The Vestibular System

Question 1

What 3 things do we need to co-ordinate to get balance?

Answer 1

Vestibular System
Vision
Proprioception

Question 2

How is the vestibular apparatus separated into 3?

Answer 2

External ear
Middle ear
Inner ear

Question 3

What bone is the vestibular apparatus located deep into?

Answer 3

The petrous part of the temporal bone

Question 4

What are the two labyrinths that the vestibular apparatus is located inside of?

Answer 4

Outside bony labyrinth and inside membranous labyrinth.

Question 5

How many semicircular canals do we have and where do they come off from?

Answer 5

3 (anterior, lateral and posterior)

Come off from the vestibule

Question 6

What does the membranous labyrinth consist of?

Answer 6

Consists of membranous sacs that are floating.

The sacs are attached to the bony labyrinth and contain&are surrounded by fluid.

Question 7

What are the two organs inside the vestibule?

Answer 7

Utricle and saccule

Question 8

What are the two fluids that are found in the labyrinth?

Answer 8

Inside the bony labyrinth (bathing the inside of the membranous labyrinth), is a fluid known as perilymph. This has a high sodium conc and a low potassium conc.

Inside the membranous labyrinth is a fluid called endolymph. This is a high potassium conc and a low sodium conc.

Question 9

What are the main cells of the vestibular system?
Where are they located?
What is the main purpose?

Answer 9

The hair cells

They are located in regions of thickened epithelium in the vestibular organs.
The basal end has a synapse. These are lots of vesicles containing glutamate here. When the cells depolarise, the vesicles exocytose the transmitter to the primary affect fibre.

Question 10

What are the most unusual things about the hair cells?

Answer 10

Release of NT is at the basal membrane.

The primary afferent fibre is an axon and not a dendrite.

Question 11

How are the hair cells arranged?

What is the tallest cell called?

Answer 11

Mainly made of actin.

Arranged in a staircase pattern up towards the tallest called the kinocilium. This creates the axis of polarity.

Question 12

What are tip links and how do they work?

Answer 12

They are ridges at the top of the stereocilia that link the stereocilia together.

The top of the shorter sterocilia are linked to the side of the taller ones via tip links.
Tip links are connected to channels at the tip of the shorter stereocilia called mechano-transducer channels. When the channels get stretched, they open.
This allows potassium and calcium to flow in (positive ions in) this causes depolarisation.
The strereocilia get bended up the axis of polarity causing the tip links to stretch. This allows the channel to open.

Question 13

Explain the difference between the hair cells bending towards their axis and away

Answer 13

Stretching of tip links towards axis of polarity = depolarisation of hair cell.
This causes firing in the afferent nerve.

Bending of stereocilia away from axis of polarity = tip links relax and channels close.

Question 14

How do the hair cells fit into the labyrinth system?

What is the ampulla?

Answer 14

Each of the semicircular canal and each end has a slightly thicker region, called the ampulla. Inside the ampulla = the ampullary crest

Question 15

What is located within the ampullary crest?

Answer 15

The hair cells are located within the ampullary crest. This is at the basal end.
The sterocilia are located within the cupula. This is a jelly like structure within the ampulla.

Question 16

How do the semicircular ducts response to movement of the head?

Answer 16

-Ampullary crest with all the hair cells with their stereocilis sticking up.

All the hair cells are arranged so that the axis of polarity are pointing in one direction.
When the sterocillia bend, they all end the same way so all the hair cells will either all depolarise of hyperpolarise.
This creates a very sensitive system.

Question 17

If we were looking on top of someones head from behind then what changes would occur when someone turned their head to the left?

Answer 17

The individual will turn their head to the left.
The fluid inside the ducts will move in the opposite direction in the left and in the right.
(If you turn the head anticlockwise, the fluid will move clockwise). The LHS stereocillia go towards their axis of polarity causing depolarisation and increased firing.
On the RHS, the sterocilia will bend away from the axis of polarity, causing hyperpolarisation and decreases firing.
This goes to the brain and this change in firing tells the brain if the head has rotated to the left or right.

THIS IS A DYNAMIC SYSTEM - you need acceleration. Need a force to change the stereocilia. These ducts are therefore known as dynamic organs of equilibrium.

Question 18

In the vestibule there is the utricle and the saccule.

Explain how each work.

Answer 18

Utricle:
The utricles have a region of thickened epithelium called the macula and this is where the hair cells are. The utricle sterocillia are embedded in the gelatinous membrane. Onto of this membrane are otoconi. These are crystals of calcium carbonate.

When upright, the macula is horizontal. If you tilt the head, the weight of the head will act on the stereocillia and the force of gravity will cause the stereocillia to be bent.
The stereocillia will then depolarise or hyperpolarise.
This means that static head positioning to the side can be detected.
As well as this, the mucula will detect linear acceleration.

The utricle stereocillia all point towards the striola (curving landmark). This means the axis of polarity for each stereocillia is in a different direction. Whatever position the head moves in, some cells with be depolarised and some hyper polarised.
This creates a complex signal to give the brain an accurate measure of head position.

Saccule:
Similar to utricle by is vertical when the body is in a standing position. This means that it reacts to vertically detected linear force e.g. up and down in a lift.

Unlike the utricle, in the saccule all the cilia orientate away from the striola. This means they are all pointing in direction directions which again gives the brain an accurate understanding of positioning.

Question 19

How is the information sent to the brain?

Answer 19

Can see the primary afferent fibres coming out the utricle and saccule. These are synapsed onto by the hair cells.
These run through the internal auditory meatus to the brain stem at the medulla region.

The cell bodies sit in the vestibular ganglion.
The neurones are bipolar (one axon to hair cells and one to the brain stem).
Fibres are all myelinated so high speed to brain stem.

Question 20

What happens to the vestibular neurons once in the brainstem?

Answer 20

Once in the brain stem… vestibular neurones will synapse onto neurons in the vestibular nuclei in the medulla in the brainstem.

4 nuclei for the vestibule. (Inferior, superior, medial and lateral).
Each has slight specialisation as to where the firing is coming from in the vestibule apparatus.
Medial and superior receive inputs from the semicircular ducts (tend to fire when you rotate the head).

Question 21

Where do the different nuclei of the brain stem connect to?

Answer 21

Nuclei connect up to other regions of the brainstem.

Lateral nucleus connects up to motor neurones to control limbs, superior and medial connect up to neurons in the neck but also to the 3 nuclei that control eye movements (3, 4 and 6 cranial nerve) which is critical to make sure eye movements can adjust to head movements called the vestibulo-ocular reflex.