June 14, 2013
Beyond the middle ear; “current” thoughts about hearing system. This includes the inner ear, the nervous system and the brain. Part two in a series of blogs about the physiology of hearing.
In our previous blog entry we discussed the transmission of sound energy from vibrations of air molecules to mechanical vibrations carrying the properties of sound in the middle ear. In order for the brain to listen to sounds, this energy must be transformed into the electrochemical signals the nervous system utilizes to gather information.
As the footplate of the stapes bone (the “stirrup”) vibrates at the oval window of the cochlea, or inner ear, all the amplified vibrations from every sound we hear at all times cause the fluid inside to vibrate as a travelling wave. Smack the surface of swimming pool water with your palm and you will cause waves. Now imagine waves travelling from one end of the inner ear to the other, often thousands of times per second. These waves cause the “basilar membrane” to rise and fall, similar to a buoy on the water’s surface.
Spaced along the basilar membrane, which separates two fluid-filled chambers, are several thousand “organs of Corti”, which are the organs of hearing. “Inner hair cells” with whip-like cilia at the end jostle in the surrounding fluid every time the membrane vibrates. If the vibration is strong enough, the cilia will bend and trigger a nerve impulse to the brain. “Outer hair cells” are not receptors, but they help ensure only the organs at the peak of each vibration (closer to the oval window for high-pitched sounds and farther for lows) transmit a signal to carry the frequency of the sound. They also act as an amplifier and are responsible for hearing soft sounds. The nerve pathway beginning at every point along the basilar membrane carries the signal to a specific brain region responsible for interpreting that frequency. This is what we interpret as the “pitch” of the sound.
These nerve “spikes” carried by individual neurons (which gather into a bundle known as the auditory nerve) travel in synchrony toward the temporal lobe of the brain. Other regions of the brainstem, midbrain and thalamus help us organize and analyze the information into coherent sounds. Finally it reaches the cortex, the outer “smart” part of the brain, specifically in the temporal lobe.
It is interesting to note the brain does not “hear” any sound vibrations at all. It only receives messages coded in the patterns of nerve impulses and the specific brain regions that are stimulated. The brain can use this information to analyze, recognize, process and remember sounds, thereby extracting all available information carried by the neuron firing patterns.
To summarize, hearing is a complex chain of events involving sound vibrations becoming mechanical vibrations becoming fluid wave motion becoming electrochemical nerve transmission. The end product is a complex interpretation in the brain. Any miscue in this hearing system can cause problems hearing or interpreting sounds. It is imperative to protect the system we have. As always we will help keep you on a “clear path” to good hearing.