Visual Pathways and Control of Eye Movements

Question 1

Signals received from eye sent to which part of brain?

What are the main landmarks of the visual pathway?

Answer 1

Visual cortex

-Eye
-Optic Nerve (retinal ganglion nerve fibres)
-Optic Chiasm
-Optic Tract– Ganglion nerve fibres
exit as optic tract
-Lateral Geniculate Nucleus
-Optic Radiation (4th order neurons from LGN to visual cortex NB retinal ganglion cells= 3rd order neurons)
-Primary Visual Cortex (Striate Cortex)

Question 2

Primary visual cortex found where?

Where do retinal ganglion axons coming down the optic nerve synapse?

Answer 2

Within occipital lobe

Lateral Geniculate Nucleus, they synapse here with 4th order neurons which form the optic radiation, this goes to the visual cortex

Question 3

Where is the lateral geniculate nucleus found?

Answer 3

Thalamus

Question 4

What are the fibres leaving the lateral geniculate nucleus called?

Answer 4

Optic Radiation

Question 5

Which order neurones are these and where do they terminate?

Answer 5

4th Order Neurones

They terminate in the primary visual cortex

Question 6

What are the first, second and third order neurones in the visual pathway?

Answer 6

First Order – photo-receptors (rods and cones)
Second Order – bipolar cells
Third Order – retinal ganglion cells

Question 7

What happens as the retinal ganglion cells enter the optic nerve, which improves the transmission of the signal?

Answer 7

They become myelinated

Question 8

What percentage of retinal ganglion cell fibres crosses the midline at the optic chiasma?

Answer 8

53%

Question 9

Describe the convergence and receptive field sizes of rods and cones.

Answer 9

Rod system has high convergence and hence larger receptive fields for the ganglion cells that they eventually converge onto
Cone system has low convergence (less cones converge to higher order neurons) = smaller receptive fields for the ganglion cells

Question 10

What is retinal convergence and how does central retinal convergence differ from peripheral retinal convergence

Answer 10

retinal convergence = Basically how many rods converge onto a higher order neuron.
Cones not involved because they are pretty much always 1:1.

central retinal convergence is lower than peripheral ie less rods converge onto one bipolar cell near the macula compared to the periphery where there is more convergence

Question 11

What is the benefit of having high convergence and a large receptive field?

Answer 11

High light sensitivity (at the cost of visual acuity)

Question 12

What is the benefit of having low convergence and a small receptive field?

Answer 12

Fine visual acuity (at the cost of light sensitivity)

Question 13

Retinal ganglion cells can be described as on-centre or off-centre. What do these two terms mean?

Answer 13

On-centre
– stimulated by light falling on the centre of the receptive field
-inhibited by light falling on the edge of the receptive field
Off-centre – they are stimulated by light falling on the edge of the receptive field and inhibited by light falling on the centre

This is important in contrast sensitivity and enhanced edge detection ie sensing difference in the colour/brightness of colour of the object which helps u to distinguish it from others in the background

Question 14

Where do the fibres that decussate at the optic chiasma originate?

Answer 14

The nasal part of the retina

These fibres are responsible for the temporal half of the visual field

Question 15

What effect do lesions anterior to the optic chiasm have on vision?

Answer 15

Affects only ONE eye (unilateral field loss)

Question 16

What effect do lesions posterior to the optic chiasm have on vision?

Answer 16

Affects BOTH eyes
Right-sided lesion: left homonymous hemianopia both eyes
Left-sided lesions: right homonymous hemianopia both eyes

Question 17

What is the effect of a lesion at the optic chiasm? What normally causes lesions at optic chiasm which causes this?

Answer 17

Bitemporal hemianopia

Pituitary gland tumour (pituitary is below optic chiasm)

Question 18

Which part of the brain does the upper division of the optic radiation travel through and which parts of the visual field is it responsible for?

Answer 18

Parietal Lobe
Responsible for the inferior visual quadrants

Each optic radiation controls the same half of visual field in both eyes. This optic radiation then splits into two branches and so each branch will be in charge of a QUADRANT of the total visual field per eye

Question 19

Which part of the brain does the lower division of the optic radiation travel through and what part of the visual field is it responsible for?

Answer 19

Temporal Lobe  (upper division goes through parietal lobe because parietal lobe is above temporal lobe and they are all going back towards the occipital lobe where V1 is)
Responsible for the superior visual quadrants

Question 20

What is homonymous hemianopia typically caused by?

Answer 20

Strokes and other cerebrovascular accidents

Question 21

Where is the primary visual cortex located?

Answer 21

Along the Calcarine Fissure in the occipital lobe

Question 22

Describe which parts of the primary visual cortex are responsible for the different fields of vision.

