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Semester 4 - Clinical Neuroscience > 5. The retina and central visual pathways > Flashcards

5. The retina and central visual pathways Flashcards

1
Q

what are 3 layers of the eye?

A

 Outermost sclera (tough and continuous with dural sheath of the optic nerve)
 Uvea (pigmented vascular layer)
 Retina

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2
Q

what does the uvea consist of?

A
  • Choroid sitting just deep to sclera

* Ciliary body and iris sitting anteriorly

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3
Q

What are the 2 layers of the retina?

A

pigmented layer and neural layer

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4
Q

What is the function of the pigmented layer?

A
  • Main site of absorption of light preventing scatter of light (improved visual acuity)
  • anchors the retina to the choroid layer (vascular tunic)
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5
Q

What cells are present in the neural layer?

A
  • Photoreceptor cells (rod and cones)
  • Horizontal cells
  • Bipolar cells
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6
Q

What are the functions of rods and cones?

A

Rods: black and white, low acuity, night vision
Cones: colour, high acuity

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7
Q

What is the function of horizontal cells?

A

Lateral inhibition (disables the spreading of action potentials from excited neurons to neighbouring neurons in the lateral direction) - detects where signal is coming from and inhibits photoreceptors on either side of the photoreceptor detecting signal

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8
Q

What is the function of bipolar cells?

A

Stimulate ganglion cells which form action potential

- between photoreceptors and ganglion cells

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9
Q

list the layers of the retina from superficial to deep

A
  • Retinal pigment epithelium
  • Photoreceptor cells
  • Bipolar cells
  • Ganglion cell layer
  • Nerve fibre layer
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10
Q

What clinical diseases can be seen on fundoscopy?

A
  • retinopathies (e.g. hypertension, diabetes)
  • vascular occlusions (branch of central retinal artery)
  • macula degeneration
  • papilloedema
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11
Q

What does occlusion of a branch of central retinal artery cause?

A

Amaurosis (dark) fugax (fleeting/temporary) - curtain blindness

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12
Q

what is normal in fundoscopy?

A

The normal appearance of the fundus, with the macula (point of highest acuity) sitting lateral to the optic disc (point of exit of ganglion cell axons). Branches of central retinal artery and vein are visible on the macula

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13
Q

Define papilloedema?

A

Swelling of the optic disc

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14
Q

Define fovea and macula.

A

Macula: yellow spot on retina at the back of the eye with high concentration of cones
Fovea: at the centre of the macula, only contains cones

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15
Q

Why is there a depression at the centre of the fovea and why is it important?

A
  • due to splayed motion of the axons

- reduced distance light has to penetrate to get to the photoreceptors

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16
Q

What are the divisions of the retinal fibres/optic nerve?

A 
Laterally = temporal fibres (upper and lower)
Medially = nasal fibres (upper and lower)
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17
Q

Which of the retinal nerve fibres form the optic chiasm?

A

The upper and lower nasal fibres deccusate to form the optic chiasm
(temporal fibres remain ipsilateral)

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18
Q

What is formed after the optic chiasm?

A

Optic tracts

- contain ipsilateral temporal fibres and contralateral nasal fibres

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19
Q

Where do the optic tracts terminate?

A

Lateral geniculate nucleus of the thalamus

20
Q

What is formed after the LGN and where do they go to?

A

Optic radiations, go to the primary visual cortex (occipital lobe)

21
Q

What are the different optic radiations and where do they go through?

A

Superior optic radiations: parietal lobe
- continuation of superior quadrant fibres (temporal and nasal)
Inferior optic radiations: temporal lobe
- continuation of inferior quadrant fibres (temporal and nasal)

22
Q

What are the superior and inferior optic radiations also called?

A
  • Superior: Baum’s loop

- Inferior: Meyer’s loop

23
Q

summarise visual pathway

A

◦ Temporal fibres run ipsilateral
◦ Nasal fibres decussate at the optic chiasm
◦ Optic Tracts run to the lateral geniculate nucleus
◦ Optic Radiations run to the primary visual cortex
- Superior – via parietal lobes
- Inferior – via temporal lobes

24
Q

Why is binocular vision important?

A

Each eye has its own set of visual fields
◦ These overlap to form our binocular vision
◦ Good for depth perception

25
Q

What are the divisions of the visual fields?

A

Temporal (lateral) and nasal (medial)

26
Q

Which retinal fibres detect light from which visual fields?

A

Nasal fibres are responsible for our temporal field of vision
Temporal fibres are responsible for our nasal field of vision

27
Q

The eye is a pinhole camera, what does it imply?

A

This implies that light from a lateral visual field is detected by the medial retina and that light from an upper visual field is detected by the inferior retina

28
Q

where do ganglion cell supplying the temporal and nasal retina supply?

A

Ganglion cells supplying the temporal retina project to the ipsilateral cerebral hemisphere whereas ganglion cells from the nasal retina project to the contralateral hemisphere via the optic chiasm (i.e. they decussate)
 This implies that the left binocular visual field projects to the right hemisphere and vice versa

29
Q

What are some different visual field defects?

A
  • monocular blindness
  • bitemporal hemianopia
  • homonomous hemianopia
  • quadrantanopias
30
Q

What would be the deficit in an optic nerve lesion, what is this called?

