3: Control of Eye Movement Flashcards
(OBJ) List and define the 7 different types of movements made by the eyes.
- Saccadic movements: small jumps; occur quickly
- Smooth pursuit eye movements: tracking slowly moving objects
- Vestibulo-ocular reflexes: produces eye movement in response to changes in head position
- Fixation reflex: fixate on moving target
- Optokinetic reflex: involuntary fixation on objects moving in relationship to head
- Vergence: the movement of the eye to focus close up or far away
- Pupillary light reflex: constriction of pupil in response to light
(OBJ) List the actions of the 6 extraocular muscles.
- Medial rectus: adducts eye
- Lateral rectus: abducts eye
- Superior rectus: elevate, intort, adduct eye
- Inferior oblique: elevate, extort, abduct eye
- -Superior rectus + inferior oblique: elevate eye - Inferior rectus: depress, extort, adduct eye
- Superior oblique: depress, intort, abduct eye
- -Inferior rectus + superior oblique: depress eye
(OBJ) Compare and contrast saccadic eye movements and smooth pursuit.
Saccadic: rapid, ballistic eye movement
- -Produced by cerebral cortex (frontal eye fields) or superior colliculus -> PPRF (horizontal) & rostral interstitial nucleus (vertical)
- -Vision blurred during movement
- -Head turns 20-40 msec later
Smooth pursuit: eyes move smoothly
- -Only occurs when tracking something or in response to head movement
- -Shares pathway with vestibulo-ocular reflex
Why do we need precise control of eye movement?
Keep visual images on equivalent location of each retina to prevent double vision
True/false: Most eye movements are voluntary.
FALSE - most eye movements are REFLEX
Distinguish between intorsion and extortion.
Intorsion: upper part of eye approaches nose
Extortion: upper part of eye approaches temporal region
Distinguish between convergence and divergence.
Convergence: bringing the pupils together
Divergence: moving the pupils away from each other
What is the MLF? PPRF? What results from damage to each?
MLF: contains ascending projections from vestibular nuclei, contributing to eye movements with changes in head position
- -Interconnects nuclei of extraocular muscles to coordinate conjugate eye movements
- -Damage: intenuclear opthalmoplegia (eye moving laterally not followed by medial movement of the other eye)
PPRF: critical area for voluntary conjugate horizontal eye movement
–Damage = prevent voluntary horizontal gaze towards the side of damage
What does the superior colliculus do? From where does it receive afferents and to where does it send efferents?
Site of relay of cortical control of eye movement
Afferents: retina, inferior colliculus, spinal cord, cerebral cortex
Efferents: spinal cord (tectospinal tract) and brainstem (tectoreticular tract) including PPRF and midbrain RF
Function: adjusts head and eyes toward a stimulus; saccadic movement to target
What is the frontal eye field? What is the occipital eye field?
Frontal eye field: part of the premotor cortex patterning voluntary horizontal gaze
Occipital eye field: a region near the junction of the occipital lobes with the posterior parietal and temporal lobes; includes areas of cortex involved in interpreting movement of objects -> eye movements respond to visual inputs
What is the pathway for voluntary horizontal gaze?
- Right frontal eye field of cortex
- (right superior colliculus ->) LEFT paramedian pontine reticular formation (PPRF)
- Left CN VI -> left lateral rectus m.
- -> MLF -> right CN III -> right medial rectus
What is the pathway for voluntary vertical gaze?
- Diffuse areas of cortex
- Rostral interstitial nucleus of midbrain (vertical gaze center)
- (-> posterior commissure) -> oculomotor & trochlear nuclei
Damage inhibits voluntary upgaze, does not affectvertical vestibuloocular reflex
What is the pathway for the vestibuloocular reflex?
- CN VIII
- Vestibular nucleus & flocculus
- CN VI & PPRF
- Through MLF to CN III, CN IV
What is the pathway for smooth pursuit? What two other movements use this pathway?
Uses some of the same wiring as the vestibuloocular reflex
- Awareness of a moving visual stimulus in occipital eye field (visual association areas in the occipital lobe) -> lock eyes onto target
- (-> frontal eye field) -> ipsilateral pontine nuclei
- -> middle cerebellar peduncle -> contralateral flocculus
- -> inferior cerebellar peduncle -> ipsilateral vestibular complex
- CN VI & PPRF (bilateral, some excitatory, some inhibitory)
- -> MLF -> CN III, IV
Also used by fixation and optokinetic reflexes
What is the pathway for vergence?
Occipital lobe -> mesencephalic reticular formation -> superior colliculus
- -Pathway unclear
- -Wired in parallel with acommodation
What is the pathway for the pupillary light reflex?
Afferents: optic nerves, optic chiasm, optic tract
- -> Pretectal nucleus (processes light)
- -> both E-W nuclei
- -> Preganglionic parasympathetic fibers running on surface of CN III
- -> Ciliary ganglion
- -> Postganglionic parasympathetic fibers -> sphincter pupillae
(OBJ) List the cranial nerves involved in regulating eye movements, including their source and what they innervate. [CN III]
CN III - oculomotor nerve - from interpeduncular fossa of midbrain
- -> GSE to medial rectus, superior rectus, inferior oblique, inferior rectus, levator palpebrae superioris
- -> GVE from E-W nucleus to ciliary ganglion -> constrictor pupillae & ciliary muscles
Name 3 places/situations in which the oculomotor nerve can be easily damaged or compressed. What is the typical first sign of pressure on oculomotor nerve?
Aneurysm of posterior communicating artery or internal carotid artery
Wall of the cavernous sinus
First sign = pupillary dilation
(OBJ) List the cranial nerves involved in regulating eye movements, including their source and what they innervate. [CN IV]
CN IV - trochlear nerve - decussates in brainstem, then exits from *dorsum of brainstem at ponto-mesencephalic junction
- -> GSE to CONTRALATERAL superior oblique
- -Long, thin - can be damaged in head trauma or lateral wall of cavernous sinus
(OBJ) List the cranial nerves involved in regulating eye movements, including their source and what they innervate. [CN VI]
CN VI - abducens nerve - from abducens nucleus in floor of fourth ventricle, exits at medullary pontine junction
–> GSE to lateral rectus
Characterize the activity of extraocular motor neurons before, during, and after eye movement.
Before: tonic activity
During: marked increase or decrease in firing rate
After: new resting tonic level of activity
Why is there a 1/5 second lag before a saccade takes place?
To process the eye movement to assure that it is accurate
Describe the characteristics of someone with a trochlear nerve lesion. How would a lateral rectus lesion differ?
Trochlear nerve (CN IV) = superior oblique
- -Eye deviates slightly medial and extorts
- -Head laterally flexed away from side of lesion
LR lesion = head tiles TOWARD side of abnormal eye
What are microsaccades?
Minute, nearly imperceptible movements of the eye that keep an image from remaining on the same photoreceptors for too long (would -> blanching in a few seconds otherwise)
What is jerk nystagmus? When is it seen?
Nystagmus with a slow and fast component
Indicates problem with vestibular system
What is pendular nystagmus?
Rapidly oscillating eye movements in which oscillations are equal in both directions
How are single eye muscles tested?
The H! Abducted -> elevation = superior rectus Abducted -> depression = inferior rectus Adducted -> elevation = inferior oblique Adducted -> depression = superior oblique Abduction = lateral rectus Adduction = medial rectus
