The vestibular system

Question 1

What is the difference between the auditory system and vestibular system functions?

Answer 1

Auditory systems:

• Mechanotransduction
• Cellular and molecular anatomy of underlying hearing

Vestibular system:

• Shares anatomy with the auditory system
• Head position, self-motion, spatial orientations
• balance

Question 2

LO

Answer 2

• Be able to describe the specialized sense organs involved in hearing and balance at a molecular and cellular level
• Be able to describe the circuitry that processes this type of sensory information in the CNS

Question 3

What does the vestibular system include?

Answer 3

• semi-circular canals
• The otolith organs
• the nerves that relay the information about balance and orientation to the brain
• the neural networks that interpret and combine the information from the different sources

Question 4

What structures does the vestibular system include?

Answer 4

The structures of both inner ears, the eyes (and visual system) and the proprioceptors/ stretch receptors in the joints and muscles

Question 5

The signals in the vestibular system are relayed to where?

Answer 5

The signals are relayed to the autonomic, motor and cognitive effector systems

Question 6

Tell me about endocochlear potential

Answer 6

• Stereo cilia are bathed in endolympth (high K+); the base is bathed in perilymph (low K+)
• Tip links join the hair cells together, flextion of hair calles causes a change the membrane portential and cause an influx of K+ and release of neurotransmitter
• low basal rate of neurotransmiter when at rest
• the priniciple of mechanotransduction that we see in the auditory hair cells are relevant to the vestibular hair cells
• hair cell bundles differ from those on the auditory system in that they bave bindles of stereocilia

Question 7

Whats the key to the function of the hair cells?

What is this structure?

Answer 7

key to the function of the hair cells is the kinocilium (pl. Kinocilia) in each bundle, this is the longest of the stereocilia and its movement controld the activation of the hair cell bundle

Question 8

Where is the vestibular system?

What does it sit on and what does this mean?

Answer 8

• the vestibular system is in the head, which sits on a pivot point on the neck.
• Pivoting of the head about the neck means that angular velocity and acceleration forces are exerted on the head and therefore the vesucllar systtem which is embedded in the temporal bone
• the vestibular system is also sbjected to linear forces (e.g. walking, and tilt/ translation when under the influence of gravity)

Question 9

Tell me about the ionic basis and what its dependent on?

What does this organisation mean?

Answer 9

• Biphasic > depolarisation and hyperpolarisation is K+ dependent
• This organisation means that the movement of the stereocilia of the hair cells will create a graded response (= generator potential)

Question 10

The vestibular system and what does the system work best under the influence of?

Answer 10

Vestibular system (system works best under influence of gravity)

Question 11

Where are the hair cells located?

Answer 11

In the ampullae, saccule and utricule. these are important structures

Question 12

Whats relative to gravity in the vestibular system/

Answer 12

sensing head position, spatial orientation and head movements

Question 13

How many vestibular systems are there and where are they located?

Answer 13

The is a vestibular system on each side of the head. in the left and right temporal bone

Question 14

What can the vestibular system be divided into?

Answer 14

The Otolith system and the semi-circular canals

Question 15

What comes under the otolith organs?

Answer 15

The utricle and saccule

Question 16

What is the otolith system comprised of?

Answer 16

Utricle

Saccule

The gelatinous otolithic membrane

Question 17

What sits on the geltinous otolithic membrane?

Answer 17

Dense calcium carbonate (otoconia) sit on a gelatinous otolithic membrane- the apical hairs of the sensory vestibular hair cells are embedded in this gel

Question 18

The mass of the otoconia is high enough that what?

What does this result in?

Answer 18

It is sensitive to the effects of gravitational pull- as the head moved there is a short lag in the movement of the otoliths due to inertial forces

This results in the detection of front-back, right-left, up-down and tilting movements of the head

Question 19

In the otolith system, how are the hair cells organised?

What are the positives for the hair cells being organised in this way?

Answer 19

The hair cells are organised in maculae (semi-circular curved regions) on the walls of the utricles and saccule

This organisation effectively enables sensing or detection of all the possible head positions – which in then interpreted in in the central nervous system

Question 20

How are the semi-circular canals organised?

Where are the hair cells located in this region?

Answer 20

Organised at right angles to each other (posterior, lateral and anterior) and detect angular acceleration.

Hair cells are located within the enlarged regions, ampullae, at the base of each canal

Question 21

What does the ampulla contain?

Answer 21

The ampulla contains the crista ampullaris the hair bundles of the sensory cells in the crista project into a region of visous proteoglycan, the cupula

Question 22

The semi-circular canals are fluid filled, what does this mean when the head pivots on the neck?

Answer 22

The canals are fluid filled, as the head pivots on the neck, there is a lag in the flud movement that causes the sensory hair cells to be deflected by the displacement of the cupula

Question 23

In both the otolith and the semi-circular organs, what does deflection result in?

Answer 23

In both the otolith and the semicircular organs deflection results in neurotransmitter release which may result in action potentials in the afferent nerves fibres

Question 24

What are vestibular hair cells divided into?

What do these types of hair cells form?

Answer 24

Vestibular hair cells are divided into type I and type II.

Both types of hair cells form ribbon synapses.

Question 25

Where are type I hair cells surrounded on?

What does this lead to?

