The cochlea is NOT the eardrum. Let's get that straight right now. The cochlea is the part of the ear that takes natural sound and separates it into its component frequencies. This isn't easy. Natural sounds are very complex. If you don't believe me, open winamp (if it's not already running). Now, get rid of those little bouncy looking bar things by clicking on them. They suck. You should get an oscilliscope display. A little squiggly line going nuts. That is what natural sound looks like, only probably uglier. Since this is music, the frequencies are organized a bit more nicely and you can probably catch some bits of regularity. Anyway, you can tell that that squiggly line is a damn sight short of a sine wave. So, there was this guy, Fourier. He discovered once upon a time that any curve could be represented by a series of summed sine waves. Seperating a curve out like this is called Fourier analysis. This is what the cochlea does (well, other things too, but that's its main auditory function), except it does it fast fast fast. Real time processing. Evolution beat Fourier to the punch, but he's still a cool guy.

So, how does the cochlea do a Fourier analysis? It's astoundingly simple (in principle). The stapes transfers sound energy to the fluid-filled cochlea through the oval window, a membranous, flexible structure. The cochlea also has a round window, which allows pressure waves to be created in the cochlea. When the oval window is pushed in, the pressure pushes the round window out. Without this mechanism, the oval window wouldn't have any give to it, since the cochlear fluid is relatively incompressible. The cochlea looks like a snail's shell. It's a long coiled tube. The end near the oval window is called the base the end furthest away (at the center of the spiral) is called the apex. This tube contains some wonderfully engineered structures. To explain how it functions, I'd better make a blocky oversimplified diagram.

    |                                                        |
    |                                                        |
    |                                                        |
    |\___                                                    |  ...
    |..  \___              Scala vestibuli (fluid)           |  ... Stria vascularis
    |..      \___                                            |
    |..          \___                                        |
    |...             \___                                    |  === Tectorial membrane
    |...                 \___                                |
    |....                    \___                            |  +++
    |...   Scala media(fluid)    \__                         |  +++ Organ of corti
    |..                             |                        |
    |..                             |                        |  {} Outer hair cells
    |..                           ++|                        |
    |..   ======================++++|________________________|   # Inner hair cell
    |+++++++{}{}{}++++++++#+++++++++/                        |
    |++++++++++++++++++++++++++++++/                         |  """ Basilar membrane
    |""""""""""""""""""""                                    |
    |                                                        |
    |                                                        |
    |                                                        |
    |              Scala tympani (fluid)                     |
    |                                                        |
    |                                                        |
    |                                                        |
    |                                                        |

That's what the 'tube' of the cochlea looks like in cross section. There are three fluid filled chambers. The pressure wave generated at the oval window travels through the scala media. Now, to explain what all the goop does. The basilar membrane is the part of the cochlea that seperates out the component frequencies of a sound. When sound enters the cochlea, it sets up a travelling wave in the the basilar membrane. At the base of the cochlea, the basilar membrane is narrow and stiff. This means that this portion of the cochlea has a natural resonance at high frequencies. (Think of guitar strings. The small tense ones make the high noises.) Further up the cochlea, the basilar membrane gradually becomes wider and looser. Thus, toward the apex the basilar membrane resonates with low frequencies. (thick guitar strings).

Now we have a physical process which is separated into frequencies. The hair cells are what turn this action into neural impulses. The body of the hair cells are embedded in the organ of corti, a structure which is attached to and moves with the basilar membrane. However, at the surface of the organ of corti, the hair cells project sterocilia, tiny fibers, which are attached to the tectorial membrane. When the basilar membrane vibrates, it creates a shearing action between the organ of corti and the tectorial membrane. This bends the sterocilia of the hair cells, which activates a physical process that opens ion channels in the hair cells. When the outer hair cell cilia are deflected in one direction, they push in that direction. This creates positive feedback and amplification of the signal. The outer hair cells recieve inputs from the auditory nerve, as well as outputting to it. The inputs allow the auditory system to regulate the amount of amplification occuring from outer hair cells. Typically many outer hair cells will output to the dendrites of a single neuron this means that they cannot precisely code individual frequencies. They may transmit information about amplitude (volume). Generally there are three rows of outer hair cells and one row of inner hair cells.

The inner hair cells have usually have one neuron attached to them, over which they transmit activity. These fibers create a tonotopic map. This simply means that the fibers' positions are organized by frequency. This is called a place coding (information about a type of input is encoded by a neuron's position in the brain). But wait! There's more! Because the inner hair cells only transmit when deflected in one direction, the action potentials in the corresponding neurons only occur when the sound wave peaks. While there isn't a spike for every peak, each spike that is transmitted corresponds to a peak. This behavior is called phase locking allows the neurons to carry information about the phase of the sound waves. (this type of coding is called rate coding, information coded in a neuron's rate of fire).

The compositions of the fluid media of the cochlea are also important to it's function. The scala media contains an extremely high concentration of K+. When the ion channels of the hair cells open, they open on the scala media side. K+ rushes in because of its high extracellular concentration, depolarizing the cell. This signal is transmitted to the cochlear neurons. The K+ leaves the hair cell into the Scala tympani, which has a low K+ concentration. Thus, it also diffuses out of the cell on a concentration gradient. One can think of the hair cells as functioning to allow current flow from the scala media to the scala tympani. Notice that the ion channels are opened by the physical force of the sound, and that the ion flow is entirely by diffusion. The hair cells do none of the work. This means that they can't get fatigued under normal conditions.

The K+ that flows into the scala tympani is constantly cycled through the spiral ligament of the cochlea (not shown) and into the stria vascularis. The stria vascularis secretes the K+ back into the scala media. That's not all it does though. Hair cells have to have nutrients to maintain their basic cellular processes. There's a problem though. Blood flow to the hair cells would create noise which would interfere with their function. To overcome this, blood flows through the stria vascularis, and the nutrients diffuse into the scala media, and are uptaken by the hair cells from there. This is why you can only hear blood flowing in your ears if your heart is beating very fast.

So that's how the cochlea works. Fucking rad if you ask me.

Back to how your brain works.
On to auditory tuning.

Coch"le*a (?), n. [L., a snail, or snail shell, Gr. a snail, fr. a shellfish with a spiral shell.] Anat.

An appendage of the labyrinth of the internal ear, which is elongated and coiled into a spiral in mammals. See Ear.


© Webster 1913.

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