AN EXAMINATION OF THE SPIRAL GANGLION NEURONS OF THE COCHLEA FOLLOWING CHRONIC USE OF A COCHLEAR IMPLANT.
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Date
2006-08-03T15:29:47Z
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Johns Hopkins University
Abstract
Cochlear implants have helped many profoundly deaf individuals regain auditory
communication. However, the extent to which hearing is restored is variable, and may
depend on fundamental changes to the auditory pathways following deafness. In the
present study, I used the congenitally deaf white cat to explore cochlear and auditory
nerve structures following cochlear implant use. Many white cats are congenitally deaf
from birth, and this was confirmed using auditory brainstem potentials in cats used for
this study. These congenitally deaf cats exhibit a lack of spike activity in the auditory
nerve, and abnormalities in a specialized auditory nerve ending called the endbulb of
Held (Ryugo et al 1996, Ryugo et al 1998). Cochlear implants (Clarion 1.2 or II,
Advanced Bionics) were placed in six congenitally deaf cats, and auditory nerve endings
and spiral ganglion neurons were examined using light and electron microscopy.
Auditory nerve endings originating in the stimulated cochlea appeared similar to those
from normal hearing cats. This result suggests that auditory nerve activity early in life
can restore normal synaptic function in the congenitally deaf animal. Surprisingly,
auditory nerve endings originating in the contralateral cochlea, which were not directly
stimulated, exhibited an intermediate phenotype, similar to that found in cats with
elevated hearing thresholds. This intermediate phenotype was not found in a cat who
received a non-functional implant. This result suggests widespread, top-down, bilateral
effects of the cochlear implant, perhaps lengthening the efficacy window for a second
implant. The spiral ganglion neurons of the cochlea did not show an effect of
stimulation in terms of total neuron number or neuron size. This is consistent with the
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finding that degree of spiral ganglion neuron survival is not closely related to clinical
benefit from a cochlear implant. Taken together, these results demonstrate that positive
effects of cochlear implantation occur at the earliest auditory synapse in the central
nervous system. The implication for clinicians is that a single cochlear implant is an
intervention which prevents abnormal central development. The bilateral effects imply
that a single implant primes the contralateral nerve for a second implant.
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Keywords
Cochlear nucleus, Endbulb of Held, Auditory nerve, Synapse, Ultrastructure, Post-synaptic density