Vestibulo-occular Reflex Flashcards
Name the parts of the vestibular apparatus and their function?
Semi-circular canals: Detect angular acceleration in 3D. Otolith Organs (utricle and saccule): Detect linear acceleration.
Where is the vestibular apparatus found?
Buried in the petrous temporal bone.
Describe the structure of the semicircular canals and how they work?
Horizontal canal - movement in the horizontal plane.
Superior canal: Movement in the lateral plane.
Posterior canal: Movement in the antero-posterior plane.
Each canal has an ampulla where transduction occurs. Here, hair cells sit on a bony promontory. They have cilia which project into the lumen and detect movement of endolymph.
Activation of hair cells generates AP’s which travel to the brainstem via the VC nerve.
This sensory input projects to extrinsic eye muscles and elicits the vestibulo-occular reflex.
Why does endolymph movement lag behind movement of the head?
Due to inertia.
What is the purpose of the vestibulo-occular reflex?
To stabilise images on the retina during head movement.
What nuclei does the VC nerve impulses pass through from the vesitbular apparatus to the eye rectus muscles?
VC nerve: Semicircular apparatus to Vestibular nucleus (pons)
Abducens nerve: Abducens nucleus (pons) to Lateral rectus muscles.
Occulomotor nerve from Occulomotor nuclei (midbrain) to Medial rectus muscles.
What effect does excitation of extra-occular muscles on one side have on the muscle of the other side?
Excitation of one side causes inhibition of the other side so there is a compensatory eye movement. e.g. R lateral rectus and L medial rectus excitation causes R medial rectus and L lateral rectus to be inhibited.
What is Nystagmus?
Involuntary eye movement e.g. dancing eyes.
Which 2 ways can the vestibulo-occular reflex be elicited?
- Barony chair
2. Caloric stimulation.
Describe the Barony chair test?
A rotating chair with head in different positions.
Rotating chair with head forward - elicits a horizontal nystagmus and ‘horizontal canal’ is activated. Room appears to be spinning horizontally.
Rotating chair with head down looking at legs. Activates the superior canal
and a ‘rotatory nystagmus’ and room spins side to side when subject lifts head.
Rotating chair with head down looking to one side. Elicits a ‘vertical nystagmus’ as the posterior canal is activated and room moves back and forwards.
Describe caloric stimulation?
Injecting water (above or below body temperature) to the external auditory meatus.
This leads to heat transfer to the semicircular canals.
The horizontal canal is closest, so you gt horizontal nystagmus due to fluid movement stimulating the VO reflex.
Makes patient feel dizzy.
When would caloric stimulation be used?
To check the integrity of the brainstem in an unconscious patient. e.g. in stroke.
Also occurs when removing ear wax by syringing with warm water and patient becomes dizzy.
