Vestibular transduction

Definitions & Key takeaways

Vestibular transduction refers to the process by which the vestibular system in the inner ear converts specific head movements into electric impulses so that we can interpret where we are in space. The vestibular system is responsible for transforming movements into electrical impulses that follow the vestibular pathway, passing through the vestibular nuclei, to reach different structures to maintain balance, such as external muscles of the eyes, the muscles of the head and neck, the structures that control muscle tone, the cerebellum, and the somatosensory cortex.

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Introduction0:00–0:47

With vestibular transduction, “vestibular” refers to balance, and transduction refers to the process by which the ear converts specific head movements into electric impulses, so that we can interpret where we are in space.
The ear is made up of three parts: the outer ear, the middle ear, and the inner ear. The first part is the outer ear which is the part you see and hang earrings on, called the pinna, as well as the ear canal.
The second part is the middle ear, which is a tiny chamber that houses even tinier ear bones—the malleus, incus, and stapes.
The outer and middle ear only play a role in hearing - however, the third part, the inner ear, deals with both hearing and balance.

Inner ear0:47–1:39

On the outside, the inner ear has a tough bony shell - the bony labyrinth; and inside the bony labyrinth, there is the membranous labyrinth.
Now, both of these sections are filled with fluid - the bony labyrinth contains a fluid called perilymph, while the membranous labyrinth contains endolymph.
The bony and membranous labyrinth make up the structure of all three parts of the inner ear. The first is the vestibule, which is like a hallway that leads up to two other parts or rooms - the cochlea, towards the front of our head, that deals with hearing, and a second room containing the three semicircular canals, towards the back.
The semicircular canals, along with two other structures - the utricle and saccule, which are located in the vestibule - make up the vestibular apparatus, that helps us detect changes in our static and dynamic equilibrium.

Utricle1:39–5:28

Our static equilibrium is a job for the utricle and saccule - also known as the otolith organs. They contain endolymph, as well as special balance receptors that detect changes in our head position in relation to horizontal or vertical acceleration.
Now, inside the utricle, there’s a region called the macula - which looks like like a bean-shaped shaggy rug lying on the floor.
The macula is where our balance receptors, called the hair cells, can be found. Each hair cell has multiple finger-like projections - called stereocilia - and a single kinocilium.
These stereocilia are arranged in rows, depending on their length - the longer ones are closer to the kinocilium, and the shorter ones are further away from the kinocilium - like a cool haircut.
Now, the tips of these cilia are imbedded in a gel, called the otolithic membrane - which has a layer of tiny ear stones called otoconia on top of it, making the otolithic membrane heavier than the endolymph.
Finally, the bottom of each hair cell is connected to sensory neurons that make up the vestibular branch of the vestibulocochlear cranial nerve, that carries balance information to the central nervous system.
Ok, so the utricular macula senses changes in our head position in the horizontal axis - like when we feel pushed towards the back of a seat in a speeding race car.
This is possible because a curved line - called the striola - divides the macular hair cells into two populations. Some of the hair cells are in front of the striola, and some are behind it.
However, the cilia on the hair cells on either side of the striola are arranged so that the kinocilia of all cells face towards the striola, and this allows the macula to sense both backwards and forwards movement of the head.
To get a better sense of this, let’s go back to our race car example. Acceleration displaces the otolith membrane towards the back of the head, which means different things for the two populations of hair cells.
For hair cells in front of the striola, each stereocilium bends towards the longer one in the next row, and ultimately towards the kinocilium - like wind blowing your hair back.
At the tip of each stereocilium there’s a protein filament called a tip link that is attached to a mechanically gated potassium channel in the membrane of the longer stereocilium in the next row.
So the tip link is like the arm of the shorter stereocilium holding a door in the membrane of a longer stereocilium. The tip link on the shorter stereocilia opens the potassium channel on the longer stereocilia - in other words, the arm opens the door.
This makes potassium ions flow from the endolymph, through the door and into the hair cell, making the cell membrane depolarize.
At the bottom of each hair cell — called the presynaptic membrane— depolarization opens voltage-gated calcium channels. Calcium ions enter the cell and cause glutamate - which is stored inside vesicles - to be released into the synaptic space.
The sensory neurons at the bottom of the hair cells interpret glutamate as a “go” signal, and depolarize, sending an electrical impulse to the brain.
However, when the otolith membrane is displaced towards the back of the head, this also makes the kinocilium of the hair cells behind the striola bend towards the stereocilia - so no action potential and, in turn, no glutamate “go” signal is generated.
If, however, the speeding car suddenly stopped, the driver would first be pushed to the front - and this means that the otolith membrane would be displaced towards the front of the head.
So this time, hair cells behind the striola generate an action potential, whereas the ones in front of it don’t. The brain processes the activation and deactivation of particular hair cells to understand in which direction we are moving.

