2. Eye Movements (4/24/18)

Gaze

Combined movement of the head and eyes When moving both the head and eyes, the eyes begin to move first since they are less massive. By the time the eyes have almost reached their target, the head begins to rotate, brining the eyes to their final position. As the head continue to rotate, an eye movement in the opposite direction occurs. This prevents the eyes from "overshooting" their target.

Dorsal Midbrain Syndrome (Parinauds Syndrome)

Compression of posterior portion of midbrain Compresses superior colliculus (normally helps generate saccades) and damage = unable to look up Induces pressure on CNIII nucleus. Can have bilateral ptosis. Convergence-retraction nystagmus Light near dissociation; inability to respond to direct light. (No constriction). But ask patient to look at something near, and pupils constrict. Why? 2 centers in the brainstem. 1 with light reflex (pretectal nucleus, PTN) 1 with constriction reflex (convergence center) The one that controls the direct light is posterior to the canal. The one that controls myosis is anterior So compress the back = lose ability to respond to direct stimulation. Other one (convergence myosis) will be intact

Apraxia

Loss of a learned skill

Skew deviation

Vertical diplopia --> Know there is a problem with the frontal lobe, CN III or CN IV On exam, CN III and CN IV are normal The vertical meridian of patient is tilted. Tilt head in direction of higher eye.

What is the role of riMLF? (notes)

Vertical saccades Projects to ipsilateral CN III and IV, as well as contralateral CN III and IV

What presents clinically in bilateral INO?

eyes will each look temporally. (opposite of crossed-eyed) = exotropia

superior and inferior recti

form an agonist/antagonist pair and their primary actions are to elevate and depress the eye, respectively. SR = CN III IR = CN III

What happens to patients with vestibular lesions?

great difficulty in executing coordinated eye and head movements (gaze), since they don't make the reverse eye movements, and they typically "overshoot" the targets they want to look at (overshoot dysmetria)

optokinetic reflexes

reflexive eye movements to a slow-moving broad visual field (train example) Unlike smooth pursuit, the optokinetic reflex does NOT require the processing of visual signals by visual cortex: cortically blind subjects sometimes exhibit these reflexes.

Vestibulo-ocular reflexes

semicircular canals and otolith organs trigger reflexive eye movements (in the opposite direction) during head rotations so that the eyes maintain a target of interest on the fovea. Operates without any visual inputs (infranuclear), and is very rapid

What is the only non-conjugate eye movement?

vergence (or convergence): when both medial rectus muscles contract to bring the eyes inward to view a target near the face

1 1/2 Syndrome (Not in syllabus; PPT only)

(In diagram, the green) Lesion of PPRF and MR PPRF: Lose ipsilateral LR Lose contralateral MR MLF: Lose ipsilateral MR Net: Lost both MR, ipsilateral LR. Only muscle remaining: Contralateral LR (Have lost 1.5 eye movements) Clinical Findings: Ipsilateral side of lesion has no movement at all. Contralateral side can only abduct

Lesion 4 (see diagram); Lesion of the left MLF

-(In diagram, the blue X) -Results in internuclear ophthalmoplegia (INO) -The control of R abducens motoneurons is intact, but the connections of the right abducens interneurons with the left oculomotor motoneurons is disrupted. -During attempted rightward eye movements, the right eye moves laterally but the left eye remains at the midline. -However, the right eye shows nystagmus with rightward gaze. Why? Double vision. Brain sending extra stimulation to whole pathway to get it going. The good eye moves extra. Bad eye stil can' t move. Nystagmus. NOTE: Lesion of the MLF is always on the side of the weak MR muscle INO is a sign of Multiple Sclerosis

Lesion 2 (see diagram); Lesion of the Right Abducens Nucleus

-Both abducens motoneurons and interneurons (projecting to the left oculomotor nucleus) are destroyed (Syllabus 44). -Movements of both eyes are compromised during rightward eye movements (Right lateral gaze palsy) -Leftward eye movements are relatively normal -Some esotropia may be present due to the elimination of inputs to the right lateral rectus muscle.

