the top ten things you should know about the oculomotor system

THE TOP TEN THINGS YOU SHOULD KNOW ABOUT

THE OCULOMOTOR SYSTEM

10. Movements of the eyes are produced by six extra-ocular muscles. If they, or the neural pathways controlling them,

are not functioning normally, eye movements are abnormal.

• Additionally, accommodation and pupillary responses are produced by intraocular muscles

Video OF Duane’s

Video of Opsoclonus

Meet the musclesMuscle Primary action Example

Medial rectus Adduction Towards themidline/nose

Lateral rectus Abduction Away from themidline/nose

Superior rectus Elevation

Inferior rectus Depression

Superior oblique Intorsion

Inferior oblique Extorsion

Meet the muscles (Cont.)

Muscle Primary action

Ciliary muscle Positive accommodation:acts against suspensoryligaments

Sphincter pupillaeiris muscle

Pupilloconstriction

Dilator pupillaeiris muscle

Pupillodilation

9. The stretch reflex is absent. Gently press on your eye and you’ll see the

world move.• Proprioceptive feedback from the extra-

ocular muscles is not used to keep track of eye position.

• The brain keeps track of eye position by keeping track of the signals sent to the motoneurons that innervate the extra-ocular muscles. This is known as efference copy or corollary discharge.

8. Except for changes in viewing distance, normal eye movements are yoked.

• Yoking: the eyes move the same amount in the same direction.

• Vertical eye movements are normally always yoked.

• Projections from the abducens nucleus to medial rectus motoneurons by way of the medial longitudinal fasciculus provides the basis for horizontal yoking. During convergence, the eyes move equal amounts in opposite directions.

Excitatory

Inhibitory

MVN - Medial vestibular nucleusNPH - Nucleus prepositus hypoglossiEBN - Excitatory burst neuronIBN - Inhibitory burst neuron

VIDEO SHOWING INTERNUCLEAR OPHTHALMOPLEGIA

7. Eye movements are controlled by distinct neurological subsystems.

• Eye movements stabilize the image of the external world on the retina

• Eye movements bring images of objects of interest onto the fovea

FUNCTIONAL CLASSES OF EYE MOVEMENTSExtra-ocular muscles

Class of EyeMovement

Main Function

Vestibular Holds images of the seen world steady on theretina during brief head rotations

Optokinetic Holds images of the seen world steady on theretina during sustained head rotation

Visual fixation Holds the image of a stationary object on thefovea

Smooth pursuit Holds the image of a small moving target on thefovea; with optokinetic responses, aids gazestabilization during sustained head rotation

Nystagmus quickphases

Reset the eyes during prolonged rotation anddirect gaze toward the oncoming visual scene

Saccades Bring images of objects of interest onto thefovea

Vergence Moves the eyes in opposite directions so thatimages of a single object are placed or heldsimultaneously on both foveas

FUNCTIONAL CLASSES OF EYE MOVEMENTS

Intra-ocular muscles


Main Function

Accommodation Focuses images on fovea

Pupillary Light Reflex Controls illumination levels of retina

6. Vestibular responses. You can’t read without them.

Your head turns in one direction with acertain velocity, and because of thevestibular ocular reflex (VOR), your eyesturn with an equal velocity (if the VORgain is 1.0) in the opposite direction. Thisreflex has a latency of less than 10millseconds.

Once the transient head rotation ceases,your eyes have turned to a new position.

They need to remain at that position andnot drift back to primary position.

To achieve this, a tonic signalproportional to the integral of the eyevelocity signal is generated and sent tothe extraocular motoneurons to maintainthe new eye position.

VOR gain is low at low frequencies

Vestibulo-ocular reflex

Increased firing rate with rightward head turns

Excitatory

Inhibitory




Main Function









5. Optokinetic responses. The world drifts without them.

When large-field stimuli move, your eyes tend totrack the overall movement. This is an adaptiveresponse to the slip of the image of the outsideworld on the retina that occurs when VOR gain isnot 1.0.

This response is mediated by neurons in thepretectum and the medial superior temporal (MST)region of cortex. These neurons indirectlymodulate vestibular neurons.

