If you read the ‘About’ page, or anything about me, you probably noticed that I work on hair cell regeneration in the cochlea. But, perhaps, some readers are not familiar with the machinations of the inner ear. So, I’ll make a quick post with some relavent info to help in understanding future hearing and ear-related postings.
The sense of “hearing” refers to the detection of sound waves in our environment. Humans are able to distinguish a lof of information out of these waves: direction (due to fact we have two ears!), loudness, pitch, timbre, and frequency. Waves are “collected” by the outer ear (what we think of as an ear) and transmitted down the ear canal, resulting in vibrations across the ear drum. The ear drum, in turn, passes vibrations on to the ossicles (tiny bones called the malleus, stapes, and incus) which serve to fine-tune and amplify the sound. The stapes sits atop the round window, which is a small membrane-covered hole in the base of the cochlea.
The name “cochlea” means “snail” and refers to its coiled shaped. (More below the fold….)
Above is an illustration showing the outer and middle ear, and below is one of the inner ear and cochlea.
As the vibrations of the stapes cause it to “hit” on the cochlea’s round window, small frequency-specific disturbances are transmitted into the fluid-filled spaces in the cochlea. Each part of the cochlea corresponds to a different frequency range, high frequencies at the base of the coil and low frequencies at the top. The sensory cells of the cochlea are called “hair cells,” so named for their hair-like projections which protrude into the fluid filled space (endolymph) in V-shapes.
Hair cells are organized into 3 rows of outer hair cells, and 1 row of inner hair cells. Inner hair cells send auditory information to the brain, while outer hair cells act to stabilize or intensify the vibrations transmitted throughout the cochlea. Deafness and hearing loss occurs (most often) in a one-to-one ratio with hair cell loss. Noise, certian chemicals, and age all act to selectively and gradualy kill off hair cells. This is a serious and prevalent disorder in America, and is currently irreversible. For more on the anatomy of the cochlea, please check out a link on my BlogRoll (sidebar to the left) called Promenade ‘Round the Cochlea. It contains a lot of helpful information about inner ear biology in a graphical interface.