Lesion 3 (see diagram); Lesion of the right PPRF

-Called "pontine gaze palsy" -See diagram pg. 44 -Prevents the rightward saccade commands from the superior colliculus and frontal eye field from reaching abducens and oculomotor motoneurons; affects LR and MR muscles - leftward eye movements would be normal -neither eye would move when rightward movements are attempted -Since the abducens motoneurons are intact, no esotropia should occur.

Oculomotor Nerve Damage

-Can be caused by Berry aneurysms of the *posterior communicating artery* -Recall, LR and SO still functioning. All other muscles knocked out - Results in *"down and out"* eye position -all movements of the affected eye to be abnormal except when looking laterally and downward with that eye -*ptosis* (levator palpebrae superioris) -*mydriasis* (pupillary dilation. Loss of parasympathetic inputs to sphincter pupillae)

Diplopia (definition)

-Double vision. An individual perceives a single object as being two, which are displaced from each other. -Often occurs when the movement of one eye is defective (strabismus) -As a result, images are projected onto different areas of the retina of the two eyes.

How does one test for Simultagnosia?

Hand patient a card. What do you see? (won't be able to see the stuff in the periphery)

Role of the superior colliculus

-Receives input from the frontal eye field, and relays it to the PPRF. -Also gives rise to the tectospinal tract, which participates in eliciting gaze movements when eye position needs to be redirected substantially **From last lecture: Receives inputs from inferior colliculus to localize sound Thus, the superior colliculus plays a key role in coordinating small changes in eye position evoked by contracting the extraocular muscles alone, as well as large changes in eye position mediated by a combined head turn and eye movement.

Pathway for smooth pursuit eye movements (syllabus)

-involves higher- order cortical visual areas (for computing the speed of the moving targe) and the cerebellum -middle temporal (also termed V5) and middle superior temporal visual areas --> internal capsule --> engage a brain stem and cerebellar circuit comprising the pontine nuclei, the flocculus, and the vestibular nuclei. Damage to the middle and superior temporal visual areas can cause a deficit in generating smooth pursuit eye movements, but not other types of eye movements.

CN VI Palsy

A common cause of *strabismus* Because of its long course along the clivus and over the sharp ridge of the petrous temporal bone, the VIth nerve can be damaged by downward traction during *elevated intracranial pressure or head trauma*.

Which part of the cortex projects to the frontal eye field?

A subregion of Brodmann area 7 near the intraparietal sulcus. In other words, the parietal cortex plays a pivotal role in choosing targets that eye movements should be directed to, whereas the frontal eye fields are more involved in coordinating the mechanics of the movements.

exotropia

Abnormal lateral deviation (abduction) of the wandering (improperly moving) eye

medial and lateral recti

Act as an agonist/antagonist pair to yoke the eye horizontally. Thus, to look to the right, the lateral rectus in the right eye contracts to abduct the eye and the medial rectus in the same eye relaxes. LR = CN VI MR = CN III

What role do abducens interneurons play?

Arrows in the diagram designate abducens interneurons that play a key role in producing the horizontal vestibuloocular reflex. Without these interneurons, the reciprocal contraction/relaxation of the medial and lateral rectus muscles that produces the reflex would not occur.

Where are vertical and torsional components of saccades mediated? (syllabus)

By neurons in the mesencephalic reticular formation (MRF).

Downgaze palsy

Cannot look down What normally helps someone look down? Frontal lobe to riMLF to CN III and CN IV. That part of the riMLF that controls downward movement is DORSAL

Upgaze palsy

Cannot look up What normally helps someone look up? Frontal lobe to riMLF to CN III and CN IV. That part of the riMLF that controls upward movement is FRONTAL

Diplopia (cause & treatment)

Cause: *Any disease process that affects the movement of an eye* can result in strabismus, including damage to an extraocular muscle, the nerve fibers innervating the muscle, or extraocular motoneurons. Treatment: surgery, use of glasses to align the inputs from the two eyes on the retina, or placing a patch on the dominant eye.

What are the eye defects of Right CN VI palsy?

