Lecture 23- Vestibular System

Question 1

What does the vestibular system do?

Answer 1

-The vestibular system provides the brain with information about head
movement and location detecting both angular (rotation) and linear acceleration of the head (become consciously aware)
-It then generates reflexes to compensate for this movement.
-Additionally this information helps to maintain the right posture and a stable image on the retina

Question 2

What is the difference between angular rotation and linear acceleration?

Answer 2

• Linear acceleration= movement of head with respect to gravity
• Angular acceleration= rotations of the head e.g. nodding

Question 3

What part of the ear is the vestibular apparatus located?

Answer 3

Inner ear

Question 4

What are the 2 parts of the vestibular apparatus?

Answer 4

• The semi-circular canals
• The otolith organs

These are part of the inner ear but not invovled in the sensation of sound

Question 5

How many semicircular canals are there in each ear and what axis of movement is each concerned with?

Answer 5

3 in each ear, so work in pairs:

• Anterior stimulated by acceleration in the sagittal plane i.e. nodding
• Posterior simulated by acceleration in the coronal plane i.e trying to crack your neck
• Horizontal stimulated by acceleration along the transverse plane i.e. looking both directions when you cross the road

Say they are aligned along 3 axes

Question 6

What type of acceleration are the semicircular canals concerned with?

Answer 6

Angular rotation

Question 7

What are the semicircular canals filled with? How does this relate to it’s ion concentrations?

Answer 7

• Filled with endolymph

- Endolymph similar to ICF so is high in potassium and low in sodium

Question 8

What structure is at the base of each semicircular canal?

Answer 8

Ampulla

Question 9

What is the Capula? What are some cells around it that offer support/ serve a function?

Answer 9

• A structure that sits inside the ampulla and when at rest sits straight up to block the duct of the semicircular canal
• Support cells sit in base, as well as hair cells and stereocilia which project out into the lumen of the semicircular canal but are also embedded in the ampulla

Question 10

What happens in regards to the Capula when you shift your head in a certain direction?

Answer 10

• Base layer shifts in certain direction
• At first the endolymph of the semicircular canals is resistant to change (due to inertia) however, eventually it will give in and shift
• As this occurs and the pressure builds up the Capula moves in a certain direction (opposite to original movement)
• Embedded hair cells + stereocilia move with the Capula

Question 11

What occurs in vestibular signal transduction? What’s the difference between Movement towards the longest or shortest stereocilia?

Answer 11

-Bending of cilia towards longest stereocilia results in the opening of mechanically gated potassium channels causes influx of potassium and subsequent depolarization. Opening of voltage gated calcium channels then causes a calcium influx and the release of neurotransmitter on to the afferent nerve fibers of vestibular nerve. This means an increase to the firing rate.

The opposite…

-Bending of cilia towards the shortest stereocilia= getting closing of mechanically gated potassium channels resulting in hyperpolarization. Voltage gated calcium channels close and the release of neurotransmitter reduces and results in a decrease in firing rate from the vestibular nerve. This is often called inhibition.

Question 12

What does the resting discharge rate of the vestibular nerve allow?

Answer 12

Vestibular hair cells can detect movement in 2 directions

Question 13

What are the names of the two Otolith organs?

Answer 13

Utricle and Saccule

Question 14

What do the otolith organs (utricle and saccule) detect?

Answer 14

Linear acceleration not angular

Question 15

Describe the position of the utricle and the saccule (separate answers) when standing and what this means for the hair cells and type of linear acceleration each detects….

Answer 15

• Utricle approximately horizontal when standing (hair cells within it are oriented
  vertically) . Detects horizontal linear acceleration (movement of endolymph horizontally will cause hair cells to bend)
• Saccule approximately vertical when standing (hair cells within it are oriented
  horizontally) . Detects vertical linear acceleration (movement of endolymph vertically will cause hair cells to bend).

Question 16

What is the structure of the otolith organs like?

Answer 16

• Support cells at base make up the macula
• Contained within these support cells are hair cells which are connected to afferent nerve fibers
• These hair cells have projections (cilia) which reach into the otolith membrane
• This otolith membrane has a layer of calcium carbonate stones on top called otoconia

Question 17

What happens in the otolith organs when there is movement of the head? What transduction pathway occurs?

Answer 17

• Head moves the macula moves in a certain way
• Hair cells bend due to this combined with the pressure from the otoconia (stones above)
• Transduction pathway is the same as the semicircular canals: if cilia bend towards the longest then you get an increase in firing rate, if they bend to the shortest you get a decrease in firing rate

Question 18

What central pathway exists from the vestibular system? What is the function of this?

Answer 18

• Axons project to vestibular nuclei in brainstem
• Then to the medial longitudinal nucleus
• Then to the oculomotor nucleus (think eyes)

Functions:

• Stabilize eyes
• Stabilize the head
• Maintain balance (specifically cerebellum + spinal chord)

Question 19

What is vertigo?

Answer 19

• Caused by disease affecting the vestibule or its afferent fibres
• Illusion of movement, dizziness

Question 20

What is motion sickness?

Answer 20

• Caused by mismatch between visual and vestibular information
• Sea legs tablets: Meclozine Hydrochloride dampens the firing of the vestibular system so reduces motion sickness

Question 21

What is bed spins?

Answer 21

• Caused by alcohol
• Ethanol infiltrates cupula, lowers density and causes it to “float”, bending hairs on hair cells and creating perception of movement

Question 22

What are three types of tumors that effect the vestibular system?

Answer 22

• Acoustic neuroma (vestibular schwannoma): Abnormal growth along vestibulocochlear nerve
• Glomus Tympanicum
• Ear Canal cancer

Question 23

What is the effect of ototoxic drugs on the vestibular system?

Answer 23

• Over 600 medications are known to be harmful to hair cells: Can result in temporary or permanent hearing loss and disorders of balance
• Antibiotics are the worst culprits

Question 24

What occurs in vestibular rehabilitation?

Answer 24

-Exercise-based program to improve balance and reduce problems related to dizziness, nausea etc

-Often involves:
• Gaze stabilization (focus on something and move head side to side)
• Habituation (just jump in a car until your body is used to it: kind of cruel)