Answer 22

The macula has a disproportionate representation in the primary visual cortex
The left primary visual cortex is responsible for the right visual field from both eyes
The right primary visual cortex is responsible for the left visual field from both eyes
Visual cortex above the calcarine fissure is responsible for the inferior visual field
Visual cortex below the calcarine fissure is responsible for the superior visual field

Question 23

What causes macular sparing homonymous hemianopia? Why is it macular sparing?

Answer 23

Lesion in primary visual cortex (eg stroke)
Macular sparing hemianopia is only present if lesion is in primary visual cortex. If lesion is prior to V1 then it is likely to affect the macula or part of it. This is because the nerve fibres spread more and more as they reach V1 and so the more anterior your lesion is, the more likely that you will damage the fibres supplying the macula as well because all optic fibres are so close together.

Two reasons for macular sparing:
1. The area representing the macula in the primary visual cortex has a dual blood supply (from both right and left posterior cerebral arteries) meaning that it is less vulnerable to ischaemia.

2. Also, since such a large proportion of the V1 is for the macula, you need a lot of damage in order to affect the macula/central vision (the nerve fibres from macula spread out and innervate a wide area of V1, so if you damage parts of V1, it is likely that you did not damage enough fibres from macula in order for you to lose central vision)

Question 24

What are the two pathways of the extrastriate cortex and what are they responsible for?
What are the consequences of damage to these pathways?

Answer 24

Dorsal Pathway – deals with motion detection/peripheral vision (WHERE)

Damage leads to motion blindness

Ventral Pathway – handles detailed object recognition/face recognition, fine central vision/colour recognition (WHAT)

Damage leads to cerebral achromatopsia

Question 25

Describe the pupil's response to light and the consequences of this response. What nerve mediates this pupil response?

Answer 25

Iric circular muscle (sphincter pupillae) contracts= pupil constriction This leads to: -Reduced rate of photopigment bleaching (light causes bleaching, constriction leads to less light entering and so rate of bleaching decreases) -Increased depth of field (distance over which an object may be moved without causing a sharpness reduction beyond a certain tolerable amount. ) Decreases spherical aberrations and glare Oculomotor (III)

Question 26

Describe the pathway that is responsible for the consensual light reflex.

Answer 26

Retinal ganglion cells from the retina send they axons back via the optic nerve The fibres that are responsible for the pupillary reflex will leave at the posterior 1/3 of the optic tract before it reaches the LGN The axons then go to the pretectal nucleus in the dorsal brainstem The afferent pathways from each eye then synapse on the Edinger-Westphal nuclei on both sides of the brainstem. Efferent: Preganglionic parasympathetic fibres (part of CNIII) from the Edinger-Westphal nuclei goes to the ciliary ganglion via Oculomotor nerve. Short ciliary nerves (contain postganglionic parasympathetic fibres of oculomotor) travel from the ciliary ganglion to the pupillary sphincter Summary: Retinal Ganglion Cell --> Pretectal Nucleus --> Edinger-Westphal Nucleus --> Ciliary Ganglion --> Short Ciliary Nerves --> Sphincter Pupillae

Question 27

What would the consequences be of a right afferent defect?

Answer 27

Light shone in right eye: no direct or consensual response | Light shone in left eye: direct and consensual response present

Question 28

What would the consequences be of a right efferent defect?

Answer 28

Light shone in right eye: no direct response (right eye does not constrict), consensual response present (left does) Light shone in left eye: direct response (left eye does constricts), no consensual response (right doesnt)

Question 29

What does RAPD mean?

Answer 29

Relative Afferent Pupillary Defect This is seen in the swinging torch test. A PARTIAL pupillary response is still present despite damage to an eye and its pupillary reflex pathway – there is some degree of constriction but not as much as normal healthy constriction, and therefore when you move the torch onto the defected eye it looks like it dilates a bit because it does not constrict as much as the healthy eye before

Question 30

What test would you do to identify RAPD? What would you expect to see in a patient showing a RAPD?

Answer 30

Swinging Torch Test When the light is shone on the good eye, there will be a direct and consensual response When the light is then swung and shone at the bad eye, there will be a paradoxical dilation of the iris in the bad eye This is because the constriction response elicited by the bad eye is weaker than the consensual response elicited by the good eye

