Did You Know?

Did you know, I mean were you aware that our ears not only hear but actually "make" sound? Why yes, they do. I have recently learned this as I have been studying the human ear for one of my classes at school.

So, inside our inner ear is a little shell-like structure called the cochlea. Inside this cochlea is the most wonderous cell in our body, it is an outer hair cell. This outer hair cell is part of a unique structure called the organ of corti which sits on the basilar membrane that runs the length of the cochlea.

When a sound wave comes into our ear and vibrates our eardrum this vibration is picked up first be the malleous bone, which is connected to the eardrum. The malleous bone is connected to the incus bone, which in turn is connected to the stapes bone. The stapes bone as it gets vibrated pushes up against the oval window(this is the opening into the cochlea). The cochlea is filled with fluid that is water-like in consistency and the pressure from the vibration of the stapes will cause a "traveling wave" to occur along the basilar membrane. Now, you remember that sitting atop this membrane is the organ of corti-so naturally the "wave" is causing the organ of corti to be displaced, both longitudinally and laterally. When this happens the cilia that sits on top of the outer hair cell stimulates the tectorial membrane (which is a gelatinous mass on the top of the organ of corti). The outer hair cell not only is a sensory cell that reactes to the "traveling wave", but it is also a motor cell that becomes activated by the vibrations. The walls of this cell have motile properties that cause the cell to contract and change shape. The cell becomes longer and is able to intensify the stimulus signal. The "inner" hair cells respond to the amplified stimulus and send the message along the neural fibers to the brain. This is how we hear.

The sound wave enters our ear canal, is translated by the bones in the middle ear to a wave the inner ear can respond to. Then the inner ear translates (through the above mentioned process) the wave into a neural signal that our brain can understand. But, you are asking-how does the ear "make" sound. I will answer.

The outer hair cell is what "makes" a sound. When it receives stimulus and has a motile reaction to it, causing it to grow and intensify the signal- that makes noise! It is called an otoacoustic emission and can be recorded by putting a little microphone into your middle ear. The outer hair cell is unique in the human body. It is the only cell that is both sensory and motile, both passive and active. The only one! I don't know about you, but that fascinates the hell out of me! And all of that happens in a little "shell" called the cochlea, which is 9mm wide at the base and 5 mm high, from base to apex. And, if we could drill right through your face to get to it, that would be approximately underneath and behind your cheek bone-but at the same plane that your ear is on.

What I find perfectly criminal is that something as wondrous, unique, and grand as the outer hair cell is simply called "the outer hair cell"- I would like to re-name it. If for no other reason than to call the worlds attention to " the amazing cell that could". I mean until today had you ever heard of this? Me neither. So, I'm open to any suggestions for the cell that not only makes hearing possible, but creates sound too! The cell that is passive when it is supposed to be passive and active when it is supposed to be active. Talk about obedient, flexible, capable-it does what no other cell in the body does, it does the impossible. Perhaps we should call it the Don Quixote cell.

The information above is referred to as "The Motor Theory of Hearing" and this theory has been around since 1948, but not officially. Let me explain. There was a scientist by the name of Bekesy, he won the Noble Prize for his discovery of "the traveling wave". He was under the impression that the inner ear was passive, linear, and broadly tuned. A scientist from England named Gold had been doing experiments of his own and he was excited to discover all the information about the outer hair cell in the organ of corti, he believed the inner ear was active, non-linnear, and narrowly tuned. In 1948 Gold traveled from Enland to Harvard to discuss his findings with Bekesy. Bekesy was not amused at this British upstart and would "have none of it". So, it took 30 more years before the world would hear about the "motor theory of hearing", when another British scientist named Kemp published a paper about his findings which coincided with Golds'.

More amazing facts about our ear. Why, you are wondering, is the cochlea shaped like a shell? In utero, the fetus is designed to form the auditory nerve, before the inner ear structures. So the auditory nerve is formed first including the fibers that will be attached to both the inner and outer hair cells, when the cochlea is finally formed it develops from the basal end first and grows in a spiral fashion toward the apex and as it grows it drags the neural fibers behind it so they are twisted into a " bundle of nerves" (sorry, I couldn't resist myself). So, we have a little shell in our ear. One of the interesting facts about this nerve bundle is that the outside fibers carry the information about higher frequencies and the fibers in the center carry information about lower frequencies. What is so cool about this is if you have damage to your auditory nerve, it will most likely only affect the outside fibers and you will still be able to hear speaking tones, since those are generally lower frequencies. (With the exception of certain talking mice that live at Disneyland.)

Ears. Amazing!