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Year 2 - BRS > 2.16 - Visual system > Flashcards

2.16 - Visual system Flashcards

1
Q

What are the components of the eye? (10 - Label slide)

A
  • upper eyelid
  • palpebral fissure
  • lateral canthus
  • lower eyelid
  • pupil
  • iris
  • sclera
  • medial canthus
  • caruncle
  • limbus (border between cornea and sclera)
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2
Q

What is the normal antero-posterior diameter of the eye in adults?

A

24mm

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3
Q

What are the three layers of the coat of the eye?

A
  • sclera - hard and opaque
  • choroid - pigmented and vascular
  • retina - neurosensory tissue
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4
Q

What is the sclera?

A
  • commonly known as the ‘white of the eye’
  • tough opaque tissue that serves as the eye’s protective outer coat
  • high water content
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5
Q

What is the uvea?

A

Vascular coat of eyeball and lies between the sclera and retina

Consists of iris, ciliary body and choroid

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6
Q

What are the three parts of the uvea and how are they connected?

A
  • iris
  • ciliary body
  • choroid
  • intimately connected and a disease of one part also affects the other portions (though not necessarily to the same degree)
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7
Q

What is the retina?

A
  • very thin layer of tissue that lines the inner part of the eye
  • responsible for capturing the light rays that enter the eye (like film in photography)
  • these light impulses are sent to brain via optic nerve for processing
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8
Q

What is the optic nerve and where is it?

A
  • transmits electrical impulses from the retina to the brain
  • connects to the back of the eye near the macula
  • visible portion is called the optic disc (blind spot)
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9
Q

What is the blind spot?

A

Where the optic nerve meets the retina there are no light sensitive cells (anatomical landmark is optic disc - visible portion of optic nerve)

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10
Q

What is the macula and where is it?

A
  • located roughly in the centre of the retina, temporal to the optic nerve
  • a small and highly sensitive part of the retina responsible for detailed central vision (e.g. reading or facial recognition)
  • (fovea is the centre of the macula and has high concentration of cone photoreceptors)
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11
Q

What is the fovea?

A

The very centre of the macula

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12
Q

What is central vision responsible for? (4)

A
  • detail day vision
  • colour vision (fovea has highest concentration of cone photoreceptors)
  • reading
  • facial recognition
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13
Q

What is central vision assessed with?

A

Visual acuity assessment

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14
Q

What does a loss of foveal vision lead to?

A

Poor visual acuity

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15
Q

What is peripheral vision responsible for? (4)

A
  • shape
  • movement
  • night vision
  • navigation vision
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16
Q

What is peripheral vision assessed with?

A

Visual field assessment

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17
Q

What would a loss of visual field cause?

A

Unable to navigate in environment, patient may need white stick even with perfect visual acuity

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18
Q

What are the three layers of the retina?

A
  • outer layer - photoreceptors (1st order neuron)
  • middle layer - bipolar cells (2nd order neuron)
  • inner layer - retinal ganglion cells (3rd order neuron)
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19
Q

What is the function of the outer layer of the retina?

A
  • detection of light
  • photoreceptors (1st order neuron)
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20
Q

What is the function of the middle layer of the retina?

A
  • local signal processing to improve contrast sensitivity
  • bipolar cells (2nd order neuron)
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21
Q

What is the function of the inner layer of the retina?

A
  • transmission of signal from eye to the brain
  • retinal ganglion cells (3rd order neuron)
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22
Q

What are the two types of photoreceptors?

A
  • rods (periphery)
  • cones (centre)
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23
Q

What do rod cells do?

A
  • 100x more sensitive to light that cones
  • slow response to light
  • responsible for night vision (scotopic vision)
  • 120 million rods per eye
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24
Q

What do cone cells do?

A
  • less sensitive to light, but faster response
  • responsible for daylight fine vision and colour (phototopic vision)
  • 6 million cones per eye
  • (think C=colour)
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25
Q

What is refraction?

