Auditory transduction and pathways

Definitions & Key takeaways

Auditory transduction refers to the process of converting sound waves into electrical signals that can be processed by the brain. The auditory nerve carries these electrical signals from the ear to the brain.

Auditory transduction starts by converting sound pressure waves into mechanical vibrations of the eardrum and ossicles. These vibrations get transmitted through the middle ear to the cochlea, where they are converted into electrical signals by hair cells. These electrical signals are sent along the auditory nerve to the brain for interpretation.

Chapters:

Watch video only

Introduction0:00–0:26

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.

Outer, Middle, and Inner Ear0:26–2:21

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.

Cochlea2:21–4:20

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 waves4:20–8:05

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. The basilar membrane has different characteristics along its length, making it “tuned” to different frequencies in different regions.
Near the base of the cochlea, the basilar membrane is narrow and stiff - meaning that the sound waves need to have more waves per unit time, so higher frequencies, in order to displace it.
Near the tip, though, the basilar membrane is wider and floppier - so fewer waves per unit time - or lower frequency sounds - are required to displace it.
This organization is called a tonotopic map, and it ranges from about 20000 Hertz at the base to 20 Hz at the tip - which is our hearing range.
When the basilar membrane is displaced, it transfers the sound wave - which is actually a pressure wave - to the scala tympani.
This pressure wave travels all the way down the scala tympani until it reaches the round window - which bulges towards the middle ear, relieving the pressure.Now, the organ of Corti is stimulated by the vibration of the basilar membrane.

Organ of Corti8:05–9:37

The organ of Corti is a layer of tissue, that’s actually as long as the cochlear duct itself. On cross-section, it’s made up of mechanosensory cells called hair cells, that have 30 to 300 finger-shaped projections called the stereocilia - which are like hair - on the top or apical end.
At the opposite side - or the basal end of the cell - there’s the presynaptic membrane, which is close to the cranial nerve.
Between the hair cells, there are supporting cells that play a supporting role. The hair cells are arranged in rows, one row of inner hair cells - closer to the modiolus, and three to five rows of outer hair cells - closer to the spiral ligament.
Inner hair cells are innervated mainly by sensory nerve fibers, so they’re the ones in charge of auditory transduction. Outer hair cells, on the other hand, are innervated mainly by motor nerve fibers that carry signals from the brain, and they contract and stretch in response to these signals - and this changes the stiffness of the basilar membrane in order to intensify or minimize the auditory signal, to help us hear a whisper, or to protect our inner ear from damage during a blasting rock concert.
The tips of the stereocilia are embedded in the tectorial membrane - which is a gelatinous membrane attached to the modiolus at one end and free at the other end - so like a flap.So when the basilar membrane vibrates, it actually pushes the organ of Corti and the hair cells up against the tectorial membrane.

Auditory transduction9:37–11:22

The apical end of the cell - near the endolymph - has stereocilia, and at the tip of each stereocilium, there is a protein filament attached called a tip link.
The tip link attaches to two things, one of them is the membrane of a particular stereocilium, and the other is a mechanically gated potassium channel in the membrane of another, longer stereocilium in the next row.
So, imagine that the shorter stereocilium is a person, and the longer stereocilium is a room, then the tip link is like the arm of the person holding the door of the room.
When the hair cells move towards the tectorial membrane, the shorter stereocilia bend towards the longer ones and the arm pulls and opens the door - or the tip link opens the potassium channel.The potassium concentration is higher in the endolymph than inside the cells, so when the potassium channel opens, potassium ions enter the hair cell in the apical end, and the cell membrane depolarizes.
At the opposite side of the cell - or the basal end - there’s the presynaptic membrane, which is close to the cranial nerve.
Depolarization goes all the way through the cell from the apical membrane to the presynaptic membrane, where it causes voltage-gated calcium channels to open.
Calcium ions then enter the cell and that triggers vesicles in the cytoplasm that are filled with glutamate to be released into the synaptic space.
Dendrites of neurons that form the auditory nerve interpret glutamate as a “go go go” signal, and depolarize, sending an electrical impulse to the brain.Once the neurons of the auditory nerve depolarize, that signal heads down the axon towards the brainstem.

Auditory pathway11:22–12:41

First, they enter the medulla on the same side of the body and synapse with neurons in the cochlear nuclei. From the cochlear nuclei, the electric impulse follows three different paths.
In one path, the electric impulse crosses over and goes up to the inferior colliculus of the midbrain on the opposite side of the body.
In a second path, the impulse also crosses over and goes up to the superior olivary nuclei of the pons on the opposite side of the body.
And the third path goes straight to the superior olivary nuclei on the same side of the body. Then, from each superior olivary nuclei the impulse goes up to the inferior colliculus of the midbrain on the same side.
Finally, from each inferior colliculus, the electric impulse goes to the medial geniculate nucleus of the thalamus, that sends the impulse to the auditory cortex in the temporal lobe—which includes Brodmann's areas 41 and 42.
Since the auditory pathway splits, some nerve fibers keep on the same side while others crossover, so the impulse reaches the auditory cortex on both sides of the brain - which then interprets the characteristics of the sound, giving us an idea of what we’re listening to.Alright, as a quick recap, the ear contains the organ of Corti, which is responsible for transforming sound waves into electrical impulses.

Review12:41–13:24

The basilar membrane - where the organ of Corti sits - is “tuned” to different frequencies in different regions - which is called a tonotopic map.
So specific frequencies cause stereocilia of specific hair cells to bend, depolarize the cell, and convert sound waves into electrical impulses.
Electrical impulses then follow the auditory pathway, passing through the cochlear nuclei, superior olivary nuclei, inferior colliculi, thalamus and finally reaching the auditory cortex where sound is finally interpreted - like that kitten