately (compare all deafened groups EABR thresholds with the controls on the 1 st measurement, Fig 6) and in the short run (at least through week 1) after this local drug treatment reflects this direct damaging effect of the drug on the auditory nerve.…”

Frequency Map for the Human Cochlear Spiral Ganglion: Implications for Cochlear Implants

“…In the basal coil, the nerve bundles from the OC take a relatively direct radial course into the modiolus, but in the middle and apical turns, the trajectory of the fibers deviates significantly because Rosenthal_s canal is much shorter than the OC and does not extend to the apical turn (Bredberg 1968;Glueckert et al 2005). Thus, represented frequency along the SG must be offset from that of the OC.…”

Neurotrophic Factors and Neural Prostheses: Potential Clinical Applications Based Upon Findings in the Auditory System

“…This is followed by demyelination and degeneration of the peripheral processes as they recede from the damaged organ of Corti, eventually leading to degeneration of the SGNs. These degenerative changes are ongoing, and ultimately result in small numbers of surviving SGNs after long periods of deafness [1][2][3][4].…”