A

As light goes from one medium to another, the velocity changes (causing the light to bend)

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26
Q

What are the two basic types of lenses?

A
  • converging lens (convex) - takes light rays and brings them to a point (focal point)
  • diverging lens (concave) - takes light rays and spreads them outward
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27
Q

What is emmetropia?

A
  • adequate correlation between axial length and refractive power
  • parallel light rays fall on the retina
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28
Q

What is ametropia (refractive error)?

A
  • mismatch between axial length and refractive power
  • parallel light rays do not fall on the retina
  • e.g. near-sightedness (myopia), far-sightedness (hyperopia), presbyopia
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29
Q

What is myopia?

A
  • parallel rays converge at a focal point anterior to the retina
  • aetiology unclear - genetic factor
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30
Q

What are the causes of myopia?

A
  • excessive long globe (axial myopia) - more common
  • excessive refractive power (refractive myopia) - lens too strong
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31
Q

What are the symptoms of myopia?

A
  • blurred distance vision
  • squint in an attempt to improve uncorrected visual acuity when gazing into distance
  • headache
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32
Q

What is hyperopia?

A
  • parallel rays converge at a focal point posterior to the retina
  • aetiology unclear - inherited?
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33
Q

What are the causes of hyperopia?

A
  • excessive short globe (axial hyperopia) - more common
  • insufficient refractive power (refractive hyperopia) - weak lens
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34
Q

What are the symptoms of hyperopia?

A
  • visual acuity at near tends to blur relatively early - nature of blur varies from inability to read fine print to clear near vision but sudden and intermittently blur
  • blurred vision more noticeable if person tired, weak printing or inadequate light
  • asthenopic symptoms - eyepain, headache in frontal region, burning sensation in eyes
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35
Q

What is the near response triad used for?

A

Eye’s adaptation for near vision

36
Q

What does the near response triad consist of?

A
  • pupillary miosis (sphincter pupillae) to increase depth of field
  • convergence (medial recti from both eyes) to align both eyes towards a near object
  • accommodation (circular ciliary muscle) to increase refractive power of lens (thicken lens) for near vision, circular ciliary contracts and zonules relax
37
Q

What is presbyopia?

A
  • naturally occurring loss of accommodation (focus for near objects)
  • onset from 40 years
  • distant vision intact
38
Q

How is presbyopia corrected?

A

Corrected by reading glasses (convex lenses) to increase refractive power of eye

39
Q

What kind of lens is used to treat myopia?

A

Concave lens

40
Q

Where does the visual pathway transmit signals from and to?

A

Transmits signal from eye to visual cortex

41
Q

Describe the order of the visual pathway from eye to visual cortex?

A
  • eye (neurons in retina)
  • optic nerve - ganglion nerve fibres
  • optic chiasm (optic nerves from both eyes converge at the optic chiasm, 53% decussate to form contralateral optic tract)
  • optic tract (ganglion nerve fibres continuation)
  • lateral geniculate nucleus (relay centre within thalamus, where ganglion nerve fibres synapse)
  • optic radiation (4th order neuron, relay signal from lateral geniculate ganglion to primary visual cortex)
  • primary visual cortex / striate cortex (within occipital lobe, relays to extra-striate cortex for higher visual processing)
42
Q

What is the order of neurons in the retina?

A
  • 1st order neurons - rod and cone retinal photoreceptors
  • 2nd order neurons - retinal bipolar cells
  • 3rd order neurons - retinal ganglion cells
  • fibres from these go to form optic nerve
43
Q

Which eyes control which visual fields?

A

For each eye, respective nasal retinas responsible for temporal fields, and temporal retinas responsible for nasal fields

L temporal retina = L nasal field
L nasal retina = L temporal field
R temporal retina = R nasal field
R nasal retina = R temporal field

44
Q

What happens at the optic chiasm?

A
  • partial decussation - 53% of ganglion fibres cross at optic chiasm
  • crossed fibres originate from nasal retina and are responsible for temporal visual field
  • uncrossed fibres originate from temporal retina and are responsible for nasal visual field
45
Q

What do lesions anterior to the optic chiasm affect?

