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.