Eye Movements and Vestibular Flashcards
(41 cards)
What tract is critical for maintaining muscle tone?
[lateral vestibulospinal tract, and it’s extensor dominant]
What is the ‘near’ response triad?
[eyes converge (disjunctive movement), pupil constricts (increases depth of field), lens thickens (increases refraction for close vision)]
What is the PPRF?
[paramedian pontine reticular formation, aka horizontal gaze center]
If you lost a pretectal nucleus on one side, what would happen to your pupillary reflex?
[probably nothing, …each pretectum receives from both eyes, each pretectum projects to both nu of Edinger-Westphal]
What is the nu of Edinger-Westphal?
[This is the pre-ganglionic parasympathetic (GVE) component responsible for pupillary constriction and lens accommodation (thickening) located in the oculomotor complex (III). Axons leave with III and reach the ipsilateral ciliary ganglion (post-ganglionic parasympathetic) just behind the eyeball]
What are the direct and consensual components of the light reflex?
[ when you shine the light into one eye, constriction of the pupil of that eye is considered the ‘direct’ response, constriction of the pupil of the other eye is the ‘consensual’ response]
What’s the Argyll-Robertson pupil?
[a pupil that exhibits the accommodative response, but not the pupil (light) response. Associated with advanced syphilis, and autopsy reveals bilateral atrophy of pretectum… therefore pupillary constriction in accommodation does not depend on pretectum to get to the nu Edinger-Westphalt]
Which is the only eye movement we make in which the eyes do not work as a ‘yoked pair’?
[vergence movements (e.g., accommodative response)]
What is nystagmus?
[rhythmic movement of the eyes, usually a vestibular (peripheral) issue; 2nd most common is cerebellar damage, rarely does it involve 1 eye; for clinical reference, the horizontal nystagmus is the most common and exhibits a slow phase and fast phase. The slow phase is true vestibular signals, whereas the fast phase is a saccade. By (strange!) convention, the nystagmus is referenced to the fast phase]
Name the 5 types of eye movements we make?
[vestibular ocular reflex, optokinetic reflex, pursuit, saccades, vergence]
What is the purpose of the vestibular ocular reflex?
[when we move our head, the vestibular system triggers an eye movement equal and opposite of the head movement to maintain gaze; that way, we maintain gaze on the same target despite the head movement. We can do VOR in the dark, but calibration of system depends on vision and cerebellum (flocculonodular lobe)]
What is the purpose of the optokinetic reflex?
[when the head translates through space, we cannot look at the same thing without generating an eye movement to counteract the spatial translation. This discrepancy is referred to as ‘retinal slip’. Optokinetic eye movements are basically, VOR with vision, keeping the eye locked on target as the head moves through space. An interesting difference between VOR and OKR for all you math fans… VOR keys on head acceleration signals, OKR on head velocity signals]
What is the ‘special’ nucleus associated with the optokinetic reflex?
[nucleus of optic tract]
What are pursuit eye movements?
[relatively slow movements we make to track moving objects… up to ~60 deg/sec. Pursuit system depends on activity in the anterior superior colliculus, which is active during pursuit, and during fixation (which is considered part of pursuit ( Yeah, I don’t know why either)]
Why does state trooper Jane tell you to watch her finger as he sweeps it back and forth?
[she thinks you’ve been drinking, and is testing your pursuit eye movement system which is very sensitive to alcohol]
What are saccades?
[the fast (up to 900 deg/sec) eye movements we make about 3 times/sec to bring the fovea onto different parts of our visual scene for detailed inspection]
Which brain structures are considered selectively critical for saccades?
[frontal eye fields, superior colliculus; course, let’s not forget we also need cranial nerves III, IV and VI, the MLF, PPRF and RiMLF]
What’s an ‘anti-saccade’?
[saccades made under testing conditions in which the subject is instructed to look away from a newly presented target in the visual field. For the record, it’s not easy to execute… takes a little practice. People with frontal lobe damage are horrible at it].
What are the frontal eye fields?
[a specialized region of the superior frontal gyrus, BA 8, whose activity drives contraversive conjugate eye movements; mediates eye movements on command: e.g., ‘look left’]
What is the ‘primate plan’?
[Specialization of the retina to have a fovea and high visual acuity and a large surround area without high acuity but sensitive to movement and contours. This ‘fovea’ is then coupled with a high-precision oculomotor plant that can place the fovea onto any target within oculomotor range with pinpoint accuracy]
What are the PPRF and RiMLF?
[paramedian pontine reticular formation, rostral interstitial nucleus of the medial longitudinal fasciculus. PPRF is the ‘horizontal gaze center’ adjacent to the abducens nu: the left PPRF executes leftward gaze shifts, the right PPRF executes rightward gaze shifts]
In the brainstem, how is a left conjugate eye movement made?
[Signals must reach the left PPRF which then triggers signals to engage abducens on that side, engage medial rectus on the right, and relax the abducens on the right and medial rectus on the left. Important! The signals going to the right medial rectus cross immediately (at the level of the pons nucleus and ascend the right MLF]
What are the consequences of loss of right abducens nerve?
[the right eye does not abduct, loss of conjugate gaze control with diplopia. Patient will maintain orbital position to minimize diplopia]
What are the consequences of loss of right abducens nucleus?
[rightward gaze shifts are abolished. The right eye will not abduct because the right abducens nu is out. The left eye will not adduct because internuclear signals from the abducens and adjacent PPRF are out. Therefore the signal to adduct the left medial rectus never occurs.
