Abstract:

It is well known that cochlear pathology produces alterations in neurons of the cochlear nucleus, but there are few data that directly relate such neuronal changes to the physiology of auditory nerve fibers and structure of auditory nerve endings. These relationships are addressed in a naturally occurring model of deafness, the congenitally deaf white cat. Cats with no intact organ of Corti exhibited no measurable hearing thresholds and had severely reduced spontaneous activity in single auditory nerve fibers. Furthermore, there was structural atrophy of endbulbs of Held in the anteroventral cochlear nucleus, and hypertrophied synapses. These hypertrophied synapses were characterized by large postsynaptic densities with highly thickened membrane specializations and markedly diminished numbers of synaptic vesicles. Cats with elevated but measurable hearing thresholds had normal-appearing organs of Corti with abundant spontaneous single unit activity but infrequent sound-evoked activity. These partially deaf cats displayed modest atrophy of their endbulbs and exhibited more normal appearing synapses. The data reveal that early onset partial or total deafness that is uncorrected for 6 months or more produces unambiguous alterations in synaptic structure. These observations are relevant to selection strategies for human cochlear implant candidates. [Work supported by NIH/NIDCD.]