The cochlea in sound perception
23 February 2019

Of all of the amazing organs we have in our bodies, there are few that are 
more fascinating than the cochlea.

The cochlea is a tiny organ located in our inner ear, and it has one very 
important job: to transduce sound vibrations traveling through the air into 
the sounds we perceive.  We are still not entirely sure how the cochlea works 
but we know enough to be able to appreciate the elegance and complexity of this
remarkable organ.

Sounds are captured by your outer ear (or pinna) and channeled down your ear 
canal until they reach the tympanic membrane, aka the eardrum.  The tymapnic 
membrane vibrates due to the oscillations of the sound waves that are hitting 
it.  These vibrations match the pitches and loudness that are being conveyed 
from the source of the sound.  

The tympanic membrane is connected to three tiny bones in your middle ear 
called ossicles.  It is the job of these ossicles to transfer the vibrations 
from the tympanic membrane to the cochlea.  One of the ossicles is attached to 
one end of the cochlea, and this attachment point is where the vibrational 
energy is received from the ossicles.

This is where the real magic happens.  The cochlea resembles a curled up sea 
shell. If you uncurled it, it would resemble a long cone, with one end of the 
cone being much smaller in diameter than the other end.  The vibrations from 
the ossicles come in at the wider end of the cone.  The cochlea is completely 
sealed up, and contains a complex microanatomy of parts that are responsible 
for transducing the sound vibrations into neural signals.

The cochlea is divided into  compartments by the basilar membrane.  This 
membrane runs down the length of the cochlea, separating the inside into two 
compartments.  Running down the length of the basilar membrane is the "organ 
of Corti", composed of long arrays of millions of cells called hair cells. 
Hair cells are highly specialized neurons that are responsible for directly 
converting sound vibrations into neural signals.  

Each hair cell has a small fiber protruding from it that can bend.  It bends 
whenever the basilar membrane on which it sits is vibrated or deformed.  The 
movement of the fiber causes ion channels to open, ultimately leading to the 
production of a neural signal that is transmitted via an axon fiber projecting 
from the hair cells.  The axon fibers from the millions for hair cells in the 
cochlea all bundle together to form the auditory nerve, which is responsible 
for sending sound information from the ears to the brain.

Sound vibrations that are received by the cochlea cause the basilar membrane 
to vibrate and contort at certain points along its length.  The hair cells 
that are located at the points where the vibrations and contortions are 
occuring will be activated, sending neural signals to the brain via the 
auditory nerve.  The brain translates these signals into perceptions of sound 
pitches and loudness.  

How the cochlea translates the physical movements of the hair cells into 
neural impulses that are perceived as pitch and loudness is not well 
understood.  Two possible theories have been suggested. The first is temporal 
theory, which suggests that the hair cells encode pitch by firing at different 
rates.  Presumeably the hair cell would fire slower for low pitched sounds and 
fire faster for higher pitched sounds.  This can't be the entire explanation 
for how pitch is encoded, however, because the ion channels that physically 
control the neural impulses can't keep up with the many different pitches we 
can perceive.

Another theory to explain pitch encoding is place theory.  Place theory posits 
that pitches are transduced accoding to where along the length of the organ of 
Corti the vibrations are the strongest.  The structure of the cochlea is such 
that sound vibrations received on one end will resonate through the cochlea in 
a manner directly related to the pitches being received.  So, a low pitched 
sound would cause the hair cells in one particular spot along the organ of 
Corti to bend, while a high pitched sound would cause hair cells further up to 
bend.  Our brains ultimately translate the hair cell locations into different 
pitches.

It is very likely that both firing rate and hair cell location are both 
involved in transducing air vibrations into sounds.  What is fascinating is 
the sheer complexity of the cochlear apparatus in our ears.  It is likely that 
animals such as bats and rodents, who rely much more heavily on their sense of 
hearing than humans do, might have even more complexity built into their 
cochleas than we do.
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