5.3.2 Vestibular Flashcards
What are neural mechanisms behind saccadic eye movements– voluntary and reflexive?
Voluntary Saccades
-Frontal Eye Field (Area 8) -> Superior Colliculus -> Reticular Formation -> CN III
Reflex Saccades (in response to a novel visual stimulus – like a flash of light)
-Retinal Ganglion Cells -> Superior Colliculus -> Reticular Formation -> CN III
What are the two basic types of nystagmus and how is each evoked?
Nystagmus from rotation (post-rotational phase of Barany Chair Test)
Internuclear opthalmoplegia - results from lesion of medial longitudinal fasiculus, MLF (young adults: due to MS; older adults: occlusion of basilar artery)
- Inability rotate the eyes medially (adduct) due to interruption of abducens interneurons that transmit signals to CN III for contraction of medial rectus in the MLF
- Affects all horizontal eye movements (VOR, saccades, pursuit) except convergence
- Will cause images on contralateral side to be horizontally displaced, producing diplopia
- Abducting eye often shows nystagmus
What is nystagmus?
Involuntary, back and forth, up and down, or rotating movement of the eyeballs with a slow pull and a rapid return jerk (inducible in normal individuals, sign of lesion if spontaneous)
What is the effect of turning the head rightward on the firing of the right and left horizontal semilunar canals?
Effect of Rightward Head Rotation
- *Right** Horizontal Canal: endolymph bends cilia TOWARD the kinocilium –> Excitation
- Left* Horizontal Canal: endolymph bend cilia AWAY from the kinocilium –> Inhibition
What are the three functional pairs of the semicircular canals that allow humans to perceive our three dimensional world?
1) Horizontal on Right <- -> Horizontal on Left
2) Anterior on Right <- -> Posterior on Left
3) Posterior on Right <- -> Anterior on Left
Describe the two main types of vestibular receptors and their stimuli.
Kinetic Labyrinth
- Three semicircular canals, oriented at 90 degrees to each other
- Adequate stimuli:
- Dynamic stimuli from rotational forces
- Head acceleration/deceleration
Static Labryinth
-Otolith organs (utricle and saccule)
- Adequate stimuli:
- Static stimuli from maintenance of head position in space
- Translational forces from horizontal displacement
What is the neural mechanism of VOR eye movements?
Vestibular Hair Cell -> CN VIII -> Vestibular Nucleus -> Oculomotor Nucleus -> CN III
What are vestibulo-ocular reflex (VOR) eye movements?
- Compensatory adjustment of gaze occuring to maintain fixation of an object as head moves
- Counterclockwise Rotation of Head -> Clockwise Rotation of Eyes
- Eyes will move an equal number of degrees as the head (but in opposite direction)
- Speed: 300 deg/sec
What are the VOR effects in post-rotational component?
In a patient who was previously spinning right (decel righ/accel left)
- Slow conjugate eye movements to the right followed by nystagmus to the left
- Increased left CN VIII firing ⇒ Increased Med Vestibular Nuc firing
- Caused decreased left CN VI and decreased right CN III
- Decreased right CN VIII firing à Increased Med vestibular Nuc firing
- Causes increased right CN VI and increased left CN III
Results in nystagmus to opposite direction results as a reflex
What are the two main types of cilia found in the vestibular hair cell. How does their relationship to one another affect the firing rate of the vestibular hair cell?
Stereocilia (multiple small cilia) and Kinocilium (single long cilium)
When stereocilia are displaced towards the kinocilium, the rate of CN VIII firing increases above the basal level as a result of depolarization of the vestibular hair cell increasing synaptic vesicle release
When stereocilia are displaced away from the kinocilium, the rate of CN VIII firing decreases as a result of hyperpolarization of the vestibular hair cell decreasing synaptic vesicle release to CN VIII
What are the two main types of vestibular receptors?
Semicircular canals and otolith organs
What are the symptoms seen with a unilateral labyrinthectomy?
Imbalance of input from the semicircular canals due to the high tonic background firing rate present in the unaffected side (elicits both a vestibulo-ocular and vestibulo-postural reflex)
- Fall towards the side of the lesion
- Nystagmus away from the side of the lesion
How is the direction of nystagmus defined?
Direction of nystagmus is based on direction of the fast phase (by convention)
What is the neural circuit of smooth pursuit eye movement?
Retinal Image -> Ret Ganglion Cells -> LGN -> Visual Cortex -> Reticular Form -> CN III
What are saccadic eye movements?
- Quick step changes in eye position that occur as the visual field is scanned
- Speed: 400-700 degrees/second
- Do not cause a blurring of visual field
What are smooth pursuit eye movements?
- Tracking movements used to maintain visual fixation of moving target as head is stationary
- Speed: 30 degrees/second
