Term
| What is "Smooth pursuit"? How does it differ from "Saccades"? |
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Definition
1) Slow, conjugate eye movement to is used to follow object to minimize retinal slip (difference between target and eye velocities)
2) Accuracy of smooth pursuit depends on visual feedback, while accuracy of Saccades depend on neural representations of eye position in the brain. |
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Term
| What is "Optokinetic nystagmus (OKN)," or the "Optokinetic Response" (OKR)? |
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Definition
Important when large portion of visual field is in motion (fence outside of moving car window).
1) Slow phase (resembles smooth pursuit) following target to reduce Retinal Slip
2) Quick phase component (resembles saccade) in opposite direction when eye reaches eccentric position. |
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Term
| How do eyes adjust for target depth? |
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Definition
| Vergence movements (convergence for close, divergence for far). |
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Term
| Explain the neurophysiological basis of the "Vestibulo-Ocular reflex (VOR)". What is "VOR gain"? |
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Definition
Eyes move opposite to head movement to keep target stable on retina (velocities are equal and opposite).
2) VOR gain is the (-) ratio of eye velocity and head velocity (Should be 1 because of opposing directions)
ex) VOR >1 would mean that eye was moving too fast for target |
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Term
| What is the insertion and action of the Superior and Inferior Oblique muscles of the eye? |
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Definition
1) SO arises from apex of orbit and passes through Trochlea. It then turns laterally and inserts on superior aspect of globe. DEPRESSION AND INTORSION (contralateral CN IV)
2) IO originates for maxillary bone in medial wall of orbit. It goes underneath the globe and inserts on lateral side, medial to tendon of LR. ELEVATION AND EXTORSION (CN III) |
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Term
| What is the CN innervation of the 6 ocular muscles? |
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Definition
SO4LR6AR3
** Superior muscles are contralateral (SO and SR, with IV and III)**
1) SO gets information from contralateral CN IV nucleus
2) LR gets information from ipsilateral CN VI nucleus
3) MR IR, IO gets ipsilateral CN III nucleus
4) SR gets contralateral CN III nucleus |
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Term
| What is the importance of the Edinger-Westphal and Central Caudal nuclei? |
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Definition
Parts of the Oculomotor nucleus complex
1) Edinger-Westphal is for parasympathetic papillary response- contraction)
2) Central Caudal is for eyelid innervation |
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Term
| How is conjugacy of horizontal eye movements achieved? |
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Definition
Need simultaneous contraction of LR (IV) from one eye and MR from contralateral eye (III).
Abducens internuclear neurons connect nuclei of IV (major input is PPRF) and III via MLF. |
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Term
| What is the difference between Strabismus and Amblyopia? |
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Definition
1) Strabismus, or "misaligned gaze/wandering eye" involves lateral (exotropia) or medial (esotropia) deviation of one eye.
2) Amblyopia, or "lazy eye," occurs when the brain adapts to visual discrepancies by suppressing vision in one eye.
Amblyopia is often caused by persistent Diplopia (double vision), and Strabismus is only one cause of Diplopia |
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Term
| How can you tell between a right VI nerve palsy and a lesion of the VI nucleus? How about a lesion of the left MLF? |
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Definition
1) In VI nerve palsy (similar to right PPRF infarct), patient will not be able to turn right eye laterally, but will still be able to turn left eye to the right-side (AIN projections from VI nucleus via the MLF to the medial rectus muscle).
2) If the lesion in the nucleus, there will be no AIN projections to the medial rectus, so rightward gaze will be compromised in BOTH eyes.
3) Lesion of left MLF (Internuclear Opthalmoplegia), where signals from right VI nucleus cannot be processed by left oculomotor nucleus, and there are therefore no projections to the left medial rectus muscle. |
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Term
| What are the possible explanations for the inability of the left eye to exhibit right-side gaze? The right eye can still turn to the right. |
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Definition
This requires the medial rectus muscle of the left eye, which is innervated by CN III.
1) Lesion of CN III motor nucleus (you would also probably see issues with contralateral superior rectus, ipsilateral inferior rectus and ipsilateral inferior oblique muscles, however)
2) Lesion of left MLF (Internuclear Opthalmoplegia), where signals from right VI nucleus cannot be processed by left oculomotor nucleus, and there are therefore no projections to the left medial rectus muscle. |
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Term
A patient presents with normal ocular movements, except that during right gaze, their left eye cannot turn to the right. There is also right-eye nystagmus.
