Neurology of the Visual System

Question 1

Describe the visual pathway anatomy or neural pathway, including the retinal pathway, and identifying the first, second, third and fourth order neurones along the way

Answer 1

1. Rod and Cone Photoreceptors - first order neurones
2. Retinal Bipolar Neurones and other intermediate neurones - second order neurones
3. Retinal Ganglion Cells - third order neurones
4. Retinal Ganglion Cells continue along the optic nerve bilaterally, becoming myelinated
5. 50% of the ganglion nerve fibres decussate at the optic chiasm - these are fibres from the nasal retina, while 50% continue ipsilaterally - these are fibres from the temporal retina
6. Continues along the optic tracts
7. Terminates, synapsing at the lateral geniculate nucleus (LGN)
8. Optic radiation - fourth order neurones then begin
9. They travel to and terminate at the Primary Visual Cortex (aka Striate Cortex)
10. The Primary Visual Cortex (Striate Cortex) can then communicate with the Extra-Striate Cortex for further visual processing

Question 2

What does the receptive field of a neurone describe?

Answer 2

• Retinal space within which incoming light can alter the firing pattern of a neuron

Question 3

1) How does the receptive field of neurones vary as you get from lower to higher order neurones?
2) Describe the receptive field of a photoreceptor
3) Describe the receptive field of a bipolar neurone cell
4) Describe the receptive field of a retinal ganglion cell

Answer 3

1)

• The receptive field increases

2)

• Small space surrounding the photoreceptors

3)

• Neighbouring photoreceptors converging on the bipolar neurone cells

4)

• Neighbouring photoreceptors converging on the retinal ganglion cells
• Therefore, also the neighbouring photoreceptors converging on all of these bipolar neurone cells

Question 4

What does the convergence of a receptive field describe?

Answer 4

Convergence – Number of lower order neurons field synapsing on the same higher order neuron

Question 5

Describe the properties that low convergence of receptive fields confers

Answer 5

• Fine visual acuity
• Low light sensitivity

Question 6

Describe the properties that high convergence of receptive fields confers

Answer 6

• Coarse visual acuity
• High light sensitivity

Question 7

How does the convergence of receptive fields of cones and rods vary, and thus how does it affect their properties they confer to vision?

Answer 7

• Convergence of cones < convergence of rods
• Therefore there is more fine visual acuity and low light sensitivity in cones (photopic vision)
• And coarse visual activity and high light sensitivity in rods (scotopic vision)

Question 8

1) What are on-centre and off-centre retinal ganglion cells?
2) What do these principles allow?

Answer 8

1)

• On-centre: stimulated by light at the centre of the receptive field. Inhibited by light on the edge of the receptive field
• Off-centre: Inhibited by light at the centre of the receptive field. Stimulated by light on the edge of the receptive field

2)

• Contrast sensitivity
• Enhanced edge detection

Question 9

Which retinal ganglion fibres cross contralaterally at the optic chiasm and which continue ipsilaterally?

Answer 9

• Nasal retinal ganglion cells cross contralaterally
• Temporal retinal ganglion cells continue ipsilaterally

Question 10

What visual field defects will occur with lesions at the optic chiasma?

Answer 10

• Damages crossed ganglion fibres from nasal retina in both eyes
• Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia

Question 11

What visual field defects will occur with lesions posterior (note this is in reverse to the direction you think it is - I mean closer to the eye not along the optic tract) to the optic chiasma, mention for each side?

Answer 11

• Right sided lesion – Left Homonymous Hemianopia in Both Eyes
• Left sided lesion – Right Homonymous Hemianopia in Both Eyes
• N.B. Homonymous Hemianopia = visual field loss on the same side of both eyes

Question 12

Give a condition that causes bitemporal hemianopia and how it causes it

Answer 12

• Pituitary adenoma - enlargement of pituitary
• Compresses at the optic chiasm just above it
• Thus causing bitemporal hemianopia

Question 13

What conditions typically give rise to homonymous hemianopia (with macular sparing)?

Answer 13

• Stroke or cerebrovascular accidents
• Lesion posterior to the optic chiasm

Question 14

What neuro-anatomical landmark can help identify the location of the primary visual cortex?