VESTIBULAR NUCLEUS NEURON

A. ROTATION IN DARKNESS (Vestibular but no Optokinetic)

B. ROTATION IN LIGHT (Vestibular and Optokinetic)

C. NO ROTATION. OPTIC FLOW. (Optokinetic but no Vestibular)

Vestibular-optokinetic interactions

When rotation stops, nystagmus starts in the opposite direction (postrotatory nystagmus, PRN). In the middle panel, an optokinetic stimulus (drum rotation to the right) causes a sustained optokinetic nystagmus (OKN), with slow phases to the right during the entire period of stimulation. When the lights are turned off during stimulation, eye movements do not stop immediately but persist as optokinetic after-nystagmus (OKAN). In the lower panel, the subject is rotated in the light (natural situation of self-rotation). This gives a combined vestibular and optokinetic stimulus. The response is a sustained nystagmus. When the chair stops rotating, eye movements stop nearly completely: postrotatory nystagmus is suppressed by the opposite-directed optokinetic after-nystagmus and by visual fixation of the stationary world.

Schematic summary of vestibular-optokinetic interaction occurring in response to velocity-step rotations. Graphs on the left show characteristics of the stimulus (head velocity during rotation or drum velocity during optokinetic stimulation); graphs on the right show the responses (slow-phase eye velocity, quick phases having been removed). R, right; L, left; t, time. In the top panel, constant-velocity rotation to the left in the dark produces slow-phase movements to the right (per-rotatory nystagmus, RN) with initial eye velocities equal to head velocity (VOR gain = 1.0).



Main Function

Vestibular Holds images of the seen world steadyon the retina during brief headrotations

Optokinetic Holds images of the seen world steadyon the retina during sustained headrotation

Visual fixation Holds the image of a stationary objecton the fovea

Smooth pursuit Holds the image of a small movingtarget on the fovea; with optokineticresponses, aids gaze stabilizationduring sustained head rotation

VOLUNTARY

Saccades Bring images of objects of interestonto the fovea

VOLUNTARY

Vergence Moves the eyes in opposite directionsso that images of a single object areplaced or held simultaneously on bothfoveas

VOLUNTARY

4. Saccadic eye movements. You can’t look at anything interesting without them.

• FAST - 40-90 MS IN TOTAL DURATION

• BALLISTIC

B A

AB

BA

Pulse of firing rate is requiredto produce the transient forceneeded to move the eyerapidly despite viscous drag

Sustained firing rate isrequired to hold the eye in anew postion despite theelastic forces that try to returnit to primary position

Right Medial Rectus Motoneuron - Saccades

Horizontal saccades are generated in the paramedian pontine reticular formation (PPRF)

VERTICAL SACCADES ARE GENERATED HERE

Vertical saccades are generated in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF)


Excitatory

Inhibitory


EBN

Burst size proportional to saccade size

OMNIPAUSE NEURON (OPN)

NI - Neural Integrator

Excitatory burst neuron- small saccade

Excitatory burst neuron- medium saccade

Excitatory burst neuron - large saccade

Omnipause neuron - various saccades

EBN

Burst size proportional to saccade size

OMNIPAUSE NEURON (OPN)

NI - Neural Integrator

THE SUPERIOR COLLICULUS PROJECTS TO THE PPRF

2-D map of contralateral saccades

SUPERIOR COLLICULUS MOTOR MAP

A block diagram of the major structures that project to the brain stem saccade generator (premotor burst neurons in PPRF and riMLF). Also shown are projections from cortical eye fields to superior colliculus. FEF, frontal eye fields; SEF, supplementary eye fields; DLPC, dorsolateral prefrontal cortex; IML, intramedullary lamina of thalamus; PEF, parietal eye fields (LIP); PPC, posterior parietal cortex; SNpr, substantia nigra, pars reticulata. Not shown are the pulvinar, which has connections with the superior colliculus and both the frontal and parietal lobes, and certain projections, such as that from the superior colliculus to nucleus reticularis tegmenti pontis (NRTP).

Disorders of the saccadic pulse and step. Innervation patterns are shown on the left, eye movements on the right. Dashed lines indicate the normal response. (A) Normal saccade. (B) Hypometric saccade: pulse amplitude (width ´ height) is too small but pulse and step are matched appropriately. (C) Slow saccade: decreased pulse height with normal pulse amplitude and normal pulse-step match. (D) Gaze-evoked nystagmus: normal pulse, poorly sustained step. (E) Pulse-step mismatch (glissade): step is relatively smaller than pulse. (F) Pulse-step mismatch due to internuclear ophthalmoplegia (INO): the step is larger than the pulse, and so the eye drifts onward after the initial rapid movement.