Answer 25

Type I hair cells are surrounded on the basolateral side by a calyx-like expansion of the primary nerve ending this leads to potassium accumulation in the synaptic cleft

Question 26

What does the Scarpa’s ganglion house?

What are these?

Answer 26

Scarpa’s ganglion houses the cell bodies of the bipolar neurons of the vestibular nerve fibres.

These are myelinated fibres; the peripheral process receives its innervation from the hair cells and the axon (central process) terminates in the cerebellum or vestibular nuclei.

Question 27

Tell me about gaze control?

Answer 27

Gaze control – exceptionally well controlled - (e.g., walking down the street, looking at a screen), head-up position (e.g. when riding round a tight bend on a bike) as well as position in space are controlled by the vestibular system.

Question 28

What can detect rotational movements and where is this found?

Answer 28

Information about the head relative to gravity: static head tilts and movements in a straight line (ampullae at the base of the semicircular canal detect rotational movement)

Question 29

What does the otoconia allow you to sense?

Answer 29

Where your head is in space relative to gravity

Question 30

How is movement detected?

Answer 30

Through the flexion of hair cells

Question 31

How are the hair cells organised in the vestibular system?

Answer 31

• Shorter hair cells on left and longest on the right-hand side
• Mid-point there is a striola, beyond those point’s, hair cells are orientated in a mirror image to those on the left (some are being excited as stretched and others are inhibited).
• Activation and inhibition acting in sync
• Left and right vestibular system are working in sync. Any unstabilization can cause dizziness
• Hair cell bundles are orientated towards the striola
• The striola acts as an axis of symmetry and hair bundles on either side of the striola have opposing morphological polarization
• This sets up a differential pattern of excitability encoding head position
• Utricle detects movements in horizontal plane
• Saccule detects movements in the vertical plane

Question 32

Tell me about the different structures of the type I and type II hair cells?

Answer 32

• Kinocilium is always the longest of the hair cells
• Movement and activation in rest of bundle happens relative to kinocilium
• Activation in vestibular system is slower to activation in auditory system. Be case kinocilium is lose in mammalian cochlea hair cells and size of bundle is greater is vestibular system hair cells

Question 33

Tell me about the structure of the semi-circular canals: ampulla house sensory epithelium?

Answer 33

• Fluid filled canals (semi-circular canals)
• Canals are at 90˚ to one another
• Head movement causes relative displacement of one or another canal
• Left and right canals
• Hair cells are aligned opposite to each other in each pair of semi-circular canals

Question 34

Vestibular reflexes to how many planes of motion?

Answer 34

3 planes of motion

Question 35

The vestibular systems detects 3 planes of motion using a similar sensor to the cochlea. Tell me about this?

Answer 35

• Vestibulo-ocular: keeps your eyes still while your head moves in space
• Vestibulo-colic: keeps your head still in space/same plane while you walk
• Vestibular spinal: adjusts posture for rapid changes in position

Question 36

Equilibrium pathway to CNS

Answer 36

Question 37

Tell me the 4 types of dizziness one can get and either give examples of what causes it or what its caused due to

Answer 37

Epidemiology of dizziness

• Inner ear dizziness (otologic) e.g. Meniere’s disease
• Medical dizziness (for example from low blood pressure)
• Neurological dizziness (central) e.g. Benign postural vertigo
• Psychiatric dizziness and undiagnosed dizziness

Question 38

Overview of vestibular implant

Answer 38

Question 39

Summary of the vestibular system

Answer 39

• Provides information about position, orientation and motion of head and body
• Receptor cells located in utricle, saccule and semicircular canals (ampulla)
• Movements cause a stimulation of distinct subsets of hair cells in the saccule, utricle and semi-circular canals
• Produces a distinct pattern of neural activity at a population level that encodes head position

Question 40

Questions from the lecture

Answer 40

Why do some people experience motion sickness and what causes it?

Not clear why some people are more susceptible than others, though when you consider the anatomy and physiology of the vestibular system you can start to speculate. It is likely caused by the integration of information from the different inputs (consider the equilibrium pathway slide) being slightly different from what would be anticipated (or has been learnt by the system) e.g. the head is being kept still (person reading) while rest of body is being shaken through the motion of the vehicle – this results in conflicting inputs. Current evidence is that if someone has an intact vestibular system there are likely to be conditions that will cause them to experience motion sickness.

What causes the dizziness when at height (e.g. up a ladder, on a roof)?

Vertigo, which can be considered as a sensation of motion when a person is not moving, typically happens at height (though can be caused by pathology, such as damage to the otoconia). Vertigo associated with height is thought to happen when the critical distance between a person and the closest stationary object is exceeded. When this point is exceeded the input from the visual system is reduced, and the amount of sway that a person experiences is increased (this may be to increase visual balance feedback) [– sway also increases when you stand and shut your eyes –] as sway increases a feeling of instability can develop, this can lead to a feeling of dizziness and fear.

Use of potassium concentrations rather than sodium from the depolarisation of the hair cells is due to what?

likely due to evolutionary pressure. There is recent evidence that the signal processing rates and duality of systems in the vestibular system reflects our evolution from slow/long-duration movements when we were water dwelling, through to the need for a system capable of responding to the forces that are applied to the vestibular system on land.