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Neurology > Neuro 14 - Structure and Function of the Eye > Flashcards

Neuro 14 - Structure and Function of the Eye Flashcards

1
Q

What is the average anterior-posterior diameter of the orbit?

A

24 mm

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2
Q

What are the three layers of the eye? Describe their propertiesand function.

A 
Sclera 
 Hard and opaque
 Maintains the shape of the eye 
Choroid  
 Pigmented and vascular 
 Provides circulation to the eye 
 Shields out unwanted scattered light 
Retina  
 Neurosensory
 Converts light into neurological impulses
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3
Q

What are the two segments of the eye separated by?

A

Lens separates anterior and posterior segments

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4
Q

Which humours are found in the two segments of the eye?

A 
Anterior = aqueous humour  
Posterior = vitreous humour
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5
Q

What name is given to the fibrous strands that suspend the lens from the ciliary bodies?

A

Zonules

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6
Q

Describe the production and drainage of aqueous humour.

A

Aqueous humour is produced by the ciliary body

It is drained via the trabecular meshwork into the canals of Schlemm

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7
Q

What is the role of aqueous humour?

A

Provides nutrients to the cornea and other tissues in the anterior chamber

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8
Q

Describe vitreous humour.

A

It is 99% water trapped inside a jelly matrix

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9
Q

What is the function of vitreous humour?

A

Mechanical support for the eye

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10
Q

Describe how the vitreous humour changes with age.

A

It loses its jelly consistency, liquefies and can become detached from the retina
Vitreous detachment in seen as floaters

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11
Q

What are the potentially disastrous consequences of vitreous humour detachment?

A

Detaching from the retina could cause a small tear in the peripheral retina
If there is a small tear, liquid vitreous could seep into the sub-retinal space and lead to retinal detachment
If untreated, it can lead to blindness

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12
Q

What are the two layers of the iris?

A

Anterior – stromal layer containing muscle fibres

Posterior – epithelium

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13
Q

Describe how the retina and choroid contribute to the different parts of the iris and ciliary body.

A

Retina gives rise anteriorly to the ciliary body epithelium and the posterior (epithelial) layer of the iris
Choroid gives rise anteriorly to the ciliary body stroma and the anterior layer of the iris (stromal layer)

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14
Q

What is the collective term for the choroid, iris and ciliary body?

A

Uvea

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15
Q

What is the normal range for intraocular pressure?

A

11-12 mm Hg

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16
Q

What is glaucoma?

A

Condition of sustained raised intraocular pressure

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17
Q

What changes can be seen in the retina in glaucoma?

A

Retinal ganglion cell death

Enlarged optic disc cupping

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18
Q

What are the consequences of untreated glaucoma?

A

Progressive loss of peripheral vision

Blindness

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19
Q

What is the most common type of glaucoma and what is it causedby?

A

Primary open angle glaucoma

It is caused by a functional blockage of the trabecular meshwork

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20
Q

State another relatively common type of glaucoma. What is it caused by?

A

Closed angle glaucoma
This can be acute or chronic
It is caused by the forward displacement of the iris-lens complex –narrowing the trabecular meshwork

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21
Q

In what type of patients does closed angle glaucoma tend to occur and what is the treatment?

A 
Small eyes (hypermetropic) 
Treatment: peripheral laser iridotomy
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22
Q

Describe the structure of the lens.

A

It has an outer acellular capsule
There are regular inner elongated fibres, which give the lens its transparency
NOTE: may lose transparency with age

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23
Q

Which two structures provide the majority of the refractive power of the eye?

A 
Cornea = 2/3 
Lens = 1/3
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24
Q

What layer of the eye is the cornea continuous with?

A

Sclera

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25
Q

Other than its role in refracting light, what else is the corneanecessary for?

A

Physical barrier – protects the eye from opportunistic infection

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26
Q

What are the consequences of prolonged contact lens wear?

A

Reduce the oxygen supply to the cornea

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27
Q

What are the three layers of the cornea?

A

Epithelium
Stroma
Endothelium

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28
Q

What is the role of the endothelial layer of the cornea?

A

It pumps out fluid from the stroma and prevents stromal oedema

29
Q

What is the role of tear film?

A

Maintains clear vision and removes surface debris

30
Q

What are the three layers of the tear film?

A

Superficial oily layer
Aqueous tear film
Mucinous layer

31
Q

What produces the superficial oily layer?

A

Meibomian glands

32
Q

What are the roles of each of these three layers?

A

Superficial Oily Layer
 Reduces tear film evaporation
Aqueous Tear Film
 Oxygen and nutrients
 Bactericide
Mucinous Layer
 Ensures that tear film sticks to the eye
 Conjunctiva is a transparent layer above the cornea that is very vascular
 The conjunctiva has goblet cells that produce mucin

33
Q

Where is the lacrimal gland located?

A

Superio-laterally to the orbit

34
Q

What are the three types of tears?

A

Basal Tears – produced at a constant level in the absence of irritation
Reflex Tears – increased tear production in response to irritation
Emotional Tears – crying

35
Q

Describe the innervation of the cornea.

A

The cornea is very sensitive and it is innervated by the ophthalmic branch of the trigeminal nerve (CN V)

36
Q

Describe the drainage of tears.