A

Affect visual field in one eye only (as fibres have not crossed) - affect temporal and nasal field

46
Q

What do lesions at the optic chiasm affect?

A
  • damages crossed ganglion fibres from nasal retina (controlling temporal field) in both eyes
  • temporal field deficit in both eyes - bitemporal hemianopia
47
Q

What do lesions posterior to the optic chiasm affect?

A
  • affect visual field in both eyes
  • right-sided lesion - left homonymous hemianopia in both eyes (cannot see left visual field both eyes)
  • left-sided lesion - right homonymous hemianopia (cannot see right visual field both eyes)

Affects ipsilateral temporal retina (ipsilateral nasal field) and contralateral nasal retina (contralateral temporal field)

48
Q

What usually causes bitemporal hemianopia?

A

Typically caused by enlargement of pituitary gland tumour (sits under optic chiasm)

49
Q

What usually causes homonymous hemianopia?

A

Stroke (cerebrovascular accident) causing primary visual cortex damage –> contralateral homonymous hemianopia with macula sparing

50
Q

Why does macular sparing occur in homonymous hemianopia from stroke?

A

Area representing the macula in primary motor cortex receives dual blood supply from posterior cerebral arteries from both sides

51
Q

What is pupillary function?

A

Regulates light input to the eye (like a camera aperture)

52
Q

What happens to pupillary function in light?

A
  • pupil constriction - parasympathetic stimulation causes circular muscles to contract
  • decreases glare
  • increases depth of field (Near Response Triad)
  • pupillary constriction mediated by parasympathetic nerve (within CN III)
53
Q

What happens to pupillary function in the dark?

A
  • pupil dilatation - sympathetic stimulation causes radial muscles to contract
  • increases light sensitivity in the dark by allowing more light into the eye
  • pupillary dilatation mediated by sympathetic nerve
54
Q

Describe the afferent pathway of the pupillary reflex.

A
  • a small sub-section of retinal ganglion cells participate in the pupillary reflex pathway
  • pupil-specific ganglion cells exit at posterior third of optic tract before entering lateral geniculate nucleus
  • afferent pathway from each eye synapses on Edinger-Westphal nuclei on both sides in the brainstem
55
Q

Describe the efferent pathway of the pupillary reflex.

A
  • oculomotor nerve efferent emerges from the Edinger-Westphal nucleus (where afferents from both sides synapse)
  • this synapses at the ciliary ganglion
  • a short posterior ciliary nerve emerges and goes to the pupillary sphincter
56
Q

What is the direct vs consensual pupillary reflex?

A
  • direct light reflex - constriction of pupil of the light-stimulated eye
  • consensual light reflex - constriction of pupil of the other (fellow) eye
57
Q

What is the neurological basis of the consensual light reflex?

A

Afferent pathway on either side alone will stimulate efferent pathway on both sides (as synapses on Edinger-Westphal nucleus on both sides)

58
Q

What happens in a right afferent defect?

And what is an example of an afferent defect?

A
  • e.g. damage to optic nerve
  • no pupil constriction in both eyes when right eye is stimulated with light
  • normal pupil constriction in both eyes when left eye is stimulated with light
59
Q

What happens in a left afferent defect?

A
  • no pupil constriction in both eyes when left eye is stimulated with light
  • normal pupil constriction in both eyes when right eye is stimulated with light
60
Q

What happens in a right efferent defect?

And what is an example of an efferent defect?

A
  • e.g. damage to right CN III
  • no right pupil constriction whether right or left eye is stimulated with light
  • left pupil constriction whether right or left eye is stimulated with light
61
Q

What happens in a left efferent defect?

A
  • no left pupil constriction whether left or right eye is stimulated with light
  • right pupil constriction whether left or right eye is stimulated with light
62
Q

What is the swinging torch test?

A

Light is shone rapidly between two eyes to test for relative afferent pupillary defect

63
Q

What happens in a relative afferent pupillary defect?

