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CONTROL OF EYE MOVEMENTS - 57 (copy)

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Describe how the six extraocular muscles move the eye?

-The six extraocular muscles move the eye in a reference frame that corresponds to the spatial geometry of the vestibular semicircular canals , i.e., the typical diagonal, 45° off the midsagittal plane orientation of vertical canals, is reflected in the pulling direction of the vertical eye muscles.

-The vertical eye muscles are superior rectus, inferior rectus, superior oblique and inferior oblique.

-the horizontal eye muscles are lateral rectus and medial rectus

-only lateral rectus and medial rectus are "straight" eye muscles, whereas all vertical eye muscles, including superior rectus and inferior rectus, are in reality "oblique" muscles.
Describe the innervation of extraocular muscles?

1. Oculomotor nucleus(CN III)
a. Superior rectus (contral)
b. Inferior rectus (ipsi)
c. Inferior oblique (ipsi)
d. Medial rectus (ipsi)

2. Trochlear nucleus (CN IV)
a. Superior oblique (contra)

3. Abducens nucleus:
a. Lateral rectus (ipsi)

NOTES:

-Abducens internuclear neuron pathway to cMR MNs
-In humans (and most vertebrates), the extraocular muscles are innervated by four ipsilaterally and two contralaterally projecting motoneurons populations: i.e., three ipsilaterally (IR, IO, MR) and one contralaterally projecting (SR) oculomotor nucleus neuron population, one trochlear motoneuron population (contralateral, SO) and one abducens motoneuron population (ipsilateral, LR).

NOTE:
All the innervations are connected by the medial longitudinal fasciculus – allows for the generation of fast key movements.
Describe the actions of the individual extraocular muscles?

The actions of the extraocular muscles, so-called kinematics, are defined as adduction, abduction, elevation, depression, intorsion and extorsion. Only in case of the horizontal eye muscle do we observe simple actions, such as adduction and abduction for medial rectus and lateral rectus, respectively. For the vertical eye muscles, their actions have to be decomposed in primary and secondary components.

NOTES:

B/c the muscles are not straight they do not do torsional eye movements but curved eye movements; each has vertical and torsional components.
The contraction of the superior rectus leads to elevation, the eye is adducted towards the nose.
Different movements of the eye; torsion means rotation around the optic axis; Also called rotational but the eye is a sphere and naturally everything is a rotation so it is not a good definition.
What determines the action of the eye muscle?

Where the eye looks determines the action of the eye muscle – the superior rectus – makes an elevation and introsion; to look to the left – there would be elevation, but to look to the right then intorsion must occur. Medial rectus adducts the eye towards the nose; lateral rectus abducts the eyes laterally, away from the nose, superior rectus – elevates the eye and brings it towards the nose; superior oblique – insertion is on top of the ye and depresses the eye, depends on the point of the insertion – intorting and depressing are the primary actions. Know that the eye muscles have different actions that allow for the understanding of certain syndromes. The eye muscle planes are inline with the semicircular – in order to make eye movements easier.
-B2 = nasal orientation
-B3 = intraaural orientation
Describe the origination of:

1. Abducens nucleus
2. Medial rectus muscle
3. Control of abducens nucleus
Abducens nucleus comes from outside the rhombomere where all other muscles come from
Medial rectus was not in earlier organisms. It came from the inferior rectus as a split off it; sharks = from sup rectus. the abducens nucleus is under the control of the hox genes.


NOTES:
The abducens nucleus does not seem to be a motor neuron but a somatic neuron that innervates the rhombomeres; the medial rectus is not present in the early days, originated as a split of the inferior rectus muscles; don’t need to know this; the abducens nucleus is under the control of the hox genes.
What are the 8 types of eye movements based on their functional contexts and the 2 major categories that these 2 fall under?
1) Saccades (fast, tracking)
2) Smooth pursuit (slow,
tracking)
3) Fixation
4) Compensatory eye
movements (vestibulo-
ocular reflex)
5) Nystagmus
6) Optokinetic reflex
7) Vergence
8) Divergence

The two major categories are:

1. Conjugate movements = eyes move into the same direction at the same time.
-Saccades
-Smooth pursuit
-Compensatory eye movements (vestibulo-ocular reflex)
-Optokinetic reflex
-“Fixation”

2. Disconjugate movements = eyes do not move into the same direction at the same time
-Vergence/Divergence

NOTES
Disconjugate = with focusing on an object
Describe nystagamus and its 2 phases?

