Visual Pathways and Control of Eye Movements

Question 1

What are the main landmarks of the visual pathway?

Answer 1

Eye
Optic Nerve
Optic Chiasm (1/2 ganglion nerve fibres cross)
Optic Tract
Lateral Geniculate Nucleus
Optic Radiation
Primary Visual Cortex (AKA. Striate Cortex)

Question 2

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

Answer 2

Lateral Geniculate Nucleus

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 optic radiations and where do they terminate?

Answer 5

4th Order Neurones

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 + 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

Rods have high convergence + large receptive fields

Cones have low convergence + small receptive fields

Question 10

Describe how the convergence of the rod system differs across different parts of the retina.

Answer 10

The rod system near that macula has lower convergence than in the peripheral retina

Question 11

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

Answer 11

High light sensitivity

Question 12

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

Answer 12

Fine visual acuity

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 at centre of receptive field + inhibited by light at edge of receptive field
Off-centre: stimulated by light at edge of the receptive field + inhibited by light at centre of receptive field
Important in contrast sensitivity + enhanced edge detection

Question 14

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

Answer 14

Crossed: Nasal part of the retina
Responsible for the temporal half of the visual field
Uncrossed: Temporal retina
Responsible for nasal visual field

Question 15

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

Answer 15

Affects only 1 eye

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
Left-sided lesions: right homonymous hemianopia

Question 17

What is the effect of a lesion at the optic chiasm? What typically causes this defect?

Answer 17

Bitemporal hemianopia

Typically caused by pituitary tumour

Question 18

What would a visual defect with respect to the horizontal line (superior/ inferior) most likely be related to?

Answer 18

Eye e.g. ganglion cells

Question 19

What would a visual defect with respect to the vertical line (not crossing vertical line) most likely be related to?

Answer 19

Neurological problem

Question 20

What is homonymous hemianopia typically caused by?

Answer 20

Strokes + other cerebrovascular accidents

Question 21

Where is the primary visual cortex located?

Answer 21

Along the Calcarine sulcus in the occipital lobe

Question 22

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

Answer 22

Macula has a disproportionate representation in the PVC
Left PVC: responsible for right visual field from both eyes
Right PVC: responsible for left visual field from both eyes
Visual cortex above calcarine fissure is responsible for inferior visual field
Visual cortex below calcarine fissure is responsible for superior visual field

Question 23

How is it possible for the macula to be spared by a stroke in the primary visual cortex leading to homonymous hemianopia?

Answer 23

Area representing the macula has a dual blood supply (from posterior cerebral arteries from both sides)
Thus is less vulnerable to ischaemia

Question 24

What are the 2 pathways of the extrastriate cortex and what are they responsible for? What does damage result in for each?

Answer 24

Dorsal Pathway: motion detection. Damage results in motion blindness
Ventral Pathway: detailed object + face recognition. Damage results in Cerebral Achromatopsia

Question 25

Describe what happens to the eyes in the light.

Answer 25

Circular muscle contracts Constriction of pupillary aperture Reduced rate of photopigment bleaching Increased depth of field

Question 26

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

Answer 26

Retinal ganglion cells send axons back via the optic nerve Fibres responsible for the pupillary reflex exit at posterior 1/3 of the optic tract before entering LGN Axons then go to the pretectal nucleus in the brainstem Afferent pathways from each eye synapses on the Edinger-Westphal nuclei on both sides of the brainstem. PNS nerve from the E-W nuclei to the ciliary ganglion forms the efferent pathway Short ciliary nerves travel from the ciliary ganglion to the pupillary sphincter

Question 27

What would the consequences be of a right afferent defect e.g. damage to optic nerve?

Answer 27

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

Question 28

What would the consequences be of a right efferent defect e.g. damage to right 3rd nerve?

Answer 28

Light shone in right eye: no direct response, consensual response present Light shone in left eye: direct response, no consensual response

Question 29

What does RAPD mean?

Answer 29

Relative Afferent Pupillary Defect | Partial pupillary response still present when damaged eye is stimulated- there is some degree of constriction

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 light is shone on the good eye, there will be a direct + consensual response When the light is then swung + 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

Duction: Movement of 1 eye Version: Simultaneous movement of both eyes Vergeance: Simultaneous movement of both eyes in opposite directions Convergeance: 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

Question 35

What is the technical term for left gaze?

Answer 35

Levoversion

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 | A form of physiological nystagmus triggered by the presentation of a constantly moving grating pattern

Question 38

State which nerve innervates each of the extrinsic eye muscles.

Answer 38

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

Question 39

Where do all the rectus muscles originate?

Answer 39

Common tendinous ring at the apex of the orbit

Question 40

Where do the rectus muscles insert?

Answer 40

Into the sclera anterior to the globe equator

Question 41

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 41

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

Question 42

Where do oblique muscles attach to the eye?

Answer 42

Into the sclera posterior to the globe equator Pull the eye forwards and nasally Because of the pulley system established by the trochlea + the oblique muscles

Question 43

The oculomotor nerve has 2 branches. State what each of these branches innervates.

Answer 43

Superior: Superior Rectus + Levator Palpebrae Superioris Inferior: Inferior + Medial Rectus, Inferior Oblique, Parasympathetic nerve that causes pupil constriction

Question 44

How would you test the extraocular muscles?

Answer 44

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

Question 45

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

Answer 45

Affected eye would point down + out Because of unopposed contraction of lateral rectus + superior oblique Ptosis: because of the loss of innervation of levator palpebrae superioris Pupil dilation: loss of PNS innervation to the eye via CN III

Question 46

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

Answer 46

When asked the abduct the affected eye, they eye will stop around midline Because the lateral rectus isn’t functioning + can’t abduct the eye Can lead to blurred vision

Question 47

Describe the organisation of the primary visual cortex

Answer 47

Columns with unique sensitivity to visual stimulus of a particular orientation Right + left eye dominant columns intersperse each other

Question 48

What is pupillary constriction mediated by?

Answer 48

Parasympathetic nerve (within CNIII)

Question 49

What happens to the pupils in the dark? Why? What mediates this?

Answer 49

Pupil dilation Increases light sensitivity by allowing more light into the eye Mediated by sympathetic nerve

Question 50

Describe the attachment of the superior + inferior rectus

Answer 50

Superior: attached to eye at 12 o'clock. Moves eye up Inferior: attached to eye at 6 o'clock. Moves eye down

Question 51

Define Torsion

Answer 51

Rotation of eye around the anterior-posterior axis of the eye