Hearing impairment is a common feature of old age. Human temporal bones from old individuals with hearing impairment show the loss of sensory and neural cells within the cochlea as well as vascular changes, strial atrophy and thickening of the basilar membrane. These changes certainly contribute to hearing impairment, however there is also evidence that the surviving neural elements of the cochlea may be abnormal. Changes in the structure and function of the surviving neurons may play as important a role in the hearing impairment associated with aging as actual cell loss. Animal models of aging have also shown the loss of sensory and neural cells within the cochlea and brainstem and atrophy of the stria vascularis in old animals. Using the rat, the present proposal will investigate the morphology and function of the surviving neural cells which project from the cochlea to the brainstem. Morphological studies will include descriptions of the peripheral and central processes of the spiral ganglion cells as seen following labeling with horseradish peroxidase. Electro-physiological studies will include recording the spontaneous and stimulus- evoked activity of the spiral ganglion cells with microelectrodes. The function of the spiral ganglion cells will also be evaluated using the 2-deoxy D-(14C)-glucose technique. Evaluation of the results of these experiments will allow us to determine whether, and in what way the information transmission function of the aging auditory nerve is compromised. These results will have implications for the development of hearing aids and cochlear implants for the elderly.