Experimental cerebellectomy completely abolishes the adaptive capability-for both the pulse size and the pulse-step match.296 Monkeys with lesions restricted to the dorsal cerebellar vermis cannot adapt the size of the saccadic pulse; they have pulse-size dysmetria .416,416a On the other hand, monkeys with floccular lesions cannot match the saccadic step to the pulse to eliminate pulse-step mismatch dysmetria.298 This evidence suggests that the repair of conjugate saccadic dysmetria is mediated by two different cerebellar structures: the dorsal cerebellar vermis and the fastigial nuclei control pulse size, and the flocculus and paraflocculus control the pulse-step match.



Main Function









• Smooth pursuit: Tracking eye movements - conjugate. Velocity of visual target

• Visual cue: retinal slip velocity of visual target.AB

BB AA

3. Smooth pursuit eye movements. You can’t track anything interesting without them

• Smooth pursuit: Tracking eye movements - conjugate. Velocity of visual target. Slow.

• Visual cue: retinal slip velocity of visual target.

SMOOTH PURSUIT PATHWAYS



Main Function









• Vergence: Eye movements in depth. Disconjugate - left and right eyes move in opposite directions.

B

BAA

A

B

2. Vergence. Without it, you can’t get a closer look.

Far viewing Near target Accommodation and convergence

Blurred images fall on non-corresponding retinal locations -

blur and disparity signals

F F F F F F

Eyes converge and lenses focus - reduced

blur and disparity signals

Far viewing Near target Accommodation and

accommodative convergence

Blurred images fall on retina - blur signal

F F F F FFEyes change focus

- reduced blur signal.Also accommodative

convergence

2-D map of saccades


Excitatory

Inhibitory


Internuclear ophthalmoplegia - adduction during convergence is not reduced

FUNCTIONAL CLASSES OF EYE MOVEMENTS

Intra-ocular muscles


Main Function

Accommodation Focuses images on fovea

Pupillary Light Reflex Controls illumination levels of retinaMODULATE ROOM LIGHTS

Pupillary light reflex

Pupillary light reflexDirect Consensual

Direct Consensual

Pupillary light reflex

1. Pupillary light reflex. If it’s absent,

there’s a problem. AFFERENT DEFECTS:PUPILS APPROX. EQUAL IN SIZE. BUT RESPONSE TO LIGHT IN ONE EYE IS LESS THAN THE RESPONSE TO LIGHT IN THE OTHER EYE.

EFFERENT DEFECTS:PUPILS MAY BE OF DIFFERENT SIZES (ANISOCORIA). PUPIL OF ONE EYE REACTS MORE TO LIGHT IN EITHER EYE THAN THE PUPIL OF THE OTHER EYE TO LIGHT IN EITHER EYE.

Pupillary light reflex: Afferent deficit

Pupillary light reflex: Afferent deficit

Neutral Density Filter (0.5 log unit)

0.5 log unit Relative AfferentPupillary Deficit (RAPD)

Pupillary light reflex: Efferent deficit

1. Pupillary light reflex. If it’s absent, there’s a problem.2. Vergence. Without it, you can’t get a closer look.3. Smooth pursuit eye movements. You can’t track anything

interesting without them4. Saccadic eye movements. You can’t look at anything interesting

without them.5. Optokinetic responses. The world drifts without them.6. Vestibular responses. You can’t read without them.7. Eye movements are controlled by distinct neurological

subsystems.8. Except for changes in viewing distance, normal eye movements

are yoked. 9. The stretch reflex is absent. Gently press on your eye and you’ll

see the world move.10. Movements of the eyes are produced by six extra-ocular

muscles. If they, or the neural pathways controlling them, are not functioning normally, eye movements are abnormal.

TOP TEN LIST

the top ten things you should know about the oculomotor system

Documents

normal eye movements

retina eye movements

vertical eye movements

optokinetic responses

vestibular responses

track of eye position

darkness vestibular

vestibular nucleus neurona