The Vestibular System

Question 1

What is the vestibular system?

Answer 1

The sensory organ for detecting sensations of equilibrium of the body (i.e. balance)

Question 2

State the functions of the vestibular system.

Answer 2

• Providing spatial reference for other sensory-motor coordinations
  
  • Motor functions include:
    
    • Stabilising the head and body during movement
    • Maintaining posture
• Providing compensatory reflexes
  
  • Vestibular ocular reflex
• Tuning cardiovascular function for re-orientations
• Helping with perception of motion in space
• Controlling balance reactions

Question 3

Describe the structure of the vestibular apparatus.

Answer 3

It is encased in the bony labyrinth and is continuous with the cochlea

Consists of:

• Semicircular canals
• Otolith organs:
  
  • Utricle
  • Saccule

Characteristics:

• Semicircular canals
  
  • Canals are filled with endolymph
  • Have swelling at the end called ampulla
    
    • ​Ampulla = sac-like swelling/enlargement of canal or duct
  • Detects angular acceleration
• Otolith organs
  
  • Each has a sensory area called a macula
  • Detects:
    
    • Linear acceleration
    • Position of the head due to gravity

Question 4

Describe the structure of the macula.

Answer 4

• The macula is a small sensory area
• Maculae are present on the utricle and saccule
• Each macula has hair cells (receptor cells) which synapse with the vestibular nerve
• These hair cells have minute projections called stereocilia and one longer projection (on one side) called a kinoclium
• The stereocilia and kinocilium project project into a gel layer that has small calcium carbonate crystals called octonia

Question 5

Describe how the macula works.

Answer 5

• Gravity pulls on the octonia (calcium carbonate crystals) OR linear acceleration causes them to move
• The movement of the otoconia in turn cause the stereocilia and kinocilium to move
  
  • When the stones accelerate, with respect to the cilia, they exert a shearing force on the cilia, causing them to bend
• This opens ion channels and causes depolarization or hyperpolarization of the hair cells
• Thus, signals are sent via the vestibular nerve to give an idea of linear acceleration and the position of the head due to gravity

Question 6

COME BACK TO THIS

What is linear acceleration?

What is the difference between type 1 and 2 hair cells?

What is angular acceleration?

Question 7

Describe the structure of the semicircular canals.

Answer 7

• Theree are three semicircular canals
• They are arranged at right angles to each other
• They each have an enlargement at one end which is called the ampulla
• They are also filled with a fluid called endolymph

Question 8

Describe how the semicircular canals work.

Answer 8

When you move your head one way:

• Your semicircular canals also move in that direction,
• However, the endolymph moves in the opposite direction

This moves a structure in the ampulla called the cupula and stimulates hair cells and vestibular nerves

The semicircular canals give us information about angular acceleration

Question 9

Explain in detail what happens inside the semicircular canals during head movement.

Answer 9

Say we are moving are head to the LEFT (rotating anti-clockwise):

• At least one of semicircular canals on both sides of your head will move to the left
• Endolymph in the semicircular canals will move to the right (clockwise)
• On the left hand side the endolymph will be moving in such a way that it stimulates the hair cells and there is increased firing
• On the right hand side the endolymph will be moving in such a way that it inhibits hair cells and decreases firing
• Thus when you move your head to one side, there is decreased firing of nerves on the opposite side