The Retina and Central Visual Pathways

Question 1

Describe the structure of the retina?

Answer 1

Pigmented layer = melanin stops light refracting too much, anchors photoreceptive cells (rods/cons)

Neural layer = contains nerves and blood vessels

Question 2

Rods are responsible for what type of vision?

Answer 2

Black and white, low level lighting

Question 3

Cons are responsible for what type of vision?

Answer 3

Colour, high definition vision

Question 4

Outline the pathway from the retina to the optic N

Answer 4

Photoreceptor cells – rods and cons

Bipolar cells (like first order neurones)

Axons of ganglion cells

Converge to form optic N

Question 5

Outline the role of horizontal cells

Answer 5

Lateral inhibition

Stop the receptors next to the point of the highest light intensity from sending any confusing info

= emphasising contrast

Question 6

What is the role of the macula?

Answer 6

Fovea = highest density of con cells = more defined vision

Thinner layer of retinal axons

Question 7

What structures can be seen on fundoscopy?

Answer 7

Fovea

Macula

Optic disk = raised ICP present with bulging blurred outline (papilloedema)

Central retinal vein/artery = can see occlusions – amaurosis fugax

Retinal venules/arterioles

Question 8

Give an overview of the central visual pathway

Answer 8

Retina

Optic nerve

Optic chiasm

Optic tract

Lateral geniculate nucleus

Optic radiation

Primary visual cortex (occipital lobe)

Question 9

What 2 fibres are present in the visual pathway?

Answer 9

Nasal fibres = temporal field of vision, cross at optic chiasm

Temporal fibres = nasal field of vision, run ipsilateral

Question 10

How do lesions present when they are located before the optic chiasm?

Answer 10

Signs are unilateral and ipsilateral

Question 11

How do lesions present when they are at or after the optic chiasm?

Answer 11

Signs are bilateral

Site of crossing over

Question 12

How do lesions present when they are after the optic chiasm?

Answer 12

Signs are contralateral

As the pathway has crossed over so will be seen on the opposite side

Question 13

Outline monocular blindness

Answer 13

Lesion of the optic N

Lesion to the optic N is on the same side as the blindness (remember it hasn’t crossed over yet)

Children = optic N glioma, retinoblastoma

Middle aged = optic stealth meningiomas

Question 14

Outline bitemporal hemianopia (tunnel vision)

Answer 14

Lesion at optic chiasm

Affects both nasal fibres = temporal fields lost

Causes = growth of pituitary, aneurysm of anterior communicating artery

Question 15

Outline left homonomous hemianopia

Answer 15

Lesion of R optic tract = R temporal and L nasal fibres affected

Lose L temporal + R nasal vision

Causes = vascular stroke most common

Question 16

Outline right homonomous hemianopia

Answer 16

Lesion of L optic tract = L temporal and R nasal fibres affected

Lose R temporal and L nasal vision

Causes = vascular stroke most common

Question 17

What is macular sparing?

Answer 17

Occipital lobe has dual blood supply = posterior cerebral A + middle cerebral A

Stroke affecting posterior cerebral A = most of occipital lobe lost

However middle cerebral A supplies occipital pole = macula = macular function spared (central vision)

Question 18

Explain optic radiations

Answer 18

Superior radiations = inferior field of vision (project into parietal lobe)

Inferior radiations = superior field of vision (project into temporal lobe)

Quadrantanopias – loss of a quarter of the visual field

Question 19

Outline the pupillary light reflex

Answer 19

Afferent = CN II optic N

Synapse with pretectal nucleus – send signals to Edinger Westphal nuclei bilaterally

Efferent = parasympathetic CN III oculomotor N

(BOTH stimulated = direct and consensual pupillary constriction)

Question 20

Outline the accommodation reflex

Answer 20

Required for near vision

1) eye converge (medial rectus)
2) pupillary constriction (constrictor pupillae)
3) convexity of lens (ciliary muscle) = increase refractory power

Afferent = retina CN II optic N

Synapse lateral geniculate nuclei – projects to visual cortex (processes finger getting closer to the face)

Communicates this back to the midbrain – Edinger Westphal and Oculomotor nuclei

Efferent = parasympathetic CN III oculomotor N

Question 21

The lateral geniculate nucleus projects to which lobe?

Answer 21

Occipital lobe

Question 22

The optic nerve has no photoreceptors so is referred to as what?

Answer 22

The blind spot

Question 23

Outline the layers of the retina

Answer 23

1) retinal pigment ep = contain melanin, stop excessive reflections, maintain photoreceptor cells
2) rod and cons = photoreceptors
3) horizontal cells = lateral inhibition
4) bipolar neurone = can think of as 1st order neurones
5) ganglion = can think of as 2nd oder neurones
6) optic N

Question 24

What is retinal detachment?

Answer 24

Most common form = photoreceptors separate from the underlying pigment ep (neural retina detaches from the pigment layers)

= fluid build up between

Presents = sudden blurring, stars, visual artefacts

Question 25

What is amacrosis fugax?

Answer 25

Sudden transient loss of vision = emboli blocks ophthalmic A

‘Curtain coming down over vision’

Question 26

If the L and R visual fields are generated in each hemispheres how do we see a complete image?

Answer 26

Commissural fibres of corpus callosum connecting the 2 visual cortex

Question 27

What does the medial longitudinal fasciculus contain with regards to the eyes?

Answer 27

Connections between: oculomotor, abducens, trochlear nuclei to coordinate eye movements

Vestibular nuclei are also attached = maintain a level horizon

All are connected to the spinal cord = enabling generation of compensatory movements

Question 28

Describe Internuclear ophthalmoplegia

Answer 28

Paralysis of the eyeballs caused by loss of connection between the cranial nuclei