Damage to the right abducens nerve results in the right eye deviating inward (esotropia), because the resting activity of the medial rectus muscle is not balanced by that of the lateral rectus (center panel). When looking leftward, the eye movement is normal, since the right medial rectus muscle has normal innervation. When attempting to look rightward, the right lateral rectus does not contract, so the right eye stays at the midline

Describe what would happen if the Medial Longitudinal Fasciculus (MLF) was damaged between the abducens and oculomotor nuclei

Damage would compromise the horizontal vestibuloocular reflex, and (as noted later) saccadic eye movements.

Direct vs. Indirect Pathway for Saccadic Eye Movements

Direct pathway: abducens interneurons and motoneurons receive direct inputs from the PPRF, causing the eye position to jump to the target of interest Indirect pathway: abducens interneurons also receive integrated PPRF commands through the prepositus hypoglossi, which sustain the adequate level of motoneuron activity to maintain the eye on the target

During convergence, what happens to the ciliary muscle? The pupil?

During vergence, the ciliary muscle contracts making the lens more convex, shortening its focal length (accommodation). The pupil also constricts in order to prevent diverging light rays from hitting the periphery of the retina and resulting in a blurred image. These responses are coordinated.

Acquired ocular motor apraxia (Balint's syndrome)

Extensive bilateral cerebral disease (parieto-occipital) Absence of reflexive saccades and pursuit Vestibular pathways intact (infra nuclear) Visual field defects Dementia Simultagnosia (will only focus on fovea. Peripheral vision missing. failure of recognition of simultaneous info sent to brain)

Oculogyric Crisis

Eyes are looking up, stuck. Chemical imbalance --> dopamine and ACh (usually at level of basal ganglia) Stuck in high tone upwards, Not moving down. Not moving R or L voluntarily But vestibulo pathway in tact. Only the supranuclear vertical gaze pathways are affected.

Where does the PPRF receive inputs from?

From the contralateral superior colliculus, which in turn receives inputs from the frontal eye field. There are also direct inputs from the contralateral frontal eye field to the PPRF

In the saccades pathway, what does "prefrontal cortex" refer to?

Frontal cortex is frontal eye fields, supplementary eye fields, and dorsolateral prefrontal cortex Frontal eye fields - visually guided, memory guided, fixation and vergence Supplementary eye fields - control of learned patterns of ocular motor behaviors DLPC - planning saccades to remembered targets

Role of frontal eye field in saccades? What happens with damage to them?

Frontal eye field abduct contralateral eye. Damage to R frontal eye field --> unable to abduct L eye. L eye will still Adduct. So look right (toward the lesion).

What happens with the vestibule-ocular reflex during gaze?

Initially during a combined eye and head movement, it is counterproductive for a vestibuloocular reflex to oppose the eye movement. Thus, the vestibulo-ocular reflex is suppressed during saccades. once the saccade (the active eye movement) is completed, the vestibulo-ocular reflex returns. In fact, the vestibulo-ocular reflex is mainly responsible for the reverse movement of the eyes during the continued rotation of the head.

CN III

Innervates all extraocular muscles except the SO and LR Carries the axons of motoneurons innervating the levator palpebrae superioris muscle, which elevates the eyelid Carries parasympathetic preganglionic axons that mediate pupillary constriction and changes in lens shape.

Describe the pathway of horizontal saccadic eye movement once at the PPRF

Mediated by neurons in the paramedian pontine reticular formation (PPRF) Activates ipsilateral abducens (CN VI) nuclei. Abducens nuclei does 3 things: 1. Excites ipsilateral LR muscle 2. Excites contralateral MR muscle(thru CN III) 3. Inhibits antagonistic muscles (contralateral abducens nuclei)

Describe eye movements in general

Most eye movements are conjugate; both eyes move in the same direction at the same velocity. However, the particular extraocular muscles that contract on each side can differ depending on the movement executed. For example, if you look to the right, the right lateral rectus and the left medial rectus contract, whereas the right medial rectus and the left lateral rectus relax.

