HNS45 Eye Movement Flashcards
Eye movements
- Complex and interesting behaviour does NOT rely on higher-order cognitive processing (like language)
- Visual perception
- Complex cognitive processes such as reading, thinking
- Interaction between sensory-motor systems
3 layers of eyeball
- Outer
- Sclera
- Cornea - Middle
- Choroid
- Ciliary body
- Iris - Inner
- Retina
Anterior chamber and Posterior chamber
Anterior chamber: Aqueous humour
Posterior chamber: Vitreous humour
Formation and drainage of aqueous humour
Aqueous humour is formed by capillary network in Ciliary body
—> drains into Schlemm’s canal (Trabecular meshwork)
—> enters blood
Importance of eye movements
- Achieve high visual acuity
- High visual acuity is restricted to Fovea (small circular region 1.2mm diameter) in the central retina —> Densely packed with photoreceptors - Direct Fovea to new objects of interest (Foveation) from a target already being foveated
Control of pupillary size
Constriction:
- Circular muscle
- Parasympathetic stimulation
Dilation:
- Radial muscle
- Sympathetic stimulation
***5 basic types of eye movements
Gaze-shifting movements (***Voluntary / Attention)
- Saccades
- Smooth pursuit
- Vergence
Gaze-stabilising movements (***Reflexive)
- Vestibulo-ocular reflex
- Optokinetic reflex
6 cardinal directions of movement to test muscle
Lateral rectus: Lateral
Medial rectus: Medial
Superior rectus: Lateral upper (only superior rectus can elevate eye when abducted)
Inferior rectus: Lateral lower (only inferior rectus can depress eye when abducted)
Superior oblique: Medial lower (only superior oblique can depress eye when adducted)
Inferior oblique: Medial upper (only inferior oblique can elevate eye when adducted)
Innervation of extraocular muscles
LMN:
CN3 Oculomotor nucleus (rostral midbrain): **Ipsilateral Others
CN4 Trochlear nucleus (dorsal midbrain): **Contralateral Superior oblique (特別)
CN6 Abducens nucleus (pons/medullary junction): ***Ipsilateral Lateral rectus
- Saccadic eye movement
- Conjugate movement (same direction)
- ***Rapid and Ballistic movements of both eyes between 2 phases of fixation in the same direction (conjugate eye movement)
- Driven mainly by Position signals (voluntary)
- 200-250ms
- Very small micro-saccades are involuntary —> keep image moving on retina —> prevents fading
- Range in amplitudes from small movement (reading) to large movements (gazing around a room)
- Smooth pursuit movement
- Conjugate movement (same direction)
- ***Slow, smooth eye movements —> track moving objects once foveation is achieved
- ***Quasi-voluntary (observer can choose whether or not to track, but cannot voluntarily generate smooth pursuit in absence of a moving target)
- 2 basic phases:
1. Pursuit initiation (0.1s, driven by retinal slip / velocity error —> slip of image across retina)
2. Pursuit maintenance
- Vergence movements
- Disconjugate movement (different direction of eye movement)
- Used to converge eyes onto targets at different distances
- Reflexive + Voluntary
- Driven by ***Binocular disparity of the target to be fixed
- Align fovea of each eye with targets located at different distance from observer
- Convergence / Divergence of lines of sight to see an object nearer / further away
- Vestibulo-ocular reflex (VOR)
- To stabilise eyes on a target in response to ***rapid head movements (快) —> Physiologic / Vestibular nystagmus
- Eye movement in direction ***opposite to head movement —> preserving image on centre of visual field
- Vestibular system detects **brief, transient changes in head position —> **rapid corrective eye movements
2 forms:
- Rotational VOR - driven by signals from Semicircular canals (sense head rotation)
- Translational VOR - driven by signals from Otolith organs (sense linear head acceleration)
Pathway:
Head movement —> Vestibular nerve (CN8) —> Vestibular ganglion —> Vestibular nucleus —> Oculomotor / Trochlear / Abducens nucleus —> CN3, 4, 6 —> Extraocular muscles
Example: Canal-related nystagmus
Canals on both sides operate as complementary pairs (One side ↑ firing, Other side ↓ firing)
—> Head turn **Right
—> endolymph turn **Left
—> ↑ excitability of **Right hair cells / ↓ excitability of Left hair cells
—> Synchronous ↑/↓ firing of CN8
—> Vestibular ganglion
—> Medial Vestibular nucleus (ipsilateral) (medulla)
—> PPRF (Paramedian Pontine Reticular Formation) (contralateral)
—> Ipsilateral CN3 nucleus (cross返轉頭via Medial Longitudinal Fasciculus) + Contralateral CN6 nucleus
—> Right MR + Left LR activated / Left MR + Right LR inhibited
—> Both eye balls turn **Left
(Vestibular Nucleus —(先cross去左)—> Contralateral Abducens nucleus (Pons) —(再經Medial Longitudinal Fasciculus cross翻)—> Ipsilateral Oculomotor nucleus (Midbrain))
Clinical tests for VOR
- Bedside VOR suppression test
- Visual fixation on outstretched hand
- Rotate chair from side-to-side
- Normal: eyes remain fixed on outstretched hands
- Abnormal: eyes move in opposite direction to rotation with catch-up saccades to re-fix gaze on outstretched hands - Caloric reflex test
- Irrigate ear with warm / cold water
- Warm water —> activate ipsilateral hair cells —> eye balls move to opposite side
- Cold water —> inhibit ipsilateral hair cells —> eye balls move to same side
—> Test if Vestibular nucleus / Medulla intact
- Optokinetic eye reflex
- Stabilise eyes during head movement
- Driven by **Retina slip (like pursuit) but **reflexive + involuntary
- Operate effectively in response to ***slow (below 1Hz) (慢) head movements (where VOR has low gain) —> Keep eyes on target (e.g. in train eyes automatically track objects until they reach end of their excursion)
- Alternating slow and fast movement of eyes in response to any stimuli —> **Optokinetic nystagmus (normal reflex of Visual and Oculomotor systems in response to **large-scale movements of visual scene)
- Optokinetic eye reflex + VOR act complementarily
—> VOR: rapid, transient head movement
—> Optokinetic eye reflex: slow head movement
