Vestibulo-ocular System, Vertigo and Vomiting

Question 1

What does this system consist of?

Where are all the hair cells located?

Answer 1

• Otolith System (Utricle and Saccule) and 3 Semi-circular Ducts and Ampullae
• AMPULLAE

Question 2

Otolith system:
What does it detect?

What is the structure of the Macula here?

What important property does the Otoconia provide to the Otolith layer? What does this mean?

How are the hair cells arranged here?

What occurs to this system when the head moves to one side? LOOK AT PICTURE

What occurs when you keep moving at that same velocity?

What is this system mainly used for?

How is posture maintained?

Answer 2

• Detection of LINER acceleration and deceleration (Side to side, Up, Down)
• OTOCONIA (crystals weighing down) on top of the Otolith (Gelatinous layer) with Hair cell stereocilia embedded within it
• INERTIA; tendency to do nothing or remain unchanged
• In an orderly pattern; at different angles, different movements activate different groups of hair cells, and all directions of movement are covered by the Utricle and Saccule
• Acceleration causes movement of Hair cell and the Stereocilia lags behind due to the Inertia Otolith layer
  o If the movement causes the Stereocilia to bend towards the longer end, the channels will open = Depolarisation
  o If they bend towards the shorter end, channels will close fully = Hyperpolarisation
• Otolith catches up to be in its resting position
• Posture
• Sends impulses along LATERAL Vestibulospinal Tract to certain muscle groups to maintain posture and brace against externally induced movements e.g. when on a train

Question 3

Semi-circular Duct/Canal:
What does it detect?

What is its structure at the Ampulla?

What occurs to this system when the head rotates? LOOK AT PICTURE

How are both semi-circular ducts affected when rotating head in one direction?

What does increased firing on one side do to the partner duct on the other side?

What are these ducts used for?

How does it do this?

Answer 3

• Detection of ANGULAR acceleration and deceleration (Rotation)
• At the centre of the Ampulla, the Endolymph in the duct is closed off by a CUPULA (Gelatinous membrane), Ampullary Crest contains the Hair cells
• Movement causes duct to rotate and the Fluid lags behind; Stereocilia moves in opposite direction = Depolarisation
• Depolarisation of Ipsilateral Semi-circular ducts, Suppression of Contralateral side
• Decreases firing of the partner duct on the other side
• Vestibulo-ocular Reflex, Head stability, Visually-guided movements
• Sends impulses along MEDIAL Vestibulospinal Tract to certain muscle groups to compensate for externally induced movements

Question 4

Vestibulo-ocular reflex:
What is its function?

What does turning the head to the left cause?

Why are these pathways vulnerable to MS? Where do the plaques commonly build up?

Cerebellar Input:
Where does the cerebellum receive input from and where does it produce output to?

What does the output of the cerebellum do?

Visual Input:
What does this do?

What pathway is taken from the eye?

What is this system capable of? How can this occur here?

What defects can occur here? What conditions can these defects occur in? What will this lead to?

Answer 4

• Ensures stability of gaze; SMOOTH PURSUIT
• Activation of Left Horizontal duct, which activates the Vestibular nuclei
  o Then the Abducens and Oculomotor Nuclei work to contract the ipsilateral Medial extraocular muscle and contralateral Lateral extraocular muscle
• Its heavily myelinated (have to be fast); plaque build-up commonly occurs in the MEDIAL LONGITUDINAL FASCICULUS (connects vestibular and abducens to the oculomotor nucleus)

• Input from Vestibular nuclei to Granule/Parallel fibres, OUTPUT from PURKINJE CELLS to Vestibular nuclei
• Controls the “gain” of the reflex, eliminating small errors

• “Fine-tunes” the cerebellar circuit
• To Accessory Optic system, which then goes to the Pontine nuclei and Olivary nuclei; both then act on the Granule/Parallel and Purkinje cells
• Synaptic Plasticity; long-term changes through Climbing fibres from Olivary nuclei onto Purkinje cells
• ↓Transmission (MS, Acoustic Neuroma) and ↓Hair cells (Age, Meniere’s disease) leads to eye drift = NYSTAGMUS

Question 5

Vestibular Dysfunction:
What normally happens when you turn your head to the Right?

What happens when there’s dysfunction in the RHS canal, for example?

What are the symptoms? How does dysfunction cause them?

Answer 5

• ↑RHS canal output, ↓LHS canal output = Eyes turn the opposite way (left)
• On dysfunction in RHS canal, Head is stationary, but there’s NORMAL LHS canal output and ↓RHS canal output
• Difference in output creates the sensation of turning head left; Eyes will turn to the right as a result = Vertigo, Nystagmus, N+V

Question 6

Motion sickness:
What is it?

What are the inputs of the Nausea and Vomiting (N & V) pathway?

What do the Vestibular nuclei receive inputs from?

So, how is N & V caused by this system?

What are the 2 treatment options for motion sickness?
→ Where do both act?

Answer 6

• Induction of N+V by motion or perceived motion; vestibular system says there’s actual movement, yet the brain thinks there isn’t = Sensory conflicts
• Inputs from Area Postrema, Vestibular system, Higher centres, Abdominal afferents
• Vestibular organ and Visual system
• Vestibular/Cerebellar dysfunction, Overstimulation, Sensory conflict all trigger the Nuclei to send outputs to the Nucleus Tractus Solitarius = N+V
• MUSCARANIC and HISTAMINE Receptor Antagonists
  → Vestibular nuclei