A

Temporal and nasal fibres on the ipsilateral side are affected
Therefore the nasal and temporal visual fields are lost on the ipsilateral side
- monocular blindness

31
Q

What would be the deficit in an optic chiasm lesion, what is this called?

A
  • Nasal fibres affected bilaterally
  • temporal fibres unaffected
  • temporal visual fields affected bilaterally
  • bitemporal hemianopia
  • tunnel vision - outer peripheral vision affected
32
Q

What would be the deficit in an optic tract lesion, what is this called?

A
  • Ipsilateral temporal fibres and contralateral nasal fibres
  • ipsilateral nasal visual field and contralateral temporal visual field lost
  • homonomous hemianopia
  • if lesion on right tract, left homonomous hemianopia as left visual field affected and vice versa
33
Q

which quadrant of vision is the superior and inferior optic radiation responsible for?

A

◦ Superior radiations are responsible for our inferior quadrant field of vision - Project into the parietal lobe
◦ Inferior radiations are responsible for our superior quadrant field of vision - Project into the temporal lobe

34
Q

What would be the deficit in a right superior optic radiation lesion, what is this called?

A
  • Ipsilateral superior temporal fibres and contralateral superior nasal fibres lost
  • Ipsilateral loss of inferior nasal visual field and contralateral loss of inferior temporal visual field
  • Quadrantanopias
  • left HOMONOMOUS INFERIOR QUADRANTANOPIA - as left side of vision affected
35
Q

What would be the deficit in a left inferior optic radiation lesion, what is this called?

A
  • Ipsilateral inferior temporal fibres and contralateral inferior nasal fibres lost
  • Ipsilateral loss of superior nasal visual field and contralateral loss of superior temporal visual field
  • right HOMONOMOUS INFERIOR QUADRANTANOPIA
36
Q

what if both superior and inferior radiations are affected??

A
  • ipsilateral Superior and inferior temporal fibres lost
  • ipsilateral superior and inferior nasal visual field loss
  • contralateral Superior and inferior nasal fibres lost
  • contralateral superior and inferior temporal visual field loss
  • HOMONOMOUS HEMIANOPIA
37
Q

What is macula sparing?

A

Visual field loss that preserves vision in the centre of the visual field
- occurs with vascular pathologies

38
Q

Why does macular sparing occur?

A

Occipital lobe has dual blood supply
◦ Posterior cerebral artery
◦ Middle cerebral artery (occipital pole)

In a stroke affectng the posterior cerebral artery…
◦ Most of occipital lobe will be lost
◦ However, middle cerebral a. supplies the occipital pole (represents the macula)
◦ Therefore macular function (central vision) will be spared

39
Q

If macular sparing occurs, then what part of the pathway has been affected?

A

Visual cortex of occipital lobe

40
Q

damage to which 5 areas of the visual pathway can cause contralateral homonymous hemianopia

A

 Damage to the optic tract causes a contralateral homonymous hemianopia
 Damage to the lateral geniculate causes a contralateral homonymous hemianopia
 Damage to both optic radiations causes a contralateral homonymous hemianopia
 Non vascular damage to the occipital lobe can cause a contralateral homonymous hemianopia without macular sparing
 Occlusion of the posterior cerebral artery causes a contralateral homonymous hemianopia with macular sparing

41
Q

what does Damage to the superior and inferior optic radiation cause?

A

 Damage to the superior optic radiations (in the parietal lobe) causes contralateral homonymous inferior quadrantanopia
 Damage to the inferior optic radiations (in the temporal lobe) causes contralateral homonymous superior quadrantanopia

42
Q

Describe the Pupillary light reflex

A
  • light stimulates the optic nerve
  • synapses in the pretectal area
  • gives rise to neurones supplying the edinger westphal nuclei bilaterally
  • both the oculomotor and parasympathetic nerves are stimulated to cause direct and consensual pupillary constriction
43
Q

What are the 3 C’s of the accommodation reflex?

A
  • Convergence (medial rectus)
  • Pupillary Constriction (sphincter pupillae)
  • Convexity of the lens to increase refractive power (ciliary muscle)
44
Q

What pathway does the first part of the accommodation reflex follow?

A

Follows the visual pathway via the lateral
geniculate nucleus to the visual cortex (image analysis must involve the cortex)
- then signals to the midbrain etc

45
Q

summarise pupillary light reflex

A

 Afferent arm: optic nerve
 Processing centres:
• Pretectal nucleus which projects bilaterally to Edinger Westphal nuclei (which contain parasympathetic preganglionics)
 Efferent arm: oculomotor nerve
 Effect: illumination of the eye leads to both direct and consensual pupillary constriction. The consensual reflex is mediated by the bilateral projections from the pretectal nucleus

46
Q

summarise accommodation reflex

A

 Afferent arm: optic nerve
 Processing centres:
• Visual cortex (via lateral geniculate nucleus), allowing processing of visual image which then project to oculomotor and Edinger Westphal nuclei
 Efferent arm: oculomotor nerve
 Effect: focusing on a near object leads to pupillary constriction, convergence of the eyes (contraction of medial recti) and thickening of the lens

Semester 4 - Clinical Neuroscience (19 decks)

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