Question 31

# Define: a. Duction b. Version c. Vergeance d. Convergeance

Answer 31

``` a. Duction Movement of one eye b. Version Simultaneous movement of both eyes c. Vergence Simultaneous movement of both eyes in opposite directions d. Convergence Simultaneous adduction of both eyes when viewing a near object ```

Question 32

What is the term for the elevation of one eye?

Answer 32

Supraduction

Question 33

What is the term for the depression of both eyes?

Answer 33

Infraversion

Question 34

What is the technical term for right gaze?

Answer 34

Dextroversion dextro=on the right side

Question 35

What is the technical term for left gaze? What is torsion

Answer 35

Levoversion levo=left Rotation of eye around the anterior-posterior axis of the eye

Question 36

What are the two types of eye movement and how are they different?

Answer 36

Saccade – short fast burst | Smooth Pursuit – sustained slow movement

Question 37

What reflex is used to assess visual acuity in preverbal children?

Answer 37

Optokinetic Nystagmus Reflex triggered by the presentation of a constantly moving grating pattern of various spatial freq Optokinetic- visual movement Smooth pursuit, fast phase reset saccade.

Question 38

State which nerve innervates each of the extraocular muscles that facilitate eye movement by attaching eye to orbit

Answer 38

Lateral Rectus = Abducens (CN VI) Superior Oblique = Trochlear (CN IV) Medial Rectus, Superior Rectus, Inferior Oblique, Inferior Rectus and Levator Palpebrae Superioris = Oculomotor (CN III)

Question 39

In what position would the eye have to be to get maximum elevation/depression due to: a. Superior and Inferior Recti b. Superior and Inferior Obliques

Answer 39

a. Superior and Inferior Recti Abducted b. Superior and Inferior Obliques Adducted

Question 40

Explain why this is with respect to the anterior-posterior axis of the eye.

Answer 40

The anterior-posterior axis of the eye is aligned with the axis of the vertical recti when the eye is abducted If the eye is adducted, the axes are not aligned and contraction of the vertical recti would cause torsion

Question 41

Where do oblique muscles attach to the eye?

Answer 41

Into the sclera posterior to the globe equator They pull the eye forwards and nasally This is because of the pulley system established by the trochlea and the oblique muscles

Question 42

The oculomotor nerve has two branches. State what each of these branches innervates.

Answer 42

``` Superior Oculomotor Nerve  Superior Rectus  Levator Palpebrae Superioris Inferior Oculomotor Nerve  Inferior Rectus  Medial Rectus  Inferior Oblique  Parasympathetic nerve that causes pupil constriction ```

Question 43

How would you test the extraocular muscles?

Answer 43

Isolate the muscle to be tested by maximising its action and minimising the action of the other muscles E.g. to test the superior rectus, make the patient abduct and elevate their eye

Question 44

Describe and explain what you would see in a patient with 3rd nerve palsy.

Answer 44

Their affected eye would point down and out This is because of the unopposed contraction of lateral rectus (innerv by 6th--out) and superior oblique (innerv by 4th-down)

Question 45

Describe and explain what you would see in a patient with 6th nerve palsy.

Answer 45

Affected eye unable to abduct and deviates inwards Double vision worsens when asked to gaze to the side of the affected eye. Eg if left eye affected and you asked patient to look left with both eyes, they will not be able to abduct on their left eye due to 6th nerve palsy but their right eye will adduct, this will lead to two eyes looking at different directions and hence double vision

Question 46

What structure in the brainstem acts as a synchronising link between the eyes, allowing paired eye movements?

Answer 46

Medial Longitudinal Fasciculus

Question 47

Consensual reflex

Answer 47

Afferent pathways on either side will stimulate effect

Question 48

Unilateral afferentdefect

Answer 48

Different response pending on which eye is stimulated

Question 49

Unilateral efferent defect

Answer 49

Same unequal response between left and right | irrespective which eye is stimulated

Question 50

How many extraocular muscles Functions of extraocular muscles

Answer 50

6 Attach eyeball to orbit Straight and rotary mvmt 4 straight muscles - rectus

Question 51

Saccade

Answer 51

Fast - Reflexive - Scanning - Predictive - Memory guided

Question 52

Smooth

Answer 52

Slow | -Driven by motion of a moving target across retina

Question 53

Superior rectus muscle eye mvmt

Answer 53

Moves eye up

Question 54

Lateral rectus

Answer 54

Attaches on termporal side of eye

Question 55

Medial rectus

Answer 55

Attached on nasal side of eye

Question 56

Superior oblique -Attached where - Mvmt of eye - In relation to superior rectus - travels through

Answer 56

Attached high on temporal side of eye Diagonal- down and in Passes under superior rectus Travels through trochlear

Question 57

Inferioir oblique in relation to inferior rectua mvmt of eye

Answer 57

- Passes over inferior rectus | - Up and out

Question 58

Eye mvmt testing

Answer 58

``` Abduction – Lateral Rectus – Adduction – Medial Rectus – Elevated and Abducted – Superior Rectus – Depressed and Abducted – Inferior Rectus – Elevated and Adducted – Inferior Oblique – Depressed and Adducted – Superior Oblique ```

Question 59

3rd order neurons path

Answer 59

3rd order neurons-retinal ganglion - Optic nerve - Partial decuss at optic chiasma 53% of gang - Optic tract - Destinations - -> Lateral Geniculate Nucleus - in thalamus- to relay visual information to visual cortex

Question 60

Homogenous hemianopia is due to

Answer 60

stroke, cerebrovascular accidents

Question 61

where do crossed fibres originate from responsible for

Answer 61

Nasal retina Temporal visual field

Question 62

Where do uncrossed fibres originate from responsible for

Answer 62

Temporal retina Nasal visual field