outer hair cell-based cochlear ampliier has its volume control
at maximum.
However, we don’t need our cochlear ampliier to hear loud
sounds, where it could potentially damage the hair cells, so it
is reassuring to know that there are at least two mechanisms
the cochlea uses to suppress hearing sensitivity and extend the
safe operational range for hearing.
The most well-understood system is efferent suppression
via a neural feedback circuit from the brainstem to the cochlea.
Brainstem processing of loud sound activates the cochlear
efferent suppression pathway, triggering the release of the neuro-
transmitter acetylcholine at the outer hair cells to suppress the
cochlear ampliier. This dynamic adaptation is switched on in
milliseconds but runs down if high sound levels are sustained
for long periods of time (tens of minutes). Only the outer hair
cells that are amplifying the loudest sound frequencies are
suppressed by the efferent ibres. This helps us to mask out
background noise (like chatter in a noisy room) to better hear
speech.
This efferent suppression also helps with sound localisation.
Sound in one ear provides a signal that suppresses hearing in the
opposite ear, so that hearing control is dynamic and balanced.
Efferent suppression is also essential to cochlear self-preserva-
tion during exposure to loud sound.
A Matter of Self-Preservation
We have discovered that transgenic mice that don’t produce the
neural protein peripherin failed to develop synapses between
cochlear type II ibres and the outer hair cells. When we directed
sound to one ear of these mice, their hearing sensitivity did not
change in the opposite ear (tinyurl.com/on775l2). This meant
that the origin of the sensory signal for efferent suppression of
hearing is actually the cochlear ampliier itself.
While our work with this new model continues, it’s already
evident that without efferent suppression, permanent hearing
loss occurs at sound levels that would normally only cause a
temporary loss of sensitivity. This supports evidence associ-
ating weaker cochlear efferent suppression with poorer hearing,
and may help us understand why, as we age, we start to struggle
to hold conversations in noisy places like restaurants.
Our transgenic approach has also enabled us to discover
another cochlear self-preservation mechanism when we are
exposed to sustained loud sound.
P2X 2 receptors are adenosine triphosphate-activated ion
channels found on cochlear sensory hair cells and adjacent cells
facing the luid space next to the hair bundles. When we
“knocked out” the P2rX2 gene to produce mice without P2X 2
receptors, and then exposed these mice to 85 dB noise for 30
minutes, we observed no reduction in their hearing sensitivity
(tinyurl.com/qf9dogz). In contrast, wild-type mice that retained
APRIL 2016|| 37It’s a Noisy World
Sound is measured in the logarithmic decibel (dB) scale, and
every 3 dB represents a doubling of sound intensity. It’s extra -
ordinary to think that hearing has the largest dynamic range of
any of our senses – about 140 dB – which ranges from just above
the thermal vibrations of molecules to the blast of a jet engine
or cannon.
We can tolerate loud sound for short periods of time. Since
sound levels double every 3 dB, it’s thought that 85 dB is the
upper limit for 8 hours in a workplace. That’s about as loud as
most lawnmowers. We can tolerate 88 dB for 4 hours, 91 dB for 2
hours, 94 dB for 1 hour and 97 dB (my weedtrimmer) for 30
minutes. Many industries have peaks in sound intensity that are
much higher than this, and we are not sure how to calculate the
risk to our hearing.
What about “recreational noise”? Most of us use cell phones
and personal music devices to listen to music and have
conversations. These devices deliver amplified sound directly to
the ear canal. The louder the background noise, the higher we
turn up the volume. Many of these devices can deliver well in
excess of 100 dB. Some devices have warnings and include
safety overrides if you want to use the higher sound levels. By
comparison, the average sound levels in Sydney night clubs are
about this level (tinyurl.com/qbwa4pd) and the music gets
louder as the night draws on. Why might that be?
The answer lies in our physiology – we “adapt” to the sound.
Our sense of hearing is not an absolute sound intensity
measuring device; rather, it is a perception that’s subject to
influence both at the source of the sound transduction (in the
cochlea) and in the brain where the conscious perception of
sound lies. We know when sound is “loud” but can’t really tell
whether it’s 85 dB or 95 dB.
Our mood can affect the perception of loudness. For instance,
I sometimes do a spin class at my gym. The music sets the
rhythm and seems to make me work harder. As a hearing
researcher I was curious about the sound levels during the class,
and it didn’t surprise me that the average sound level was about
95 dB for the 45-minute session. That’s probably OK for me and
all the other cyclists in the session, but what about the
instructor, who does several of these sessions daily? I now use
ear plugs, and have discovered that the sound is still loud but
actually also clearer. When I thought the speakers were
distorting at the loudest music levels – it was actually my ears!facebook.com/austscience
Follow us @austscience