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Ageing and the auditory nerve: Hearing sensitivity and endbulb synapses
David K Ryugo1, Satoshi Nishitani2
1Hearing Research, Garvan Institute of Medical Research, Darlinghurst, NSW 2010, Australia; Department of Otolaryngology, Head, Neck and Skull Base Surgery, St Vincent's Hospital, Darlinghurst, NSW 2010 Australia; School of Biomedical Sciences, University of New South Wales, Sydney, NSW 2052 Australia.
Hearing Research
|January 20, 2026
Summary
Hidden hearing loss, characterized by auditory nerve fiber loss, affects speech understanding despite normal hearing tests. This study reveals synaptic changes in older mice, highlighting the need for new diagnostic methods.
Area of Science:
- Neuroscience
- Audiology
- Gerontology
Background:
- Age-related hearing loss (presbycusis) significantly impacts quality of life, affecting socialization and cognitive function.
- Standard audiometric thresholds may not detect subtle hearing deficits like impaired speech understanding in noise and tinnitus.
- These deficits are linked to the loss of high-threshold auditory nerve fibers and synaptic changes in the auditory pathway.
Purpose of the Study:
- To investigate the structural changes in auditory nerve synapses associated with age-related hearing loss.
- To correlate hearing difficulties with synaptic morphology in the anteroventral cochlear nucleus.
Main Methods:
- Utilized age-graded cohorts of mice for the study.
- Employed serial section electron microscopy to examine the structure of auditory nerve synapses.
- Compared synaptic morphology in younger and older mice with varying degrees of hearing loss.
Main Results:
- Identified a pathologic expansion and flattening of auditory nerve synapses against spherical bushy cells in older mice with hearing loss.
- Observed these synaptic changes specifically in the rostral anteroventral cochlear nucleus.
- Demonstrated a correlation between age-related hearing loss and structural alterations at the synapse.
Conclusions:
- The observed synaptic changes suggest impairments in auditory signal processing.
- These findings underscore the importance of identifying "hidden" hearing loss, which is not detected by conventional audiograms.
- Highlights the potential for novel diagnostic approaches and therapeutic interventions for age-related hearing impairments.
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