(Damage to afferent pathway is usually incomplete/relative)

A
  • partial pupillary response still present when damaged eye is stimulated
  • elicited by swinging torch test - alternating stimulation of right and left eye with light
  • both pupils constrict when light swings to undamaged eye
  • both pupils paradoxically dilate when light swings to damaged side (not neurologically dilate, just in comparison to constriction)
64
Q

What is eye movement necessary for?

A

Acquiring and tracking visual stimuli

65
Q

How many extraocular muscles are there and what three cranial nerves are involved?

A

Eye movement facilitated by six extraocular muscles innervated by three cranial nerves (III, IV, VI)

66
Q

What do the extraocular muscles attach?

A

Attach eyeball to orbit

67
Q

What do the extraocular muscles allow?

A

Straight and rotary movement

68
Q

What are the four straight extraocular muscles?

A
  • superior rectus
  • inferior rectus
  • lateral rectus
  • medial rectus
69
Q

Where is the superior rectus attached to and what is the function?

A
  • attached to the eye at 12 o’clock
  • moves the eye up
70
Q

Where is the inferior rectus attached to and what is the function?

A
  • attached to the eye at 6 o’clock
  • moves the eye down
71
Q

Where is the lateral rectus (AKA external rectus) attached to and what is the function?

A
  • attaches on the temporal side of the eye
  • moves eye towards outside of head (abducts eye)
72
Q

Where is the medial rectus (AKA internal rectus) attached to and what is the function?

A
  • attaches on the nasal side of the eye
  • moves eye towards middle of head (nose) - adducts eye
73
Q

Where is the superior oblique and what is the function?

A
  • attached high on the temporal side of the eye
  • passes under superior rectus
  • travels through the trochlea
  • moves the eye in a diagonal pattern down and out
74
Q

Where is the inferior oblique and what is the function?

A
  • attached low on the nasal side of the eye
  • passes over the inferior rectus
  • moves the eye in a diagonal pattern up and out
75
Q

What does the superior branch of the oculomotor nerve CN III innervate?

A
  • superior rectus - elevates eye
  • levator palpebrae superioris - raises eyelid
76
Q

What does the inferior branch of the oculomotor nerve CN III innervate?

A
  • inferior rectus - depresses eye
  • medial rectus - adducts eye
  • inferior oblique - elevates eye
  • parasympathetic nerve - constricts pupil
77
Q

What does the trochlear nerve CN IV innervate?

A

Superior oblique - depresses eye

78
Q

What does the abducens nerve CN VI innervate?

A

Lateral rectus - abducts eye

79
Q

What muscle do we isolate when testing abduction?

A

Lateral rectus

80
Q

What muscle do we isolate when testing adduction?

A

Medial rectus

81
Q

What muscle do we isolate when testing elevated and abducted?

A

Superior rectus

82
Q

What muscle do we isolate when testing depressed and abducted?

A

Inferior rectus

83
Q

What muscle do we isolate when testing elevated and adducted?

A

Inferior oblique

84
Q

What muscle do we isolate when testing depressed and adducted?

A

Superior oblique

85
Q

What happens in CN III palsy?

A
  • muscles innervated by oculomotor nerve do not work leading to overaction of muscles innervated by CN IV and VI
  • lateral rectus (CN VI) –> outward motion of eye
  • superior oblique (CN IV) –> downward motion of eye
  • ptosis (droopy eyelid) due to loss of levator palpebrae superioris action
  • dilated pupil due to lack of parasympathetic stimulation
86
Q

What happens in CN IV palsy?

A

Muscles innervated by trochlear nerve (superior oblique) do not work –> eye cannot look down/out, unopposed action of eye looking up and in

87
Q

What happens in CN VI palsy?

A

Muscles innervated by abducens nerve (lateral rectus) do not work –> eye cannot look outwards (abduct), unopposed action of eye looking in (adducted)

Year 2 - BRS (84 decks)

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