Saccule5:28–6:41

Now, on the other hand, we’ve got the saccule - which has its own macula. The saccular macula has the same structure as the utricular macula - hair cells embedded in an otolith membrane, but there’s a twist.
This time, the macula is vertically oriented, so basically hanging on the saccular wall like a tapestry. The saccular macula also has a striola, which divides the hair cells in a superior and an inferior group.
However, the cilia on these hair cells are arranged so that the kinocilium of each hair cell is further away from the striola than the stereocilia - so all the kinocilia point away from the striola.
So for hair cells above the striola, the kinocilia point upwards, whereas for hair cells below, the kinocilia point downwards.
This allows the macula to sense linear acceleration in the vertical axis. For example, when we are inside an elevator going up, the otolithic membrane moves in the opposite direction - so downwards.
This movement stimulates hair cells below the striola and inhibits hair cells above the striola. The brain then processes the activation and deactivation of particular hair cells to understand in which direction we are moving.

Semicircular canals6:41–10:44

Ok now, let’s switch gears and look at the three semicircular canals, which are shaped like three letter U’s oriented in the three directions of space, with each of them forming a 90 degree angle with the other two - kinda like the corner of box, where the 3 sides meet.
So there’s an anterior, a posterior, and a lateral semicircular canal - the anterior one arches upwards - like the handle of a bag.
The posterior one arches back - like the handle of a mug. And the lateral one arches to the side - like the handle of a drawer.
Inside each semicircular canal is a membranous semicircular duct - which contains endolymph, and opens in the utricle. At one end of these canals, there is an enlarged portion called the ampulla, that detects changes in our head rotation - so our dynamic equilibrium.
Inside the ampulla, there is a region called the crista ampullaris, which houses balance receptors - also called hair cells.
A gelatinous pear-shaped mass called the cupula - similar to the otolithic membrane - lies above the hair cells. However, there are no otoliths above the cupula, so it has the same density as the endolymph - thus they’re not affected by gravity, but respond mostly to the movement of endolymph inside the canals.
Beneath the hair cells are also dendrites of sensory neurons that make up the vestibular branch of the vestibulocochlear cranial nerve that synapse with the basal end of hair cells.
Now, the semicircular canals detect different directions of movement when we rotate our head, and help maintain dynamic balance.
For example, the lateral canals detect rotation of the head from side to side —like when a ballet dancer starts spinning.
So, let’s take the left lateral semicircular canal and see it from above. Let’s say the dancer spins counterclockwise - so to the left.
The endolymph takes a while before it moves along - due to inertia - and it would appear as if it moved in the opposite direction - so clockwise.
Since the cupula is attached to the moving membranous labyrinth, the endolymph drags the cupula towards the same clockwise direction.
So, in the left semicircular canal the cupula bends towards the utricle, thus stereocilia bend towards the kinocilium - which depolarizes the hair cells and increases the frequency of nerve impulses that arise from this side of the head.
However, in the right semicircular canal the cupula bends away from the utricle, thus stereocilia bend away from the kinocilium - which hyperpolarizes the cell and decreases the frequency of nerve impulses that arise from this side of the head.
The brain then interprets the activation of hair cells in the left semicircular canal and deactivation of hair cells in the right semicircular canal as counterclockwise rotation.
As the dancer keeps spinning, the endolymph eventually catches up and start moving along the movement of the head, so it no longer drags the cupula.
When the dancer stops rotating, again the endolymph doesn’t take the hint and keeps on moving for a bit - which can make the dancer feel dizzy since the positional information and the visual information are sending different signals to the brain for that brief time.
Now, the other two pairs of semicircular canals are vertically oriented, and they detect nodding and tilting of the head.
For example, when we do a somersault, the direction of movement of the endolymph is opposite to the movement of the head - so backwards.
This stimulates hair cells on both posterior canals and inhibits hair cells on both anterior canals. However, when we do a cartwheel to the left, the direction of movement of the endolymph is towards the right side - so it stimulates hair cells on both anterior and posterior canals on the right side and inhibits hair cells on both canals on the left side.
This way with the three semicircular canals on each side of our head, we cover the information of every direction of rotational movement.

Vestibular transduction10:44–11:55

Now let’s see how the information actually gets from the different parts of the vestibular apparatus to the brain. When the hair cells depolarize and release glutamate into the synaptic space, they depolarize neurons of the vestibular nerve, which send their axons to a part of the brainstem called the medulla and synapse with neurons on the vestibular nuclei on the same side.
Then the nerve fibers cross to the opposite side and go a number of different ways to stimulate a number of muscles in order to maintain balance.
Some neurons send their axons to the nuclei of oculomotor nerves to coordinate movement of the eyes according to the movement of the head.
Other neurons synapse with those on the nuclei of the accessory nerve, to control movement of the head and neck. Another group of neurons send their axons to the spinal cord to control muscle tone in order to maintain balance, and some neurons also send their axons to the cerebellum.
Finally, yet another set of neurons of the vestibular nuclei synapse with neurons on the thalamus —which then finally carry the impulse to the somatosensory area on the brain cortex - keeping the brain posted about where we are in space.
Alright, as a quick recap, the ear is the organ that receives the stimuli necessary to maintain balance. It contains the vestibule apparatus, which consists of the semicircular canals and the otolith organs - which includes the utricle and the saccule.

Review11:55–12:31

It is responsible for transforming movements into electrical impulses that follow the vestibular pathway, passing through the vestibular nuclei, to reach different structures in order to maintain balance, such as external muscles of the eyes, the muscles of the head and neck, the structures that control muscle tone, the cerebellum, and the somatosensory cortex.