Ageing and the auditory nerve: Hearing sensitivity and endbulb synapses

Age-related hearing loss impairs speech understanding for socialization and music appreciation for enjoyment, both of which compromise quality of life and can lead to cognitive decline. It has previously been shown that while standard audiometric hearing thresholds can remain normal over time, speech understanding in noise is more difficult and there is the emergence of tinnitus. These specific hearing difficulties are not revealed by standard audiograms but we now know that they have been attributed to loss of high threshold auditory nerve fibers caused by the disappearance of terminal endings under inner hair cells. This loss can be measured by a reduction of evoked activity in the auditory nerve and atrophy of central auditory nerve endings in the anteroventral cochlear nucleus called endbulbs of Held. In the present study, we used age-graded cohorts of mice to compare hearing loss to the structure of auditory nerve synapses using serial section electron microscopy. We demonstrated a pathologic expansion and flattening of their synapses against spherical bushy cells in the rostral anteroventral cochlear nucleus in older mice with hearing loss. These changes portend impairments in sound processing and emphasize the importance of identifying "hidden" hearing loss for potential rehabilitation.