Answer 14

The primary visual cortex lies along the calcarine fissure

Question 15

What is a distinctive feature upon examination of the primary visual cord, which also gives rise to its alternate name, the ‘striate cortex’?

Answer 15

• Characterized by a distinct stripe derived from the myelinated fibre of the Optic Radiation projecting into the Visual Cortex

Question 16

Describe the representation of various aspects of the visual field in the primary visual cortex (PVC)

Answer 16

• Disproportionately large section representing the macula
• Superior visual field projects to below the calcarine fissure
• Inferior visual field projects to above the calcarine fissure
• Right hemifield from both eyes projects onto left side of PVC
• Left hemifield from both eyes projects onto right side of PVC

Question 17

What characteristic visual deficits does stroke cause, and explain how it causes this?

Answer 17

• Contralateral homonymous hemianopia with macular sparing
• Causes injury posterior to the optic chiasm therefore causing homonymous hemianopia, and due to the decussation of the fibres, the visual field deficit will be contralateral
• However, since there is dual blood supply from right and left posterior cerebral arteries perfusing the macula, it is spared of injury and therefore of defect

Question 18

1) Where is the extrastriate cortex?
2) What is the function of the extrastriate cortex?

Answer 18

1)

• Surrounds the striate cortex but contained within the occipital lobe

2)

• Higher visual processing

Question 19

1) What is the dorsal pathway from the primary visual cortex (PVC) - what is the neural pathway?
2) What is the function of this pathway?
3) Thus what would damage to this pathway cause?

Answer 19

1)

• PVC → projects to the posterior parietal cortex

2)

• Motion detection

3)

• Motion blindness

Question 20

1) What is the ventral pathway from the primary visual cortex (PVC) - what is the neural pathway?
2) What is the function of this pathway?
3) What condition could damage to this pathway cause?

Answer 20

1)

• PVC → projects to the inferiotemporal cortex (temporal visual cortex)

2)

• Object representation facial recognition
• Detailed fine central vision
• Colour vision

3)

• Cerebral achromatopsia - inability to perceive colour despite normal functioning cone photoreceptors

Question 21

Describe the pupillary responses to light and dark environments, how it does this mechanically and neurally (neural stimulation) and why it does this (what adaptational benefits does it confer)?

Answer 21

In light:

• PUPIL CONSTRICTION: iris muscle contracts to constrict the pupillary aperture
• Mediated by parasympathetic nerve within CN III (oculomotor nerve)
• Decreases spherical aberrations and glare
• Increases depth of field
• Reduces light entering the eye
• Reduces bleaching of photo-pigments

In dark:

• PUPIL DILATION: Radial muscle contracts to cause this
• Mediated by sympathetic nerve
• Allows more light into the eye

Question 22

Outline the neural afferent and efferent pathways from and to the pupil

Answer 22

Afferent

• Follows the optic pathway initially, then breaks away at a point
• So Rod and Cone photoreceptors → Bipolar cells → Retinal Ganglion cells → along Optic Tract → some pupil-specific ganglion cells exit the Optic Tract before reaching the Lateral Geniculate Nucleus (LGN) → instead synapses at the pretectal nucleus in the brainstem → then to the Edingar-Westphal Nucleus bilaterally from each eye

Efferent

• Edinger-Westphal Nucleus → Oculomotor Nerve (CN III) efferent → synapses at Ciliary Ganglion → Short Posterior Ciliary Nerve → Pupillary Sphincter

Question 23

1) What is the direct light reflex?
2) What is the consensual light reflex?
3) What is the physiological basis for the consensual light reflex?