Nystagmus – called an eye movement, an expression of a laboratory or pathological issue; Specialized eye movement circuits. There is no “nystagmus center” per se. Nystagmus occurs as a result of a disequilibrium in neuronal circuits, or as a testing vehicle in the laboratory. Composed of a quick phase and a slow phase; used for diagnosis. Note video: quick phases and slow phases shown in video – characteristic of nystagmus. Graph is an expression of the pathology for diagnosis. There are slow and quick phases of contraction and dilation with nystagmus
What happens if movements are not properly space-time coordinated?
double vision – diplopia - will result.

-Humans are foveated animals and if images don’t get to fovea we have diplopia
Describe poststimulatory nystagamus and optokinetic nystagmus?

1. If you spin a person around in a chair – and then stop it – de-celaration and cupula deflection occurs and results – post-stimulatory nystagmus –

2. optokinetic nystagmus is a visually induced nystagmus; as you go from acceleration sensitivity – mechanical properties makes the acceleration into a velocity signal that goes to the brain; second integration is made in the brainstem done near the vestibular nucleus; Therefore, being unable to produce high accelerations scientists used deceleration to test for nystagmus
Information gets to the brain and we turn velocity signal to position signal. Resulting eye movement is opposite to the movement and an integration (second one) is done in the secondary integration system in the brainstem.

--In the pic above, Person in striped room = optogenetic nystagmus
Is a sensation that you are turning youself = flicker
Describe the vestibulo-ocular reflex circuitry (the three - neuron arc)?

-In general, passively induced eye movements, such as the vestibulo-ocular reflex are three-dimensional.

-We already know that the pulling directions of the eye muscles correspond with the orientation of the 3 vestibular semicircular canals so there is also a conservation of the principal neuronal connections for the production of compensatory eye movements.

-Within this framework,
1. excitatory connections are formed between the anterior canal and the ipsilateral superior rectus and the contralateral inferior oblique muscles

2. between the posterior canal and the ipsilateral superior oblique and the contralateral inferior rectus muscles

3. horizontal canal and the ipsilateral medial rectus and the contralateral lateral rectus muscles.

-Since the antagonists to these muscles will have to relax at the same time, we observe the existence of inhibitory connections to these antagonists arriving from the same semicircular canals. This innervation scheme has been termed the "elementary vestibulo-ocular reflex arc", or the "three-neuron arc".

- The three neurons involved in this reflex arc are :

1. primary vestibular neurons
2. second-order vestibular neurons
3. the respective extraocular motoneurons

NOTES:

Three neuron arc – from the vestibular canals to the eye muscle – connectivity from each canal to the eye muscle; red = excitatory, blue – inhibitory.
A – The anterior semicircular canal acts on the vestibular motor neurons activates the superior oblique and inhibits the inferior oblique in the right eye when the superior rectus of the left eye is activated. The antagonist of the muscles must relax as the others are excited, thus receive inhibitory stimulation and allow for proper movements.

B – excitiation of the inferior olbique and inhibition of the superior oblique by the posterior semicircular canal;

C – Breakup of connectivity – abducens internuclear pathway, the horizontal canal has a surrogate nucleus that connects the medial rectus motor neurons to the vestibular signals that arrive in the abducens motor nucleus;
Describe the role of the abducens internuclear neuron (IN)?

Abducens inter-nuclear pathway in the horizontal canals connects Medial Rectus neurons to vestibular neurons in Abducens nucleus. This is because the superior rectus and Superior Oblique project contralaterally and Medial Rectus is ipsilateral so it needs the abducens inter-nuclear pathway to connect it to its contralateral pathway
All circuits shown are for voluntary control. Therefore, there is Specialized horizontal eye movement circuitry with three-neuron-arc link to abducens motoneurons and surrogate neuron, i.e., the abducens internuclear neuron (IN) to contralateral medial rectus motoneurons for the production of horizontal conjugate eye movements (Hering’s Law of equal innervation of eye movers).
What happens if there is a lesion of the abducens internuclear pathway (AIN)?
It causes the symptom of internuclear opthalmoplegia, i.e. a palsy of the medial rectus muscle innervated by the affected abducens internuclear neuron, leading to a deficit in adduction in the affected eye and horizontal double vision. Therefore, the interneucler neuron is prone to lesions and leads to a lack of the ability to adduct the eye across the midline towards the nose because the medial rectus is not supplied and this can occur bilaterally due to a vascular incident.
Describe the Ocular Tilt Reaction (OTR)syndrome?

The OTR is an eye-head response consisting of head tilt, skew deviation and ocular torsion. The syndrome seems to be based on an isolated lesion of otolith/posterior canal pathways resulting in an isolated contraction of one inferior oblique and the superior rectus muscle of the other eye.
1. Describe the acceleration phase in the vestibular system and relate it to optokinetic input?