A

Tears are drained into two puncta via two tiny holes in the upper and lower medial lid margins
It then drains into superior and inferior canaliculi –> common canaliculus –> tear sac
Tear sac then drains through the nasolacrimal duct, which opens up inthe inferior meatus

37
Q

What two things regulate how much light reaches the retina?

A

Pupil

Pigmented Uvea

38
Q

What is the term given to perfect focusing ability?

A

Emmetropia

39
Q

What is the technical term for long-sightedness?

A

Hypermetropia

40
Q

How can long-sightedness be corrected and what is it caused by?

A

Convex lenses
It is usually caused by having short eyeballs
It is occasionally caused by a flat corneal surface

41
Q

What is the technical term for short-sightedness?

A

Myopia

42
Q

How can short-sightedness be corrected and what is it caused by?

A

Concave lenses
It is usually caused by having a long eyeball
It is occasionally caused by having a highly curved cornea

43
Q

What is astigmatism?

A

The cornea is oval rather than round
This means that the refractive power varies in different planes (in some planes you will be hypermetropic, and in others you would be myopic)

44
Q

Which nerve is responsible for accommodation?

A

Oculomotor Nerve (CN III)

45
Q

What is the near response triad?

A

Pupillary Miosis (constriction of sphincter pupillae) – increases the depth of field
Convergence – the medial recti of both eyes contract
Accommodation – ciliary muscles contract to thicken the lens and increase its refractive power

46
Q

What is the term given to naturally occurring loss of accommodation with age?

A

Presbyopia

47
Q

What are the four branches of vessel arcades radiating from the optic disc?

A

Superior Temporal
Inferior Temporal
Superior Nasal
Inferior Nasal

48
Q

Describe the difference in perfusion between the outer and inner parts of the retina.

A

Inner 2/3 of the retina = retinal arteries

Outer 1/3 of the retina = choroidal vasculature

49
Q

What part of the retina is responsible for central vision?

A

Fovea (it has the highest concentration of cones)

50
Q

What is peripheral vision responsible for?

A

Shape, movement, night vision

51
Q

Describe the structure of the retina.

A

Just inside the choroid you have the retinal pigment epithelium
Then you have the neuroretina, which consists of:
 Outer Layer – photoreceptors (rods and cones)
 Middle Layer - bipolar cells
 Inner Layer – retinal ganglion cells

52
Q

What is the function of the retinal pigment epithelium?

A

Transports nutrients from the choroid to the photo-receptor cells and removes metabolic waste from the retina

53
Q

Describe how the fovea appears on a cross-section of the macula.

A

It appears as a foveal pit due to the absence of overlying retinalganglion cells

54
Q

State the two classes of photo-receptor and their properties.

A

Rods
 Longer outer segment with photo-sensitive pigments
 100 times more sensitive to light than cones
 Slow response to light
 Responsible for night vision (scotopic vision)
 120 million rods
Cones
 Less sensitive to light
 Faster response to light
 Responsible for daylight vision and vision and colour vision (photopic vision)
 6 million cones

55
Q

Describe the recycling of photopigments.

A

Photopigments are synthesised in the inner photo-receptor segment and then are transported to the outer segment discs
The distal discs with deactivated photo-pigments are shredded from the tips and phagocytosed by retinal pigment epithelial cells
The deactivated photo-pigments are regenerated inside the retinal pigment epithelial cells and are then transported back to the photo-receptors

56
Q

What is the rod photopigment?

A

Rhodopsin

It is a G-protein coupled receptor system

57
Q

What is the co-factor for this photopigment?

A

11 cis retinal (vitamin A derived)

58
Q

To what wavelength does rhodopsin react maximally in humans?

A

498 nm

59
Q

What is another term for night vision?

A

Scotopic vision

60
Q

What is another term for day-time vision?

A

Photopic vision

61
Q

Describe the distribution of rods and cones across the retina.

A

Rods have the highest density just outside the macula
They decrease in density the further you move away from the macula
There are NO rods in the macula
Cones are ONLY found in the macula
The highest density of cones is in the fovea

62
Q

What does a hill of vision represent?

A

Sensitivity of vision over a visual field

63
Q

Where is the physiological blind spot located?

A

20 degrees temporal to central vision

64
Q

What are the three types of cone photopigment and which colours do they respond maximally to?

A

S-cone – short wavelength – BLUE
M-cone – medium wavelength – GREEN
L-cone – long wavelength – RED

65
Q

What is the most common colour vision deficiency and what is it caused by?

A

Deuteranomaly

It is caused by the shifting of the M-cone towards the L-cone

66
Q

What is the term given to shifted peaks?

A

Anomalous trichomatism

67
Q

What test is used to diagnose colour blindness?

A

Ishihara Test

68
Q

Describe how light sensitivity changes in dark adaptation.

A

Light sensitivity increases in dark adaptation

69
Q

How does retinal light change in light adaptation and what is responsible for this effect?

A

Light sensitivity decrease in dark adaptation
This suppression of light sensitivity is caused by photopigment bleaching and neuro-adaptation inhibiting rod and cone function

Neurology (19 decks)

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