Within the ear the cochlea is considered the most complex of all the components involved. Its primary function is to take all of the vibrations that are caused by sound waves and turn them into electrical information that the brain will interpret as a distinctive sound.
There are three connecting tubes that make up the structure of the cochlea, which are all separated by some of the most sensitive membranes. All of these tubes are coiled into shapes similar to that of a snail shell, but it is a lot easier to comprehend what is going on if you picture them all laid flat. It also becomes a lot clearer if you think of two of the tubes as one single chamber. The membranes that are between the tubes are extremely thin, so this way the sound waves are able to travel throughout the tubes as if they were all connected.
Your stapes are going to move side to side, which creates waves of pressure within the cochlea. The window that separates the cochlea from the middle ear provides the fluid with a place to go. As the stapes move inward, the window moves outward and vice versa.
The basilar membrane is the middle membrane. It has a rigid surface that covers the entire length of your cochlea. Whenever your stapes move inward and outward, they help to push and pull all of the parts of the membrane located just underneath the window. This movement creates a wave that moves along the length of the membrane. It is almost like a ripple travelling on a pond that moves the wave from the window and down to the cochlea.
There is a very strange structure that makes up the basilar membrane. In fact, there are between 20 and 30 thousand fibers that reach all the way across the cochlea width. They are very short and stiff, and they are located near the window. As you make your way along the tubes, you will notice the fibers tend to get a lot longer and more flexible.
This entire process works together to give the fibers varying frequencies. Specific frequencies help to resonate all of the fibers perfectly at a designated point, which causes them to vibrate extremely quickly. It is this principle that makes a kazoo and a turning fork work effectively. When you have a specific pitch in place the tuning fork will begin to ring and hum in such a manner that the reed within the kazoo will begin to vibrate.
While the waves are travelling across the membranes they are not able to release a lot of energy because they are too tense. However, once the waves reach the fibers with the identical frequency the energy is immediately released. Due to the increase in the length of the fibers and the decrease in how rigid they are, the higher frequency waves are able to vibrate the fibers that are in closer proximity to the window. The lower frequency waves are able to vibrate all of the fibers at the opposing end of the membrane.
It is not until one of the waves reach the fibers and sends out a frequency that the basilar membrane will move. Whenever the waves make their way to the resonating point the membranes will then release a large burst of energy within the area. That energy is potent enough to push the hair cells at that moment.
As the cells in the hair are moved, they are able to send any impulse into the nerve of the cochlea. That nerve works to send an impulse into the cerebral cortex, which is where the brain is able to interpret them. It is the responsibility of the brain to determine what the level of pitch is. It does this based upon a certain position of the cells that are sending the impulses. Louder sounds are going to send off more energy at the resonating point along the membrane, as well as move a larger amount of cells within the area. Your brain will know that the sound is louder because there will be an increase in the number of hair cells that are activated within a specified region.
Dr. Lynn O’Bray-Donohue is a N.Y.S Licensed Audiologist and Hearing Aid Dispenser, and has been providing Audiologic Care at East End Hearing since 1987. She has been co-owner of East End Hearing, with her partner Dr. Michael Qualley, since 1991. She is often invited by civic organizations to speak on behalf of hearing loss and hearing aids. On a personal note, Dr. O’Bray-Donohue spends her free time reading, traveling and relaxing with her family. She lives in the local community with her husband and daughter.