2. Differentiate btw central field and peripheral field?

1. Acceleration phase in darkness is only one detected in the vestibular system; in light acceleration and velocity are picked up .Optokinetic input is able to come in then to allow for that; it is in the opposite direction to the vestibular system
In real life there are no sustained velocity for a long time? Unless in a plane or something.

2. The central field gives object motion and the peripheral field gives you self-motion.
1. What brain regions are involved in optokinetic nystagmus?

2. Give an example of optokinetics?
1. The associative visual system and primary input from the motor visual cortex – MT and MST within the temporal area and VIP – ventral interparietal area, frontal eye fields (FEF) located in the precentral gyrus; V4 – located in the temporal cortex and activation of the cerebellum;

The MT and MST, VIP, and the vestibular cortex (PIVC) – the PIVC is inhibited; Areas in the cerebellar hemispheres (culmen).

2. Head-bobbing is an optokinetic response.
The structure of retinal flow depends on what?
The eye movement and on the 3D structure of the environment. Eyes are round so natural movement is rotational.
Describe saccadic eye movements?
It is important for fixation in looking at objects and the saccadic eye movements shift the fovea rapidly to a target with a speed of up to 900 deg/sec.
Describe the saccade generator network?

1. Horizontal saccades around the abducens nucleus: Horizontal saccades are generated in the pontine reticular formation (PPRF).

2. Vertical saccades in the midbrain: vertical saccades are generated in the Mesencephalic Reticular Formation.


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1. Describe the role of superior colliculus in voluntary saccades?

2. Why is substantia nigra important?

3. What is area LIP?

4. What happens if there is a lesion to the frontal cortex?

1. Voluntary saccades are controlled by the cerebral cortex (frontal eye fields/supplementary eye fields) via the superior colliculus.

2. Substantia nigra pars reticulata suppresses superior colliculus output. The substantia nigra is under the control of the caudate nucleus (basal ganglia).

3. Area LIP (Lateral Inter Parietal) in the parietal cortex modulates visual attention of saccade generation.

NOTES:

Superior colliculus provides a map of the world, does not provide the time frame of the eye movement which is provided within the reticular formation. The basal ganglia becomes involved once inhibition of the network is removed by the substantia nigra pars reticulate.

4. Humans with lesions to the frontal cortex have difficulties suppressing unwanted saccades.
Frontal eye fields have large cognitive functions
1. What is the role of smooth pursuit eye movements?

2. Control of smooth pursuit involves what?

3. Input for smooth pursuit comes from where?

1. Smooth pursuit eye movements keep the image of a moving target on the fovea. So, eye movements that react to targets and follow targets.

2. Control of smooth pursuit involves the cerebral cortex, the cerebellum and the pons.

3. Inputs arise from the motion-sensitive regions in the superior temporal sulcus (MT/MST: V5) and the Frontal Eye Fields.

In the pic above,Smooth eye movement circuit in yellow. Goes from the striate cortex to the MT and MST, to the frontal eye fields the DPN à to the cerebellum .

NOTE:
Smooth pursuit is hard to do voluntarily cos you need moving target to do this
1. Describe the "fixation" eye movement?

2. What area of the brain is involved in fixation?
1. The fixation system holds the eye still during intent gaze to allow examination of stationary objects. Therefore, Must keep eyes on target to allow for fixation–holds eye still.

- For that purpose, the eyes must stay still in the orbits. This action requires active suppression of eye movements.

2. The most rostral portion of the superior colliculus has a representation of the fovea. Neurons here discharge strongly during active fixation.
1. Describe the idea of retinal disparity for stereo-vision?

2. What 2 eye movements are involved in retinal disparity for stereo-vision?

1. When we look at an object that is close to us, our eyes rotate close to each other, or converge; when we look at an object further away, the eyes rotate away from each other, or diverge. This mechanism ensures that objects of interest are on the same place on both retinae and this is called retinal disparity for stereo-vision.

2. The 2 eye movements involved are vergence and divergence.

-Regarding the picture above:
-In top image, you focus on the mountain and the trees are blurred and you see them as one.
-In bottom image you may see two mountains as you focus on the tree.

Eye further away has to cover a steeper angle than the other eye to look at someone.

Vergence/divergence neurons are found on top of the oculomotor (III) nucleus.
Relate "Listing's Law" to involuntary eye movements?
-Involuntary (reflex) eye movements are three-dimensional, voluntary eye movements are two-dimensional.
-Voluntary eye movements lack the torsion component because they obey Listing’s Law.