What is going on? |
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Definition
Internuclear Ophthalmoplegia (INO).
There is a lesion in the left MLF, and the left Oculomotor nucleus cannot receive signals from the contralateral (right) VI nucleus, and can therefore not innervate the medial rectus of the left eye during right-gaze. |
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Term
| Explain the neural mechanisms controlling Saccadic eye movements (specifically how horizontal eye saccades are achieved). |
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Definition
SHORT- Activity of VI nerve is sum of EBN burst (direct) and integral of EBN burst (indirect path through prepositus hypoglossi nucleus (pph) in medullary RF).
-Direct gets eye to new location and Indirect keeps it there.
1) Eccentric movement arises from Excitatory Burst neurons (EBN) in ipsilateral PPRF, which synapse on left VI nerve and cause lateral rectus contraction.
2) EBNs also relay to neurons in ipsilateral nucleus prepositus hypoglossi (pph) located with vestibular nuclei in medullary RF (Neuronal Integrator). This neuronal integrator is responsible for "tonic" firing on to VI during fixation (keeping rotated eye in place)
**Remember, directional components are segregated at the level of the brainstem.
Paramedian Pontine RF= horizontal components
Mesencephalic RF= vertical and torsional components |
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Term
| How are the horizontal, torsional and vertical components of eye saccades segregated within the brainstem? |
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Definition
Paramedian Pontine RF= horizontal components
Mesencephalic RF= vertical and torsional components
**Both are inputs into VI nucleus** |
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Term
| Explain the neural mechanisms controlling the Vestibulo-ocular reflex (VOR) |
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Definition
3 synapse path (7ms) to maintain eye stability during head movement.
1) During head movement to right, primary afferents from CN VIII and neurons in the medial vestibular nucleus (vn) increase discharge onto contralateral Abducens motor-neurons.
2) Abducens motor-neurons synapse onto LR muscle
3) Lateral rectus muscle contracts and left eye turns to left. Neural integrator converts output of vestibular nuclei (which encode head velocity) into a position command, to give tonic discharge when head stops.
** Also need right medial rectus contraction from right oculomotor nucleus via MLF** |
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Term
| How is the vestibular-ocular reflex (VOR) maintained when the head stops turning to the right? |
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Definition
1) Like the saccadic system, a Neural Integrator output of vestibular nuclei neurons (encoding head velocity) into a position command, which is given to VI neurons and provides tonic discharge to hold eye.
2) During stationary head positions, vestibular afferents discharge constantly (both leftward and rightward), creating no net movement (Push-Pull system). |
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Term
| What happens to the regulation of eye movements during constant head rotation (i.e. when a car in spinning)? |
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Definition
1) Nystagmus is observed (similar to optokinetic stimulation) and VOR compensates.
2) If VOR makes eyes go eccentric, quick-phase saccade will move he eye in the opposite direction.
AND REPEAT |
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Term
A patient's head is not moving, but his right eye is exhibiting slow movements to the right followed by fast movements in the opposite direction.
What is going on? |
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Definition
Nystagmus in the absence of head moving suggests a lesion in the right lateral vestibular afferents or nuclei, disrupting the typical push-pull system balance between VOR and saccade.
If lesion is in neural integrator, the slow phase will be exponential instead of linear towards a central position. |
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Term
| What is "gaze-evoked nystagmus"? How doe does it differ form what would be seen with cerebellar damage? |
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Definition
Physiological condition where horizontal movement of eyes cause eyes to begin to drift back in the opposite direction (repeating cycle).
In the pathological case, you would also see downward nystagmus evoked by directing gaze in vertical eccentricities (downbeat). |
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Term
| What is the neurophysiological basis for "Smooth pursuit"? |
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Definition
Remember, this is slow, conjugate eye movement that is used to follow objects and to minimize retinal slip. It REQUIRES VISUAL INPUT
1) Sensory signals are sent from retina through LGN to V1.
2) From V1, information goes to V5 (middle temporal) and medial superior temporal cortex (MST), which encode direction of movement.
3) V5 and MST send signals to oculomotor structures
4) V5 also projects to Frontal eye fields and SC, which are also important for Smooth Pursuit** |
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