Structural alteration of hair cells in the contralateral ear resulting from extracochlear electrical stimulation

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Abstract
Chronic electrical stimulation of the auditory nerve in patients with profound sensori-neural deafness is becoming increasingly routine. Therefore, it is important to understand more about the long-term consequences of this procedure. Hitherto, structural studies in animals after electrocochlear stimulation have concentrated on the stimulated cochlea1–6. Here we have examined the effects of unilateral extracochlear electrical stimulation on the spiral organ of both the ipsilateral and contralateral ears of the mature guinea pig, and have found alterations in the structure of the outer hair cells and their efferent nerve terminals in the contralateral as well as the ipsilateral cochlea. This is the first evidence for a structural influence of efferent activity on the cochlea. Although the importance of the efferent system, consisting of the crossed and uncrossed olivo-cochlear bundles, is well established in providing central control of the sensory pathways7, its exact role in hearing is incompletely understood. However, it is known that the outer hair cells and their efferent innervation are important in their contribution to inner hair cell responses8,9 and in modulating the micromechanics of the whole cochlea10,11. These efferent functions now appear to be related to an important part of cochlear morphology, and are also relevant to our understanding of cochlear neurobiology, normal development and the management of hearing disability in both adult and child.