In our July 2017 newsletter, we described how birds and amphibians have the capacity to regenerate cells for hearing throughout their life. We noted that humans are born with 15,000 hair cells per ear and throughout our lifetimes, these hair cells die off. Unlike birds and amphibians, the dead hair cells in our ears never regenerate, which leads to hearing loss. Researchers are working on the regeneration of these hair cells to restore hearing; however, there is more to the treatment of hearing loss beyond cell regeneration. Read on to learn more about how our brains play a big role in what we actually hear.
The ear has three divisions: the external ear, the middle ear, and the inner ear. The external ear collects sound waves and funnels them down the ear canal, where they vibrate the eardrum. Within the middle ear, the eardrum is connected to the middle ear bones, the ossicles. These are the smallest bones in the body and they mechanically carry the sound waves to the inner ear. The inner ear contains the cochlea; this is the organ that converts sound waves into neural signals, which are passed to the brain via the auditory nerve.
There’s a population of neurons in the brain that sends signals out to the middle ear mechanisms that increase or decrease the transfer of sound wave amplitudes arriving at the cochlea from the ear canal. One way that the brain controls the transfer of sound level to the cochlea is by relaxing or tightening the eardrum, thus increasing or decreasing its motion, respectively. Another way the brain controls sound levels arriving at the cochlea is through the control of a small muscle in the middle ear called the Stapedius, which connects to the Stapes (or Stirrup – its more common name).
By relaxing or contracting this muscle, the brain can regulate the amount of vibration transfer to the cochlea. So, the brain has control over turning up or down the level of sound that reaches our cochlea, similar to a volume control.
The brain also controls which neurons in the inner ear are actively sending signals to the brain and which are temporarily set to “standby mode”. Neurons in the inner ear are arranged by frequency and the brain can increase or decrease certain frequencies that it transmits. Similar to a zoom function, our brain is able to selectively attend to specific frequencies and timbres. For example, when listening to an orchestra, we can focus specifically on a flute.
When we are in a loud restaurant we can focus on the voices of those at the table, though the general ambient noise may be much louder. The brain allows us to hear details of a particular sound that may be 25 dB lower than ambient levels.
This focusing control mechanism of our hearing sensitivity allows us to perceive energy levels as small as the movement of a molecule of hydrogen up to the roar of an airplane turbine engine during takeoff. Treatments that assist the hearing-impaired community will eventually combine both hair cell regeneration and rejuvenation of the middle ear skin cells to regain some of the control we were born with. Now that’s news worth listening to!