Neuro6 - The Retina & Central Visual Pathways

Question 1

5 features of the retina

Function
Blood Supply
Optic Disc
Macula
Optical Coherence Tomography

Answer 1

1.) Function - receives and converts focused light into neural signals which are sent to the occipital lobe

2. ) Blood Supply - central retinal artery/vein
   - branch of the ophthalmic artery
   - branch out from the optic disc
   - amaurosis fugax is transient loss of vision due to reduced blood supply to the retina
3. ) Optic Disc - where ganglion cells axons leave the eye
   - after leaving the eye, they form the optic nerve
   - no photoreceptors so is a blind spot
   - optic disc swelling (papilloedema) occurs in raised ICP
4. ) Macula - centre of the retina containing the highest concentration of cones allowing for high visual acuity
   - fovea is in the centre of the macula
   - macular degeneration impairs high res, colour vision

5.) Optical Coherence Tomography - imaging of retina

Question 2

4 types of cells/layers in the retina

Pigmented Layer
Photoreceptor Cells
Horizontal Cells
Bipolar Cells

Answer 2

1. ) Pigmented Layer - main site of light absorption
   - helps modulate amount of light you are receiving
   - lack of pigment (albinism) leads to photophobia
   - also anchors the photoreceptor cells
2. ) Photoreceptor Cells - rods and cones
   - rods: B/W and low visual acuity
   - cones: colour and high visual acuity
3. ) Horizontal Cells - lateral inhibition of photoreceptors
   - narrows signal being sent, preventing overload of impulses to the brain

4.) Bipolar Cells - connects photoreceptors to axons

Question 3

5 features of the central visual pathway

Optic Nerve
Optic Chiasm
Optic Tract
Optic Radiations
Optic Radiation and LGN Blood Supply

Answer 3

1. ) Optic Nerve - retina –> optic chiasm
   - nasal (medial) and temporal (lateral fibres)
   - each eye is split into quadrants e.g. upper and lower nasal and upper and lower temporal fibres for each eye
2. ) Optic Chiasm - where L/R optic nerve cross over
   - temporal fibres remain ipsilateral
   - nasal fibres decussate
3. ) Optic Tract - chiasm –> lateral geniculate nucleus
   - contains temporal fibres from ipsilateral side
   - contains nasal fibres from contralateral side
   - LGN is the relay centre in the thalamus
4. ) Optic Radiations - LGN –> primary visual cortex (PVC)
   - superior optic radiations (Baum’s loop) run through the parietal lobe
   - inferior optic radiations (Meyer’s loop) run through the temporal lobe
   - e.g. RIGHT superior optic radiation contains:
   - upper right temporal and upper left nasal fibres
5. ) Optic Radiation Blood Supply
   - superior optic radiation supplied by superior MCA
   - inferior optic radiation supplied by inferior MCA
   - lateral geniculate nucleus supplied by PCA

Question 4

3 features of the visual field

Peripheral Vision
Visual Fields
Lesions

Answer 4

1. ) Peripheral Vision - relates to the visual fields
   - each eye has it’s own set of visual fields which overlap to form our binocular vision (depth perception)
2. ) Visual Fields - temporal and nasal
   - nasal fibres are responsible for our temporal field
   - temporal fibres are responsible for our nasal field

3.) Lesions - a point in the visual pathway corresponds to a pattern of visual field loss

Question 5

3 lesions of the optic nerve/chiasm/tract

Monocular Blindness
Bitemporal Hemianopia
Homonomous Hemianopia

Answer 5

1. ) Monocular Blindness - lesion of an optic nerve damages both the temporal and nasal fibres of one eye
   - eg: lesion of the left optic nerve:
   - loss of entire vision of the left eye
2. ) Bitemporal Hemianopia - lesion of the optic chiasm e.g. a pituitary adenoma
   - damages the nasal fibres of the L and R eye:
   - loss of both temporal visual fields

3.) Homonymous Hemianopia - lesion of an optic tract or PVC
damages the temporal fibres from the ipsilateral side and the nasal fibres of the contralateral side
- e.g. lesions of the right optic tract:
- loss of the nasal visual field of the right eye
- loss of the temporal visual field of the left eye

Question 6

3 lesions of the optic radiations

Homonomous Superior Quadrantopia
Homonomous Inferior Quadrantopia
Homonomous Hemianopia

Answer 6

1. ) Homonomous Superior Quadrantopia - lesion of the inferior optic radiation (temporal lobe)
   - damages lower temporal fibres of the ipsilateral eye
   - damages lower nasal fibres of the contralateral eye
   - e.g. lesion of the LEFT inferior optic radiation:
   - loss of the upper nasal field of the L eye
   - loss of the upper temporal field of the R eye
2. ) Homonomous Inferior Quadrantopia - lesion of the superior optic radiation (parietal lobe)
   - damages upper temporal fibres of the ipsilateral eye
   - damages upper nasal fibres of the contralateral eye
   - e.g. lesion of the RIGHT superior optic radiation:
   - loss of the lower nasal temporal field of the R eye
   - loss of the lower temporal field of the L eye
3. ) Homonymous Hemianopia - lesion of the inferior and superior optic radiations e.g. a stroke
   - damages both temporal fibres of the ipsilateral eye
   - damages both nasal fibres of the contralateral eye

Question 7

What is macular sparing?

How does this help differentiate whether the lesion is in the optic tract or the primary visual cortex (occipital lobe) in homonomous hemianopia?

Does this hold for non-vascular lesions e.g. tumours?

Answer 7

Occipital lobe has dual blood supply from the:

• posterior cerebral artery supplies majority
• middle cerebral artery supplies the macula

In a stroke affecting the posterior cerebral artery, most of the occipital lobe will be lost apart from the macula
- therefore, macular function (central vision) is spared

Macula sparing is seen in a lesion in the visual cortex due to the dual blood supply

In non-vascular lesions e.g. a tumour, the entire occipital lobe will be affected so you do not get macular sparing

Question 8

4 features of the medial longitudinal fasciculus (MLF)?

What is it?
Function
Lesions
Clinical Signs x2

Answer 8

1. ) What is it? - pair of crossed over (decussated) tracts on each side of the brainstem
   - it ascends to the lateral wall of the 3rd ventricle, just above the cerebral aqueduct
2. ) Function - conjugate eye movements
   - connects CNIII, CNIV, CNVI nerves together
   - connects CNVIII for information about head movement
3. ) Lesions - MS or lesion of the brainstem
   - MS: demyelination of MLF axons (younger patients)
4. ) Clinical Signs
   - diplopia: due to uncoordinated eye movements
   - internuclear ophthalmoplegia: affected eye shows reduced adduction whilst other eye shows nystagmus

Question 9

How does a tumour in the cerebral aqueduct in the midbrain affect the accommodation reflex?

Location
Bilateral Symptoms
Pupil Convergence
Pupil Constriction
Lens Fattening

Answer 9

1.) Location - cerebral aqueduct is close to the oculomotor nerve and Edinger Westphal nucleus

2. ) Bilateral Symptoms - both eyes affected
   - due to a midline lesion
3. ) No Pupil Convergence - damage to CNIII
   - damage to medial rectus so no adduction
4. ) No Pupil Constriction - damage to EDW
   - parasympathetics affected so no sphincter pupillae
5. ) No Lens Fattening - damage to EDW
   - parasympathetics affected so no contraction of ciliary muscle