H

earing loss is the most prevalent sensory

disability in our society and the second

leading cause of disability after depression,

relecting a larger non-fatal burden than

alcohol-related health issues, osteoarthritis

and schizophrenia (tinyurl.com/ofzpqfu). Hearing loss is getting

worse in our society because we are exposing our ears to more

noise over our lifetime, and this stress exacerbates hearing loss

with ageing.

It’s now possible to examine differences in the vulnerability

of hearing to noise stress in the lab by measuring hearing in

mice with particular gene mutations. Using this approach, we

have identiied new aspects of cochlear physiology that explain

why some people may be more susceptible to hearing loss than

others.

The Amazing Process of Hearing

The middle ear ossicles, which are the smallest bones in our

body, transfer the sound vibrations collected by the ear drum

to our cochlea, causing tiny bundles of hairs to lex. This lexing

causes small ion channels at the tips of the hair bundles to open

and let positive ions from the surrounding luid enter for just

long enough to trigger the release of glutamate neurotrans-

mitter. Glutamate receptors on cochlear nerve ibres detect the

neurotransmitter and trigger action potentials that travel along

the cochlear nerve to the brainstem cochlear nucleus in 1/500th

of a second.

The cochlea has two types of hair cells. There is a single row

of about 3000 inner hair cells, each of which is speciically inner-

vated by about a dozen type I sensory nerve ibres. However, our

hearing is largely dependent upon three adjacent rows of outer

hair cells that establish our “cochlear ampliier”. These cells

have a piezo-electric protein in the cell wall that twists in

response to hair bundle-mediated changes in voltage.

The outer hair cells literally dance to music, amplifying the

sound vibration so that the inner hair cells are better able to

detect the quietest sounds. The reason we can literally hear a “pin

drop” when we are in bed in the quiet of night is because our

36 | APRIL 2016

GARY HOUSLEY

Transgenic mice have revealed how the cochlea protects itself from loud noise and why some

people may be more susceptible to hearing loss than others.

Keeping the

Noise Down

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