Answer 23

1)

• Pupillary constriction of the light-stimulated eye

2)

• Pupillary constriction of fellow eye of the light-stimulated eye - i.e. stimulating one eye with light will cause pupillary constriction in both

3)

• Afferent pathway from pupils on either side alone will stimulate efferent pathway on both sides

Question 24

Keeping in mind the direct and consensual pupillary reflexes, consider what will happen when you stimulate the right and left eyes in right afferent defect e.g. damage to the optic nerve

Answer 24

• No direct pupillary constriction in right eye when shone in right eye
• No consensual pupillary constriction when shone in right eye
• Intact direct pupillary constriction in left eye when shone in left eye
• Intact consensual pupillary constriction in both eyes when shone in left eye

Question 25

Keeping in mind the direct and consensual pupillary reflexes, consider what will happen when you stimulate the right and left eyes in right efferent defect e.g. damage to CN III (oculomotor nerve)

Answer 25

• No direct pupillary constriction in right eye when light shone in right eye
• Consensual reflex is intact when light shone in right eye i.e. the left eye will constrict consensually but remember the right won’t be able to despite detecting light
• Intact direct pupillary constriction in left eye when light shone in left eye
• No consensual reflex when light shone in left eye - i.e. the right eye won’t constrict

Question 26

Describe the RAPD (relative afferent pupillary defect) swinging torch test

Answer 26

• Alternating stimulation of right and left eye with light
• Both Pupils constrict when light swings to undamaged side
• Both Pupils paradoxically dilate when light swings to the damaged side

Question 27

What does duction mean, in terms of eye movement?

Answer 27

• Movement in one eye

Question 28

1) What does version mean, in terms of eye movement?
2) What does dextroversion mean?
3) What does levoversion mean?
4) What does supraversion mean?
5) What does infraversion mean?

Answer 28

1)

• Movement of both eyes in the same direction

2)

• Gaze to the right

3)

• Gaze to the left

Question 29

What do vergence and convergence mean, in terms of eye movement?

Answer 29

• Vergence = movement of both eyes in opposite directions simultaneously
• Convergence = simultaneous adduction (inward) movement in both eyes when viewing a near object

Question 30

List the 7 extraocular muscles, what movement they produce (direction and on what) and what nerves they are supplied by

Answer 30

1. Levator Palpebrae Superioris - elevates the eyelid - CN III (oculomotor)
2. Superior rectus - raises the eye - CN III (oculomotor)
3. Inferior rectus - suppresses the eye - CN III (oculomotor)
4. Lateral rectus - abducts eye - CN VI (abducens)
5. Medial rectus - adducts eye CN III (oculomotor)
6. Superior oblique - down and in on eye CN IV (trochlear)
7. Inferior oblique - up and out on eye - CN III (oculomotor)

Question 31

What occurs in 3rd nerve palsy that causes the eye to be directed oddly - like what signs are there indicating it?

Answer 31

• Extraocular muscles innervated by CN III are defective
• So those not affected are unopposed too
• Ptosis (droopy eyelid) due to defective levator palpebrae superioris
• The affected eye is down and out - down due to unopposed superior oblique muscle and out due to unopposed lateral rectus muscle

Question 32

What occurs in 6th nerve palsy that causes the eye to be directed oddly - like what signs are there indicating it?

Answer 32

• Abducens defect
• Therefore defect in lateral rectus as this is supplied by the abducens
• Inability to abduct - if you ask the patient to try they cannot
• The affected eye is excessively adducted at rest due to unopposed action of medial rectus
• Double vision worsens upon gazing to the side of the affected eye

Question 33

What cranial nerve palsy does this patient have, and how have you deduced this?

Answer 33

• 3rd nerve palsy (oculomotor)
• The affected eye is down and out
• Due to unopposed actions of the superior oblique and lateral rectus muscles respectively
• Also there is ptosis (droopy eyelid) due to defective levator palpebrae superioris

Question 34

What cranial nerve palsy does this patient have, and how have you deduced this?

Answer 34

• 6th nerve palsy (abducens)
• The eye is adducted at rest, due to unopposed action of the medial rectus unopposed by the defective lateral rectus which is innervated by the abducens nerve

Question 35

1) Describe the optokinetic nystagmus reflex
2) How can this be used as a clinical test?

Answer 35

1)

• You put up a grating pattern on a screen for the patient to observe
• Notice their eyes will demonstrate smooth pursuit (slowly follows the direction of motion), followed by fast phase reset saccade to reset the gaze back

2)

• Can be used to test visual acuity in pre-verbal infants i.e. to see if the child can see well when you can’t simply ask them as they can’t talk back
• They will demonstrate a present optokinetic nystagmus reflex