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Y2 Neuro (LCRS2) > Structure and Function of the Eye > Flashcards

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 properties and 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 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 Uvea?

A

Vascular coat of eye ball

Comprised of the choroid, iris and ciliary body

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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

Optic neuropathy with characteristic structural damage to the optic nerve, associated with progressive retinal ganglion cell death, loss of nerve fibres and visual field loss

Increased intraocular pressure is a risk factor

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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 cornea necessary 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 layers of the cornea?

A

1 – Epithelium

2 – Bowman’s Membrane

3 – Stroma

4 – Descemet’s Membrane

5 – 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

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29
Q

What is the role of tear film?

A

Tear film maintains smooth cornea-air surface

  • Oxygen Supply to Cornea – Normal cornea has no blood vessels
  • Removal of Debris (Tear film and Blinking)
  • Bactericide
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30
Q

What are the three layers of the tear film?

A

Superficial oily layer

Aqueous tear film

Mucinous layer

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31
Q

What produces the superficial oily layer?

A

Meibomian glands

‘my-bow-me-an’

32
Q

What are the roles of each tear film layer?

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

33
Q

Where is the lacrimal gland located?

A

Anterio-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

Hyperopia

40
Q

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

A

Convex lenses

It is usually caused by having a short globe (axial hyperopia)

It is occasionally caused by a flat corneal surface or insufficient refractive power (refractive hyperopia)

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 globe (axial myopia)

It is occasionally caused by having a highly curved cornea or excessive refractive power (refractive myopia)

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)

Parallel rays come to focus in 2 focal lines rather than a single focal point

Cause : refractive media is not spherical–>refract differently along one meridian than along meridian perpendicular to it– >2 focal points ( punctiform object is represent as 2 sharply defined lines)

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

‘pris-be-opia’

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 (1st Order Neuron) = Detection of Light

Middle Layer – Bipolar Cells (2nd Order Neurons) = Local Signal Processing to improve contrast sensitivity, regulate sensitivity

Inner Layer – Retinal Ganglion Cells (3rd Order Neurons) = Transmission of Signal from the Eye to the Brain

52
Q

What is the function of the retinal pigment epithelium?

A

Transports nutrients from the choroid to the photoreceptor cells and removes metabolic waste from the retina

Phagocytosis of shed discs from photoreceptor cells

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

1) 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

2) 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 rod 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 20-40o outside the macula

They decrease in density the further you move away from the macula

There are NO rods in the macula

Cones are almost exclusively 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

Biphasic Process

  • Cone adaptation 7 minutes
  • Rod adaptation 30 minutes – regeneration of rhodopsin. Rods become more sensitive than cones after 10 mins
69
Q

How does retinal light sensitivity 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

70
Q

What is the reflex pathway of reflex tear production

A

Cornea irritation V1 afferent PNS efferent

71
Q

Label this

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72
Q

Label this

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73
Q

Label this

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74
Q

Label this

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75
Q

Label this

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76
Q

What is the conjunctiva?

A

The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye.

It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids.

It is nourished by tiny blood vessels that are nearly invisible to the naked eye.

The conjunctiva has goblet cells that produce mucin

Y2 Neuro (LCRS2) (19 decks)

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