Cochlear implants, electrical prosthetic devices that stimulate inner ear neurons of individuals who have lost their cochlear sensory cells, restore usable hearing to deaf patients. Cochlear implant electrodes are placed in the fluid-filled scala tympani of the cochlea, at a significant distance from the spiral ganglion and even from the spiral ganglion dendrites.
Stimulation via a cochlear implant electrode pair is therefore likely to activate large numbers of neurons concurrently. This may decrease the resolution and dynamic range of information transmitted in patients with cochlear implants. The low precision of electrical neural activation, compared to the precise activation that occurs in the normal cochlea, may explain why increasing the number of electrodes on a cochlear implant beyond 8–10 does not improve functionality.
However, if cochlear neurons could be induced to extend neurites toward a cochlear implant, it might be possible to stimulate more discrete subpopulations, and to increase the resolution of the device. Prof. Allen F. Ryan and colleagues from University of California exposed spiral ganglion explants from neonatal rats to soluble neurotrophins, cells transfected to secrete neurotrophins, and/or collagen gels.
Researchers found that cochlear neurites grew readily on collagen surfaces and in three-dimensional collagen gels. Co-culture with cells producing neurotrophin-3 resulted in increased numbers of neurites, and neurites that were longer than when explants were cultured with control fibroblasts stably transfected with green fluorescent protein.
It is suggested that extracellular matrix molecule gels and cells transfected to produce neurotrophins offer an opportunity to attract spiral ganglion neurites toward a cochlear implant.
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