i found this on a site: "http://discovermagazine.com/2010/oct/26-ringing-in-the-ears-goes-much-deeper"
Normally, we hear sounds only when they make our eardrums vibrate. The vibrations cause nerve hairs in the inner ear to shiver, and that triggers electric signals that travel along the auditory nerve into the brain. One of their first stops is a patch of gray matter called the auditory cortex. Each nerve hair is tuned to a particular frequency of sound and excites only certain neurons in the auditory cortex. As a result, the neurons in the auditory cortex form what is known as a tone map. The neurons at one end of the auditory cortex are tuned to low frequencies; the farther you go toward the other end, the higher the tuning of the neurons.
This sound system comes with an elaborate feedback mechanism. Neurons do more than just relay signals forward into the brain. They also signal back down the line, reaching out to neighboring neurons tuned to nearby frequencies, exciting some and muzzling others. These feedback controls allow us to sift through incoming sounds for the most important information, so that we are not overwhelmed by meaningless noise. In young brains, the neurons and their feedback controls grow and link up to each other. Even in adulthood, experiencing new sounds can rewire the auditory cortex. If a rat is trained to recognize sounds at a particular frequency, the corresponding region of the tone map will get bigger.
Tinnitus arises when this flexibility goes bad. Things may start to go awry when toxic drugs, loud noises, or even whiplash cause damage to the nerve hairs in the ears. The injured nerve hairs can no longer send signals from the ear to the tone map. Bereft of incoming signals, the neurons undergo a peculiar transformation: They start to eavesdrop on their neighbors, firing in response to other frequencies. They even start to fire sometimes without any incoming signals. As the brain's feedback controls get rewired, the neurons end up in a self-sustaining loop, producing a constant ringing. That is why tinnitus often doesn't go away when people get their auditory nerve surgically cut.
Normally, we hear sounds only when they make our eardrums vibrate. The vibrations cause nerve hairs in the inner ear to shiver, and that triggers electric signals that travel along the auditory nerve into the brain. One of their first stops is a patch of gray matter called the auditory cortex. Each nerve hair is tuned to a particular frequency of sound and excites only certain neurons in the auditory cortex. As a result, the neurons in the auditory cortex form what is known as a tone map. The neurons at one end of the auditory cortex are tuned to low frequencies; the farther you go toward the other end, the higher the tuning of the neurons.
This sound system comes with an elaborate feedback mechanism. Neurons do more than just relay signals forward into the brain. They also signal back down the line, reaching out to neighboring neurons tuned to nearby frequencies, exciting some and muzzling others. These feedback controls allow us to sift through incoming sounds for the most important information, so that we are not overwhelmed by meaningless noise. In young brains, the neurons and their feedback controls grow and link up to each other. Even in adulthood, experiencing new sounds can rewire the auditory cortex. If a rat is trained to recognize sounds at a particular frequency, the corresponding region of the tone map will get bigger.
Tinnitus arises when this flexibility goes bad. Things may start to go awry when toxic drugs, loud noises, or even whiplash cause damage to the nerve hairs in the ears. The injured nerve hairs can no longer send signals from the ear to the tone map. Bereft of incoming signals, the neurons undergo a peculiar transformation: They start to eavesdrop on their neighbors, firing in response to other frequencies. They even start to fire sometimes without any incoming signals. As the brain's feedback controls get rewired, the neurons end up in a self-sustaining loop, producing a constant ringing. That is why tinnitus often doesn't go away when people get their auditory nerve surgically cut.
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