-The principle that when eye turns from looking at one object and fixes upon another, it revolves about an axis perpendicular to a plane cutting both the former and the present lines of vision.
Descrive visual feedback?
Each time neurons are used there is no feedback control, the feedback takes the form of visual feedback – adaptation to a change in glasses may occur depending on whether your glasses are to big or too small; eye movements begin to match the prescription of the glasses and resembles motor learning.
1. Describe Marr - Albus theory?

2. Describe the cerebellar modules?

Marr-Albus Theory – Visual input comes in by the climbing fibers through the inferior olive and change input from parallel mossy fibers to others and they synapse in the purkinje cells to correct vestibular-ocular reflexes for eye movements
THIS IS NOT TRUE!!! The theory was disproven.


2. - Horizontal eye muscles and vestibular canals work in one module. Vertical ones work in another module… not sure if is true. Module is there but learning is in the vestibular nuclei. Cerebellum tells brainstem how to adjust and makes the nuclei there. So, the neurons down in the brainstem learn not the climbing fibers acting on the cerebellum.

---ALL YOU NEED TO KNOW IS: Cerebellum is necessary for the motor learning of the brainstem
Inferior olive is also important for learning and if is not there then there is no learning


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Describe strabismus (squint)?
Strabismus (squint):
Muscle imbalance resulting in misalignment of the visual axes of the two eyes. Usually causes diplopia, and in children one image will be centrally suppressed resulting in later diminished visual acuity of the disused eye (amblyopia ex anopsia).

-Amblyopia = weakness of the visual system
Plasticity does a lot of work in correcting eye problems
Describe the pupillary light reflex circuit?

This results when light shined into one eyes causes both pupils to constrict. The light enters and is transmitted via optic nerve and goes directly to pretectum nuclei of the midbrain. The pretectum nuclei will then project crossed and uncrossed fibers to the rostral Edinger - Westphal nucleus of the midbrain and then to the cilliary ganglion to cause constriction of muscles of the eye. It is a good test of oculomotor output and brain functioning.
Describe how horner's syndrome occurs?

The pupillary dilation pathway is mediated by the sympathetic division of the autonomic nervous system and it consists of a three neuron arc. The sympathetic innervation of the pupil emanates from the hypothalamus, and involving the superior cervical ganglion.
-Lesion of the central or peripheral sympathetic nervous system specifically lesion of the superior cervical ganglion leads to Horner’s Syndrome, i.e., miosis (narrow pupil), ptosis (hanging of the upper lid), anhidrosis, and enophthalmus.
Describe these eye movement recording devices:

1. Nystagmo- graphy (EOG: electro-oculogram)

2.Video-Oculography

3. Frenzel Goggles

4. Scleral Eye Coil

5. Laser ophthalmoscope
1. Nystagmography (EOG: electro-oculogram): electrodes placed on both sides to differentiate between the back and front of the eye; can do all directions of the eye; Electrodes for back and front of eye (one side is neg and the other is pos)

2.Video-Oculography: visual system where the eyes can be recorded.

3. Frenzel Goggles: makes the eyes very big – the examiner can see the eyes of the patient very well;good to suppress fixation in the patient

4. Scleral Eye Coil: Put a plastic lens inside the eye, connected to wire and records eye movement; This is the method of choice and the wire comes out and is plugged into a machine to detect eye movement and done usually for scientific purposes.

5. Laser ophthalmoscope: method of choice for measurement of ocular torsion.
State 8 eye movements tests?

1. Cover test – cover dominant eye;

2. Ocular following test – follow finger;

3. Saccade test – follow snapping noise without moving head;

4. Smooth pursuit test – follow movement with eyes only;

5. Gaze holding test – tests whether a gaze can be held;

6. Vergence divergence – bring eye in close and take it away;

7. Optokinetic test – rotating cylinder to generate optokinetic stynagmus;

8. Head jerk test – if lesion is present, contralateral eye movement would be delayed;

NOTES:
strabism– non-dominant eye looks away when both eyes are open but looks at target when dominant eye is closed
Get optokinetic nystagmus when looking at rotating object– cannot be suppressed so is a good test for ppl who want to pretend they are blind. The picture above shows head jerk normal people would make saccadic eye movement. The Bottom left – healthy control of the test; right – delayed movement .
What is cervical vertigo?
Compression of basilar artery when they bend their necks
Get vertigo when they look to the left– jerking of the eye ball (“right beating”)
Has to be upright all the time as a result
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