Stereocilia are the bundles of hair like fibers in the ear that pick up mechanical
vibrations and via their associated cells convert them to electrical energy
which the brain then perceives as sound. They are incredibly tiny, in the order of 5
microns in length.
The bad news is that in mammals, once damaged, they don't regenerate.
Over a life time, they deteriorate. The outer ones, which are involved in high-frequency
hearing, tend to go first.
Through mechanisms not entirely understood, the auditory cortex can try and
compensate and turns up the gain and in some individuals they perceive a
constant ringing, referred to as tinnitus. Since the advent of MRI, researchers
have shown that tinnitus originates within the brain.
Now that fun gadgets such as MP3 players have become ubiquitous, some
professionals have suggested that unfortunately we will see a rise in the number of
individuals who suffer from hearing loss and tinnitus, as the generation
of people who adopted them begin to age.
Since tinnitus and hearing loss can be permanent, non-repairable conditions,
turning down the volume is good advice.