Optokinetic nystagmus

Normal reflexive response to large-scale movements When the eyes reach the end of the orbit, a saccade occurs in the opposite direction and smooth pursuit or optokinetic reflexes continue This is NOT pathological

Progressive Supranuclear Palsy

Progressive = gets worse Supranuclear palsy = problem with supranuclear pathway b/t cortex and brainstem Inability to look down (recurrent falls) Then inability to look up Then Axial rigidity. (Parkinsonian features but different distribution) Then lose eye movements

Smooth pursuit

Require the presence of a moving target Begins with a saccade to capture the target on the fovea. The smooth pursuit movement keeps the target on the fovea. Visual feedback is critically important for maintaining smooth pursuit.

superior oblique and inferior oblique

SO = CN IV IO = CN III The superior oblique arises from the apex of the orbit and passes through a fibrocartilaginous ring, called the trochlea, turns laterally and inserts into the superior region of the globe, underneath the superior rectus Allows the superior oblique muscle to *depress and intort the eye* (rotate towards the nose). The inferior oblique muscle originates from the maxillary bone in the medial wall of the orbit. The muscle traverses ventrally along the globe, underneath the inferior rectus, and inserts on the lateral side of the globe, medial to the tendon of the lateral rectus. Contraction results in *elevation and extortion* of the eye

How do ocular motor units differ from other motor units?

The number of muscle fibers/motor unit is very small (5-10). No stretch reflexes No recurrent inhibition

Explain the role of the prepositus hypoglossi

The previous circuit explains the jump in eye position to a new location. However, typically we want to maintain the eyes in the new location after they have moved. Thus, the prepositus hypoglossi is necessary. It integrates the "burst signal" from the PPRF, and transforms it into a "step signal" (continuous firing).

Compare the speeds of the Vestibulo-ocular reflexes, saccades, and smooth muscle pursuits

The vestibulo-ocular reflex is very rapid (latency of 7-15 msec). In comparison, saccades have a latency of about 200 msec, and smooth pursuit movements are even slower than saccades. Although smooth pursuit can also aid in stabilizing an image on the retina during head movements, the accuracy is limited (particularly during rapid head movements, due to the slow latency of the responses).

What happens if strabismus is not corrected?

They develop amblyopia, or lack of visual perception from one eye. To prevent double vision, the brain ignores the input from the "lazy eye." The loss of input from the lazy eye can become permanent if the condition is not corrected by surgery, use of glasses to align the inputs from the two eyes on the retina, or placing a patch on the dominant eye.

Pursuit Movements (PPT notes)

Visual pathway to occipital lobe --> projects to Parietal-Occpital-Temporal (POT) junction --> projects to Frontal Eye Field and Dorsolateral Pontine Nuclei --> These cross to contralateral cerebellum and then vestibular nuclei --> cross back, to CN III, IV, and VI Clinically: Double decussation results in clinical appearance of ipsilateral control R hemisphere controls objects moving to R, L hemisphere controls objects moving to L

Describe eye movements of a Right INO

With a Right INO, the right eye is not able to look medially (to the left)

esotropia

abnormal medial deviation (adduction)

Saccades

conjugate eye movements that can be generated either reflexively (abrupt appearance of target) or voluntarily. The goal of saccades is to fixate new targets for optimal visual processing by the fovea. Saccadic eye movements are ballistic: the saccade-generating system cannot respond to subsequent changes in the position of the target during the course of the eye movement.

Vergence (PPT)

• Afferent pathways project to striate cortex which project to • the Pretectal Nuclei which project to • CN III which results in Miosis, Convergence, Accommodation

Internuclear ophthalmoplegia

•Lesion of MLF blocks information from the contralateral CN VI nucleus to the ipsilateral CN III nucleus •Adduction Deficiency away from lesion •Abduction Nystagmus •Named for side of MLF lesion, side of weak medial rectus

Vestibulo-ocular-reflex (PPT notes)

•Right horizontal semicircular canal information delivered to the right vestibular nucleus •Right vestibular nucleus projects to left CN VI nucleus •Eyes move to left