Auditory transduction and pathways

In auditory transduction, auditory refers to hearing, and transduction is the process by which the ear converts sound waves into electric impulses and sends them to the brain so we can interpret them as sound. And the ear itself is made up of three parts: the outer ear, the middle ear and the inner ear, and all three play a role in hearing.

You can think of the ear like a house, with a porch, a living room and a short corridor that leads to two bedrooms at the end. The porch would be the outer ear, made up of the pinna and the external auditory canal. The middle and inner ear, would be the actual “house”, carved inside the temporal bone. The middle ear is like a living room, furnished with the tiny ear bones - called the malleus, incus, and stapes - that articulate or touch one another. The inner ear is the rest of the house, made up of a corridor and two rooms -  where the corridor is the vestibule, and the two rooms are: the cochlea, which is anterior to the vestibule - so towards the front of our head -  and the semicircular canals - posterior to the vestibule, so towards the back.

Now, the outer, middle and inner ear are functionally connected to one another, which is crucial for hearing. Between the outer and middle ear there is the tympanic membrane - or eardrum - and between the middle and inner ear there are two windows: the oval window, above, and the round window, below. So, when you hear the wind rustling through the leaves, the resulting sound waves are directed by the pinna into the external auditory canal, and they reach the eardrum, making it vibrate. The malleus is attached to the eardrum, so the vibrations are transmitted along the tiny bones - from the malleus to the incus, and then from the incus to the stapes. The foot of the stapes rests on the oval window - and since the oval window is about 20 times smaller than the eardrum, the sound waves are amplified as they vibrate their way across the tiny bones. From the oval window, the vibrations are transmitted to the inner ear. The part of the inner ear that transforms sound waves into electrical impulses is the cochlea.

The cochlea is a snail-shaped structure that coils around a bony axis called the modiolus. The base of the cochlea is contiguous with the middle ear - through the vestibule - and its tip goes deep into the temporal bone. The cochlea has an outer bony shell that contains a fluid called perilymph. Inside the bony shell, there is a membranous duct called the cochlear duct - which contains a fluid called endolymph. So the cochlea is actually made up of three fluid-filled tubes - arranged one above the other. In the middle, there is the cochlear duct - or scala media. Above it, there’s the scala vestibuli, and below it, the scala tympani. However, the cochlear duct ends right below the tip of the cochlea, leaving an opening called the helicotrema right above - so the scala vestibuli and the scala tympani communicate with each other through the helicotrema. 

Now, let’s look at a cross section of the cochlea. The cochlear duct is shaped like a triangle with the sharpest angle facing the modiolus. The upper side of this triangle is the vestibular membrane - and that separates the cochlear duct from the scala vestibuli. The lower side is the basilar membrane - and that separates it from the scala tympani. And the outer side, opposite the modiolus, is the spiral ligament. The vestibular membrane is flexible and allows the motion of sound waves to travel from the perilymph and transmit into the endolymph. The spiral ligament is covered by a specialized epithelium called the stria vascularis - that secretes the endolymph into the cochlear duct. There are also some cells called marginal cells which pump potassium ions into the endolymph, making it a fluid with high potassium concentration. Finally, above the basilar membrane lies the organ of Corti - which is the key to auditory transduction.

Now, before we dive into how the organ of Corti works, let’s first talk about sound. Sound is produced by a vibrating object - such as a tuning fork, or the larynx, and it propagates through a medium - which can be gas, liquid or solid. For example, let’s say your kitten meows at 6 am for some food - even though her bowl is actually still half full. Well, when her vocal cords vibrate, that disturbs the air molecules, which form areas of high pressure - where the air molecules are more compressed - and areas of low pressure - where they are less compressed. Kinda like what happens when you throw a rock in a calm pond. This is referred to as a series of molecular compressions and rarefactions. The cat’s meow propagates through air towards our inner ear, and it’s called a sound wave. Sound waves can be simply represented as a sine wave - and, like any respectable wave, they have a frequency, a wavelength and an amplitude. Frequency is the number of waves per unit time, while wavelength refers to the distance between two consecutive wave crests. Sounds with higher frequencies (so more waves per unit time) have a shorter wavelength, and we perceive them as high pitch - like your voice on helium. Sounds with lower frequencies (so less waves per unit time) have longer wavelengths, and we perceive them as low pitch - like a whale’s call. Finally, there’s amplitude, and that’s the height of the wave, and we interpret it as loudness. So low amplitude, might be your lover waking you up with a whisper, whereas high amplitude, might be them banging two pots together when you don’t get up.

So, to see what happens to the sound waves, let’s uncoil the cochlea. When the footplate of the stapes hits the oval window, the oval window amplifies and transfers the sound waves to the scala vestibuli. Amplification happens because the oval window is about 20 times smaller than the eardrum, so the vibrations are concentrated in a smaller space. This is important because it’s more difficult for waves to propagate in the fluid of the inner ear compared to the air. So now that the sound waves are strong enough, they transfer the pressure to the perilymph in the scala vestibuli. The sound wave travels towards the helicotrema, making the perilymph molecules vibrate along the way. However, for us to hear, sound waves take a shortcut through the cochlear duct and are transferred to the scala tympani. This shortcut makes the vibrations displace the basilar membrane towards the scala tympani.